CA1113553A - High pass switched capacitor filter section - Google Patents
High pass switched capacitor filter sectionInfo
- Publication number
- CA1113553A CA1113553A CA328,323A CA328323A CA1113553A CA 1113553 A CA1113553 A CA 1113553A CA 328323 A CA328323 A CA 328323A CA 1113553 A CA1113553 A CA 1113553A
- Authority
- CA
- Canada
- Prior art keywords
- operational amplifier
- filter
- input
- output
- phase
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H19/00—Networks using time-varying elements, e.g. N-path filters
- H03H19/004—Switched capacitor networks
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H15/00—Transversal filters
Abstract
HIGH PASS SWITCHED CAPACITOR SECTION
Abstract of the Disclosure A high-pass switched capacitor biquadratic filter based on the bilinear z-transform. The filter comprises first and second integrating operational amplifiers connected in series and in combination with a third operational amplifier that serves as a sample and hold and also generates one simple pole and zero pair in the circuit transfer function thereby enabling the circuit to provide for a high degree of filter efficiency in a preselected frequency range. The operational amplifiers are connected to and operate in cooperation with capacitors of a predetermined size which are switched on and off continuously by two phase clock signals supplied to the circuit. The loss characteristic of the filter can be programmed by varying the clocking frequency. Higher order filters can be obtained by the tandem connection of second order circuit sections followed by one or more first order pole-zero section.
Abstract of the Disclosure A high-pass switched capacitor biquadratic filter based on the bilinear z-transform. The filter comprises first and second integrating operational amplifiers connected in series and in combination with a third operational amplifier that serves as a sample and hold and also generates one simple pole and zero pair in the circuit transfer function thereby enabling the circuit to provide for a high degree of filter efficiency in a preselected frequency range. The operational amplifiers are connected to and operate in cooperation with capacitors of a predetermined size which are switched on and off continuously by two phase clock signals supplied to the circuit. The loss characteristic of the filter can be programmed by varying the clocking frequency. Higher order filters can be obtained by the tandem connection of second order circuit sections followed by one or more first order pole-zero section.
Description
i3 1 S P E C I F I C A T I O ~ ¦
3 This invention relates to electronic filters for data 4 transmission or communication systems and electronic control equip-¦
ment and more particularly it relates to a sampled data high pass 6 filter that can be implemented as an integrated circuit semi-7 conductor device.
8 Background of the Invention 9 The design of electronic data transmission systems re-0 quires adequate filters for frequency selective filtering. Such t filters preferably implemented as integrated circu~t devices must 12 not only be compatible with other system components but they should¦
1 utilize a minimum of silicon chip area, have a high dynamic range, ¦
14 provide gain in the passband and should have zeros of transmission I
1 at zero frequency in order to realize high pass filtering. I -16 Prior to the present invention, filters were suggested 17 using s~itched capacitors and opèrational amplifiers. The basic ~8 building block of such circuits was usually a sampled-data inte-19 grator, obtained by replacing the resistor in an R-C active inte-20 l grator with a switched-capacitor resistor. This approach, however, 21¦ presented certain problems because the mere replacement by switched
3 This invention relates to electronic filters for data 4 transmission or communication systems and electronic control equip-¦
ment and more particularly it relates to a sampled data high pass 6 filter that can be implemented as an integrated circuit semi-7 conductor device.
8 Background of the Invention 9 The design of electronic data transmission systems re-0 quires adequate filters for frequency selective filtering. Such t filters preferably implemented as integrated circu~t devices must 12 not only be compatible with other system components but they should¦
1 utilize a minimum of silicon chip area, have a high dynamic range, ¦
14 provide gain in the passband and should have zeros of transmission I
1 at zero frequency in order to realize high pass filtering. I -16 Prior to the present invention, filters were suggested 17 using s~itched capacitors and opèrational amplifiers. The basic ~8 building block of such circuits was usually a sampled-data inte-19 grator, obtained by replacing the resistor in an R-C active inte-20 l grator with a switched-capacitor resistor. This approach, however, 21¦ presented certain problems because the mere replacement by switched
2 capacitors does not simulate the equivalent resistors exactly.
23 Distortion in the frequency response of such circuits occurred be-24 cause of the imperfect mapping o~ the frequency variables when transformed from the s to the z plane. Circuits utilizing a groundt 26 ed capacitor in conjunction-with two switches to replace a resistor 27 are discussed in I~EE Journal of Solid State Circuits, Vol. SC-12, 28 ~ o. 6, pp. 392-599 and pp. ~00-608, Sec. 1977. For such circuits, 29 the mapping between the frequency variables is given-by the ormula 30~ s -~ (z- ~/T, which is equivalent to replacing derivatives in the
23 Distortion in the frequency response of such circuits occurred be-24 cause of the imperfect mapping o~ the frequency variables when transformed from the s to the z plane. Circuits utilizing a groundt 26 ed capacitor in conjunction-with two switches to replace a resistor 27 are discussed in I~EE Journal of Solid State Circuits, Vol. SC-12, 28 ~ o. 6, pp. 392-599 and pp. ~00-608, Sec. 1977. For such circuits, 29 the mapping between the frequency variables is given-by the ormula 30~ s -~ (z- ~/T, which is equivalent to replacing derivatives in the
3~ 3 "
.
differential equation of a continuous system with forward differ-ences. In order to keep a close match ~etween the performances of the continuous and discrete-time systems, the clock rate l/T
must be chosen much higher than the highest frequency present in the signal. In another prior technique, the switched capacitor that replaced the resistor had a special configuration based on the 7 trapezoidal integration. Thus, it performed a conformal mapping from the s plane to the z plane and eliminated the disadvantages 9 mentioned above. The resulting discrete-time response is related 1 to that of the continuous-time model through the bilinear trans-1 formation defined by:
1 2 -1 .': ' ' 1 s ~ > T l_z Equation (l) A significant disadvantage of this latter approach is that the 16 difference of two signals or the negative of a signal cannot be 17 obtained as easily as with the grounded switched-capacitor 1 "resistor." In order to exploit this ease of design inherent in 9 the grounded switched-capacitor circuit, and at the same time 2 compensate for the s-to-z plane mapping defects, a direct 2 z-domain synthesis should be carried out. The present invention 2 describes a filter section that provides a solution to this 23 problem.
24 Another problem which has been overcome by the present invention is that of providing a third order filter section that 2 eliminates analog components in an output sample-and-hold sub-27 section.
28 ~ -29 ~Brief Summary of the Invention -3 In accordance with the principles of the present inventio : - . ' ~ l ~ 3-lj a ovel switched-capacitor circuit or biquadratic section (biquad) ¦
2 is provided, which is suitable for high-pass filters, based on 3 direct z-domain synthesis.
.
differential equation of a continuous system with forward differ-ences. In order to keep a close match ~etween the performances of the continuous and discrete-time systems, the clock rate l/T
must be chosen much higher than the highest frequency present in the signal. In another prior technique, the switched capacitor that replaced the resistor had a special configuration based on the 7 trapezoidal integration. Thus, it performed a conformal mapping from the s plane to the z plane and eliminated the disadvantages 9 mentioned above. The resulting discrete-time response is related 1 to that of the continuous-time model through the bilinear trans-1 formation defined by:
1 2 -1 .': ' ' 1 s ~ > T l_z Equation (l) A significant disadvantage of this latter approach is that the 16 difference of two signals or the negative of a signal cannot be 17 obtained as easily as with the grounded switched-capacitor 1 "resistor." In order to exploit this ease of design inherent in 9 the grounded switched-capacitor circuit, and at the same time 2 compensate for the s-to-z plane mapping defects, a direct 2 z-domain synthesis should be carried out. The present invention 2 describes a filter section that provides a solution to this 23 problem.
24 Another problem which has been overcome by the present invention is that of providing a third order filter section that 2 eliminates analog components in an output sample-and-hold sub-27 section.
28 ~ -29 ~Brief Summary of the Invention -3 In accordance with the principles of the present inventio : - . ' ~ l ~ 3-lj a ovel switched-capacitor circuit or biquadratic section (biquad) ¦
2 is provided, which is suitable for high-pass filters, based on 3 direct z-domain synthesis.
4 In general, the filter circuit is comprised of a first pole-zero section having a capacitor ~etween the negative input of 6 an integrating operational amplifier and a switch means connected 7 to the input source. When the switch means is closed or "on", the 8 side of the capacitor connected to the operational amplifier is hel~
9 at virtual ground while the other side or intermediate node is charged to the input signal. When the switch turns off, the inter-11 mediate node ls held at that voltage at the next clock pulse or 12 switch "on" cycle and it is charged to the next value of the input.
13 The net charge of the voltàge at that point is the difference of 14 the input sample at the present time and the input sample at the previous time cycle.
1~ -- The output of the first operational amplifier has two 17 zeros and two complex conjugate poles in its transfer function be-18 cause of feedback from a second operational amplifier. The output ~9~ of the first operational amplifier is connected to a second and 20 similar pole-zero section that includes a third operational ampli-21 fier controlled by switch means which is "on" during the ~ phase.
22 Thus, this second section is out of phase with the first section an ~3 operates to block the feed through of the undesirable analog compo-24 nents from the input source, while the third operational amplifier 25 also generates one simple pole and zero pair in the circuit trans-26 fer to increase its filtering ~fficiency.
~7 In brief, the o~jects of the invention are; to provide an 28 i~proved switched-capacitor high pass filter; to provide such a hig Z9 pass filter that is xeadily adaptable for implementation as an inte 30 grated circuit device with a minimum of chip area; to provide a ~ ~ .
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9 at virtual ground while the other side or intermediate node is charged to the input signal. When the switch turns off, the inter-11 mediate node ls held at that voltage at the next clock pulse or 12 switch "on" cycle and it is charged to the next value of the input.
13 The net charge of the voltàge at that point is the difference of 14 the input sample at the present time and the input sample at the previous time cycle.
1~ -- The output of the first operational amplifier has two 17 zeros and two complex conjugate poles in its transfer function be-18 cause of feedback from a second operational amplifier. The output ~9~ of the first operational amplifier is connected to a second and 20 similar pole-zero section that includes a third operational ampli-21 fier controlled by switch means which is "on" during the ~ phase.
22 Thus, this second section is out of phase with the first section an ~3 operates to block the feed through of the undesirable analog compo-24 nents from the input source, while the third operational amplifier 25 also generates one simple pole and zero pair in the circuit trans-26 fer to increase its filtering ~fficiency.
~7 In brief, the o~jects of the invention are; to provide an 28 i~proved switched-capacitor high pass filter; to provide such a hig Z9 pass filter that is xeadily adaptable for implementation as an inte 30 grated circuit device with a minimum of chip area; to provide a ~ ~ .
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Il . .
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5;~
filtex with a high dynamic range and one that provides gain in the pass-band; to provide a hi~h pass filter that yields frequency responses that have a low sensitivity to the filter coefficients; to provide a sampled-data filter of the third order that provides an output with no analog components from an analog input signal.
Thus, in accordance with one broad aspect of the invention, there is provided a high-pass sampled-data filter comprising: a first integrating operational amplifier; input means connected to said first operational amplifier for supply-ing a continuous signal voltage to be filtered, said input means having a first switch means adapted to be connected with and controlled by a first phase of a continuous two-pha~e alter-nating clock generator, a first switched capacitor connected to ground and to said input means, and a first gain multiplying capacitor in said input means between said first switched capaci-tor and the input to said first operaticnal amplifier; ~a bypass lead connected between the input and output of said first oper-ational amplifier for providing a feedback thereto and contain-ing an alternating second switch means connected to said clock : generator with a second switched capacitor connected to ground s ecO ~d and to said~switch means so that it is charged during a second clock phase and discharged as a feedback during each first clock phase; a second integrating operational amplifier; means for connecting the output of said first operational amplifier to an input of said second operational amplifier including a third switched capacitor and third switch means connected to said clock generator so that said third switched capacitor is charged during each said second clock phase and is discharged to said second operational amplifier during each said first clock phase;feedback means connecting the output of said second operational _5_ ~.
amplifier to said input means of said first operational ampli-fier including a fourth switch means connected to said clock generator, a fourth switched capacitor connected between said fourth switch means and ground, said fourth switch means being connected so that the output of said second operational ampli-fier is fed back to charge said fourth switched capacitor during each second clock phase and said fourth capacitor is discharged during each first clock phase; a third integrating operational amplifier; conductive means for connecting the out-put of the first operational amplifier to an input terminal ofsaid third operational amplifier including a fifth switch means connected to the second clock phase of said clock generator, a fifth switched capacitor connected to said conductive means and a second gain multiplying capacitor between the fifth switch means and said input terminal; feedback means connected between the input and output of said third operational amplifier including a sixth switched capacitor and a sixth switch means connected thereto and to said clock generator so that said sixth capacitor is charged during each first phase clock cycle and is discharged to feedback during each second phase clock cycle, In accordance with another broad aspect of the invention, there is provided a sampled-data high pass filter comprising a first operational amplifier having its own feed-back loop; input means to said first operational amplifier;
a second operational amplifier; means connecting the output of said first operational amplifier to the input of said second ; operational amplifier; means for providing a feedback loop from the output of said second operational amplifier to said -first operational amplifier; a third operational amplifier having its own feedback loop; means connecting the output of -5a-.
S~3 the first operational amplifier to the input of said third operational amplifier; a plurality of switched capacitors in said feedback loops and said connecting means; switch means operated by a continuous two~phase clock generator for control-ling the charging and discharging of said switched capacitors so that said filter produces a predetermined filter characteristic with third order transfer function.
Other objects, advantages and features of the present invention will become apparent from the following detailed description which is presented in conjunction with the accompany-ing drawing:
Description of the Drawing ..
Fig. 1 is a circuit diagram of a filter section embodying the principles of the present invention;
Fig. 2a is a wave form of the voltage input Vin;
Fig. 2b is a wave form of the voltage output (VO) from the first operational amplifier;
Fig. 2c is a wave form of the voltage output (VO) from the third operational amplifier; and ~; 20 Fig. 2d is a wave form of the clock signals for the circuit;
Fig. 3 shows a diagram illustrating a method for pre-warping the loss characteristic to compensate for non-linear frequenc~ warping; and Fig. 4 is a diagram of the loss response for a typical filter according to the present invention.
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; ~5 ~3~3 1 Detailed Description of thc Embodiment 2 With reference to the drawing, Fig. 1 represents 3 a circuit diagram for a sampled-data, switched capacitor high-4 ass filter 10 accordinq to the ~resent invention. As used 5 in a typical data transmission system the circuit is
filtex with a high dynamic range and one that provides gain in the pass-band; to provide a hi~h pass filter that yields frequency responses that have a low sensitivity to the filter coefficients; to provide a sampled-data filter of the third order that provides an output with no analog components from an analog input signal.
Thus, in accordance with one broad aspect of the invention, there is provided a high-pass sampled-data filter comprising: a first integrating operational amplifier; input means connected to said first operational amplifier for supply-ing a continuous signal voltage to be filtered, said input means having a first switch means adapted to be connected with and controlled by a first phase of a continuous two-pha~e alter-nating clock generator, a first switched capacitor connected to ground and to said input means, and a first gain multiplying capacitor in said input means between said first switched capaci-tor and the input to said first operaticnal amplifier; ~a bypass lead connected between the input and output of said first oper-ational amplifier for providing a feedback thereto and contain-ing an alternating second switch means connected to said clock : generator with a second switched capacitor connected to ground s ecO ~d and to said~switch means so that it is charged during a second clock phase and discharged as a feedback during each first clock phase; a second integrating operational amplifier; means for connecting the output of said first operational amplifier to an input of said second operational amplifier including a third switched capacitor and third switch means connected to said clock generator so that said third switched capacitor is charged during each said second clock phase and is discharged to said second operational amplifier during each said first clock phase;feedback means connecting the output of said second operational _5_ ~.
amplifier to said input means of said first operational ampli-fier including a fourth switch means connected to said clock generator, a fourth switched capacitor connected between said fourth switch means and ground, said fourth switch means being connected so that the output of said second operational ampli-fier is fed back to charge said fourth switched capacitor during each second clock phase and said fourth capacitor is discharged during each first clock phase; a third integrating operational amplifier; conductive means for connecting the out-put of the first operational amplifier to an input terminal ofsaid third operational amplifier including a fifth switch means connected to the second clock phase of said clock generator, a fifth switched capacitor connected to said conductive means and a second gain multiplying capacitor between the fifth switch means and said input terminal; feedback means connected between the input and output of said third operational amplifier including a sixth switched capacitor and a sixth switch means connected thereto and to said clock generator so that said sixth capacitor is charged during each first phase clock cycle and is discharged to feedback during each second phase clock cycle, In accordance with another broad aspect of the invention, there is provided a sampled-data high pass filter comprising a first operational amplifier having its own feed-back loop; input means to said first operational amplifier;
a second operational amplifier; means connecting the output of said first operational amplifier to the input of said second ; operational amplifier; means for providing a feedback loop from the output of said second operational amplifier to said -first operational amplifier; a third operational amplifier having its own feedback loop; means connecting the output of -5a-.
S~3 the first operational amplifier to the input of said third operational amplifier; a plurality of switched capacitors in said feedback loops and said connecting means; switch means operated by a continuous two~phase clock generator for control-ling the charging and discharging of said switched capacitors so that said filter produces a predetermined filter characteristic with third order transfer function.
Other objects, advantages and features of the present invention will become apparent from the following detailed description which is presented in conjunction with the accompany-ing drawing:
Description of the Drawing ..
Fig. 1 is a circuit diagram of a filter section embodying the principles of the present invention;
Fig. 2a is a wave form of the voltage input Vin;
Fig. 2b is a wave form of the voltage output (VO) from the first operational amplifier;
Fig. 2c is a wave form of the voltage output (VO) from the third operational amplifier; and ~; 20 Fig. 2d is a wave form of the clock signals for the circuit;
Fig. 3 shows a diagram illustrating a method for pre-warping the loss characteristic to compensate for non-linear frequenc~ warping; and Fig. 4 is a diagram of the loss response for a typical filter according to the present invention.
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; ~5 ~3~3 1 Detailed Description of thc Embodiment 2 With reference to the drawing, Fig. 1 represents 3 a circuit diagram for a sampled-data, switched capacitor high-4 ass filter 10 accordinq to the ~resent invention. As used 5 in a typical data transmission system the circuit is
6 adapted to receive an input signal Vin from a connected data
7 source from which it is desired to filter out all frequeneies
8 below a preselected level, so that the circuit output VOUt
9 is comprised only of the filter signal above the pass level.
As shown, the input Vin is supplied via a lead 12,through a switch means 14, and through a capacitor 16 to the 12 negative input of a first integrating ope~ational amplifier 13 18. The plus terminal of the amplifier 18 is connected to 14 ground.
~he circuit 10 operates wîth alternating 0 and 16 clock phases whlch are provided from a suitable elock generator ~7 (not shown) at a predetermined frequency (e.g. 128 kilo hertz).
18 The switeh means 14, as shown, is preferably implemented 19 as a MOSFET device whose gate is connected to the 0 clock 20 phase.
~1 In a lead 22 connected to the lead 12 at a node Vl 22 between the switch means 14 and the eapaeitor 16 is a eapaeitor 124 whose other side is ~onneeted to ground.
24 ¦ A bypass lead 26 eonneeted to a capacitor 28 is also 25 ¦eonneeted to the input lead 12 and to an output lead 30 26 ¦from the operational amplifier 18. In parallel with the lead 27 ~26 and the capacitor 28 is a lead 32 eonnected to an alternating 28 ¦switch means 34 represented by a pair of MOSFET elements whose 29 Igates are connected to ~ and ~ clock phases A terminal 30 ¦between these elements iS connected through a eapacitor 36 , : ~
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. - . -Il ; 3 1 ¦ to ~round.
2 ¦ The output VO of the first operational amplifier 18 is 3 ¦ connected by a lead 38 to the lead 32 and is also supplied via 4 ¦ lead 30 to an alternating switch means 40 comprised of another pair 5 ¦ of MOSFET devices whose gates are connected to the 0 and 0 clock 6 ¦ phases. A terminal between these latter switch ele~ents is con-7¦ nected through a capacitor 42 to the terminal of another switch 8 ¦ means 44 comprised of another pair of MOSFET devices whose gates 9¦ are also connected to the 0 and 0 clock phases. The 0 phase MOSFET
of switch means 40 is connected to ground, as is the 0 phase MOSFET
11¦ of switch means 44.
12 The 0 phase MOSFET of switch means 44 is connected via a 3¦ lead 46 to the negative in~ut of a second operational amplifier 48 4¦ whose positive terminal is connected to ground. A bypass lead 50 5¦ to an output lead 54 from the operational amplifier 48 which pro-6¦ vides an output V2.
7¦ A feedback lead 56 is also connected from the output of 18 ¦ the second operational amplifier to the 0 phase MOSFET of an 19¦ alternating switch means 58 whose other 0 phase MOSFET is connected by a~lead 60 to the input lead 12 for the first operational ampli-21¦ ~ier 18. A terminal between the pair of MOSFETS of the switch 22¦ means 58 is connected to one side of a capacltor 62 whose other 231 side is connected to ground.
24¦ Connected to the lead 36 from the- output of the first operational amplifier 18 is a lead 64 connected to one contact of 26¦ a switch means 66 in the form of a MOSFET whose gate is connected 2 to the 0 clock phase. The other side of this switch means is 28¦ connected to one side of a capacitor 68 whose other side is con-¦ nected to the negative input of a third operational amplifier 70 having its posltive terminal connected to ground. A lead 72, . ~ ~
:
,.1 -I ;' ~ 3 ¦ connected to the lead 64 ~etween the switch means 66 and the 21 capacitor 68 is also connected through a capacitor 74 to ground.
3I A bypass lead 76 for the operational amplifier 70 is connected 41 between its input lead 64 and its output lead 78 through a 51 capacitor 80. In parallel with this ~apacitor is a feedback loop I comprising an alternating switch means 82 comprised of a pair of -71 MOSFET elements whose gates are connected to 0 and 0 clock phases.
81 A terminal between these two MOSFETS Is connected through a 9¦ capacitor 84 to ground.
0¦ The three operational amplifiers 18, 48 and 70 are also ¦ preferably implemented with MOS elements in a suitable circuit 2¦ configuration connected to Vss and VDD voltage sources. A particu-3¦ lar operational amplifier circuit is not shown in detail since a 4¦ suitable one can be readily selected by one skilled in the art.
51 In the operation of the filter circuit 10 ! the two-phase 6¦ non-overlapping clock is constantly supplying pulses at the pre-7¦ selected sampling frequency fc = 1 (e.g. 128 Kilo Hertz). At 8¦ time In-l) T, when clock 0 is "on," the voltages at nodes V1, V0, 19 ¦and V2 are held at Vin (n-l~, V0 (n-l) and V2 (n-l), respectively.
1 20 IAt the end of the 0 "on" period, capacitors 36 (~ 1 C1) and 42 21 ¦ (~2 C2) are charged to VO (n-l), the output voltage of the first 22 ¦operational amplifier. The capacitor 58 (~ llCl) connected to the 23 ¦output of the second operational amplifier 48, is charged to the 24 ¦level V2 (n-l). When 0 phase turns "off" and 0 phase turns "on,"
25 ¦the capacitor 24 (C) charges up to Vi (n) and the following charge 26 Iconservation equation is valid for operational amplifier 18:
28 ~ v ~n)=C v (n~ Clv0(n~ C1v2(n-1) ~3Cl[ in in I - . .. .
29 Equation (2) 301 ~ -1:
~, t ~ 8-j~
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I .,' ~ 3 ¦ Taking the z-transforms of both sides of Eq. (4) and assuming 21 l al yields ~}1 4¦ ~0(z) = (l-al) z-lvo(Z) -~lz~lV2(z) -~3Vi~(2) (l-z-l) :
- 1 Equation (3) 61 or -l ¢lV2 (Z) ~~t3Vin (Z) (z-l) 8 VO(z) = z-(l-al) 9¦ Constructing a similar charge-conservation equation for operational ¦ amplifier 2 and keeping in mind that~2C2 is an inverting switched l¦ capacitor we have ..
13 2(Z) z-l ~ Equation ~4) Combining Eqs. (5) and (6) we obtain l6 ¦ ~ V2(Z) -a2 ~3(z-l) Equation (5) 18 I in Z z -z(2-al) + l - al+~l¢2 19 ¦ and the overall transfer function 2l ~ H (z) ~ ( ) = a3(z l) Equation (6) 22 ~ ~ in Z z -z(2-~l) + l -~1 + al~'2 23 ¦ Eq. (6) is the transfer function of a high-passfilter with two 24 ¦ complex conjugate poles and two simple zeros at DC (z=l).
25 I If the filter circuit lO did not have the sub-section 26 ¦ with the third operational.amplifier 70 and the capacitors 68 and 27 ¦ 74, it would have a major problem. During the pexiod when 0 is 28 ¦ "on" and capacitor 16 (~3 Çl) is connected to Vin, there is an 29 undesirable direct feedthro~gh from Vin to the output voltage VO.-30 ¦ Therefore, the output of the first operational amplifier 18 is not-: , :~
g_ .1`-I .;' ¦ a sampled and held signal. The voltage V0 consists of two compo-2 ¦ nents - a discrete-time signal governed by the transfer function 31 (Eq. 6) and an unwanted analog output signal given by - ~3Vin, 41 which occurs during the period when ~ is "on." This is illustrated sl in the drawing, wherein Fig. 2a depicts an analog input signal 61 Vin; Fig. 2b depicts the output V0 having analog components 71 on alternate clock phases; Fig. 2c depicts the output V0l without 81 the analog components; and Fig. 2d shows the corresponding clock 9¦ signals for the wave forms.
0¦ The elimination of the analog signal components is ¦ accomplished in the circuit l0 by blocking them with the output 2¦ sample and hold section comprising the third operational amplifier I -3¦ 70 which is clocked out of phase with the original filter section 4¦ comprised of the first and second operational amplifier. Thus, the 5¦ output section is controlled by the MOSFET switch 66 which is 6¦ closed during phase 0. During this phase 0 "on" condition, the 7¦ output V0 of operational amplifier 18 is held constant because l8¦ ~ phase is off. Therefore, when 0 is "on" and the output of the l9¦ flrst operational amplifier is varying with the input Vin, that 201 signal is blocked by switch 66 and is not transferred. But when 2l¦ ~ goes "off" and 0 goes "on," the input signal is fed directly 22¦ from capacitor 16 (~ 3cl) through capacitor 28 tcl3 and the output 231 at node V0 is constant and the capacitor 68 ( ~5C3) is charged.
2~1 In addition to its-analog component blocking function as ¦ described, the third operationai amplifier also serves to operate 26¦ one additional pole-and-zero pair so that a third order filter is 271 provided. Using the charge-conservatlon equation, the relationship 28 be~ween the z-transforms of v0l and v0 is given by 29 -d 5 ~Z~ z , 3a vol(z) z - (l- ~4) ( ) ~quation (7)
As shown, the input Vin is supplied via a lead 12,through a switch means 14, and through a capacitor 16 to the 12 negative input of a first integrating ope~ational amplifier 13 18. The plus terminal of the amplifier 18 is connected to 14 ground.
~he circuit 10 operates wîth alternating 0 and 16 clock phases whlch are provided from a suitable elock generator ~7 (not shown) at a predetermined frequency (e.g. 128 kilo hertz).
18 The switeh means 14, as shown, is preferably implemented 19 as a MOSFET device whose gate is connected to the 0 clock 20 phase.
~1 In a lead 22 connected to the lead 12 at a node Vl 22 between the switch means 14 and the eapaeitor 16 is a eapaeitor 124 whose other side is ~onneeted to ground.
24 ¦ A bypass lead 26 eonneeted to a capacitor 28 is also 25 ¦eonneeted to the input lead 12 and to an output lead 30 26 ¦from the operational amplifier 18. In parallel with the lead 27 ~26 and the capacitor 28 is a lead 32 eonnected to an alternating 28 ¦switch means 34 represented by a pair of MOSFET elements whose 29 Igates are connected to ~ and ~ clock phases A terminal 30 ¦between these elements iS connected through a eapacitor 36 , : ~
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. - . -Il ; 3 1 ¦ to ~round.
2 ¦ The output VO of the first operational amplifier 18 is 3 ¦ connected by a lead 38 to the lead 32 and is also supplied via 4 ¦ lead 30 to an alternating switch means 40 comprised of another pair 5 ¦ of MOSFET devices whose gates are connected to the 0 and 0 clock 6 ¦ phases. A terminal between these latter switch ele~ents is con-7¦ nected through a capacitor 42 to the terminal of another switch 8 ¦ means 44 comprised of another pair of MOSFET devices whose gates 9¦ are also connected to the 0 and 0 clock phases. The 0 phase MOSFET
of switch means 40 is connected to ground, as is the 0 phase MOSFET
11¦ of switch means 44.
12 The 0 phase MOSFET of switch means 44 is connected via a 3¦ lead 46 to the negative in~ut of a second operational amplifier 48 4¦ whose positive terminal is connected to ground. A bypass lead 50 5¦ to an output lead 54 from the operational amplifier 48 which pro-6¦ vides an output V2.
7¦ A feedback lead 56 is also connected from the output of 18 ¦ the second operational amplifier to the 0 phase MOSFET of an 19¦ alternating switch means 58 whose other 0 phase MOSFET is connected by a~lead 60 to the input lead 12 for the first operational ampli-21¦ ~ier 18. A terminal between the pair of MOSFETS of the switch 22¦ means 58 is connected to one side of a capacltor 62 whose other 231 side is connected to ground.
24¦ Connected to the lead 36 from the- output of the first operational amplifier 18 is a lead 64 connected to one contact of 26¦ a switch means 66 in the form of a MOSFET whose gate is connected 2 to the 0 clock phase. The other side of this switch means is 28¦ connected to one side of a capacitor 68 whose other side is con-¦ nected to the negative input of a third operational amplifier 70 having its posltive terminal connected to ground. A lead 72, . ~ ~
:
,.1 -I ;' ~ 3 ¦ connected to the lead 64 ~etween the switch means 66 and the 21 capacitor 68 is also connected through a capacitor 74 to ground.
3I A bypass lead 76 for the operational amplifier 70 is connected 41 between its input lead 64 and its output lead 78 through a 51 capacitor 80. In parallel with this ~apacitor is a feedback loop I comprising an alternating switch means 82 comprised of a pair of -71 MOSFET elements whose gates are connected to 0 and 0 clock phases.
81 A terminal between these two MOSFETS Is connected through a 9¦ capacitor 84 to ground.
0¦ The three operational amplifiers 18, 48 and 70 are also ¦ preferably implemented with MOS elements in a suitable circuit 2¦ configuration connected to Vss and VDD voltage sources. A particu-3¦ lar operational amplifier circuit is not shown in detail since a 4¦ suitable one can be readily selected by one skilled in the art.
51 In the operation of the filter circuit 10 ! the two-phase 6¦ non-overlapping clock is constantly supplying pulses at the pre-7¦ selected sampling frequency fc = 1 (e.g. 128 Kilo Hertz). At 8¦ time In-l) T, when clock 0 is "on," the voltages at nodes V1, V0, 19 ¦and V2 are held at Vin (n-l~, V0 (n-l) and V2 (n-l), respectively.
1 20 IAt the end of the 0 "on" period, capacitors 36 (~ 1 C1) and 42 21 ¦ (~2 C2) are charged to VO (n-l), the output voltage of the first 22 ¦operational amplifier. The capacitor 58 (~ llCl) connected to the 23 ¦output of the second operational amplifier 48, is charged to the 24 ¦level V2 (n-l). When 0 phase turns "off" and 0 phase turns "on,"
25 ¦the capacitor 24 (C) charges up to Vi (n) and the following charge 26 Iconservation equation is valid for operational amplifier 18:
28 ~ v ~n)=C v (n~ Clv0(n~ C1v2(n-1) ~3Cl[ in in I - . .. .
29 Equation (2) 301 ~ -1:
~, t ~ 8-j~
~ ~ 1, .
I````
I .,' ~ 3 ¦ Taking the z-transforms of both sides of Eq. (4) and assuming 21 l al yields ~}1 4¦ ~0(z) = (l-al) z-lvo(Z) -~lz~lV2(z) -~3Vi~(2) (l-z-l) :
- 1 Equation (3) 61 or -l ¢lV2 (Z) ~~t3Vin (Z) (z-l) 8 VO(z) = z-(l-al) 9¦ Constructing a similar charge-conservation equation for operational ¦ amplifier 2 and keeping in mind that~2C2 is an inverting switched l¦ capacitor we have ..
13 2(Z) z-l ~ Equation ~4) Combining Eqs. (5) and (6) we obtain l6 ¦ ~ V2(Z) -a2 ~3(z-l) Equation (5) 18 I in Z z -z(2-al) + l - al+~l¢2 19 ¦ and the overall transfer function 2l ~ H (z) ~ ( ) = a3(z l) Equation (6) 22 ~ ~ in Z z -z(2-~l) + l -~1 + al~'2 23 ¦ Eq. (6) is the transfer function of a high-passfilter with two 24 ¦ complex conjugate poles and two simple zeros at DC (z=l).
25 I If the filter circuit lO did not have the sub-section 26 ¦ with the third operational.amplifier 70 and the capacitors 68 and 27 ¦ 74, it would have a major problem. During the pexiod when 0 is 28 ¦ "on" and capacitor 16 (~3 Çl) is connected to Vin, there is an 29 undesirable direct feedthro~gh from Vin to the output voltage VO.-30 ¦ Therefore, the output of the first operational amplifier 18 is not-: , :~
g_ .1`-I .;' ¦ a sampled and held signal. The voltage V0 consists of two compo-2 ¦ nents - a discrete-time signal governed by the transfer function 31 (Eq. 6) and an unwanted analog output signal given by - ~3Vin, 41 which occurs during the period when ~ is "on." This is illustrated sl in the drawing, wherein Fig. 2a depicts an analog input signal 61 Vin; Fig. 2b depicts the output V0 having analog components 71 on alternate clock phases; Fig. 2c depicts the output V0l without 81 the analog components; and Fig. 2d shows the corresponding clock 9¦ signals for the wave forms.
0¦ The elimination of the analog signal components is ¦ accomplished in the circuit l0 by blocking them with the output 2¦ sample and hold section comprising the third operational amplifier I -3¦ 70 which is clocked out of phase with the original filter section 4¦ comprised of the first and second operational amplifier. Thus, the 5¦ output section is controlled by the MOSFET switch 66 which is 6¦ closed during phase 0. During this phase 0 "on" condition, the 7¦ output V0 of operational amplifier 18 is held constant because l8¦ ~ phase is off. Therefore, when 0 is "on" and the output of the l9¦ flrst operational amplifier is varying with the input Vin, that 201 signal is blocked by switch 66 and is not transferred. But when 2l¦ ~ goes "off" and 0 goes "on," the input signal is fed directly 22¦ from capacitor 16 (~ 3cl) through capacitor 28 tcl3 and the output 231 at node V0 is constant and the capacitor 68 ( ~5C3) is charged.
2~1 In addition to its-analog component blocking function as ¦ described, the third operationai amplifier also serves to operate 26¦ one additional pole-and-zero pair so that a third order filter is 271 provided. Using the charge-conservatlon equation, the relationship 28 be~ween the z-transforms of v0l and v0 is given by 29 -d 5 ~Z~ z , 3a vol(z) z - (l- ~4) ( ) ~quation (7)
10~ .:
- .
, . . ~ i . .
. . , , .
. , .
l`\
;' ~ S~3 1 Combining Eqs. (6b) and (7), we get 4 Hul(z) Vi (Z) [Z (1- ~)][z2 _ z(2- ~) + 1 - ql + ~l ~2]
51 Equation (8) 61 which is the transfer function of a third-order high-pass filter.
7¦ In the actual design of a filter using the principle of 81 the invention, the conformal mapping from the s-plane to the 9 z-plane is the bilinear transformation defined by Eq. (l). Since 0 the entire j Q axis of the s-plane is mapped onto the unit circle in the z-plane, the aliasing error inherent in other design methods 12 using an analog model is eliminated. However, there is a nonlinear 13 relationship between the an~alog frequency Q and the discrete-time 14 frequency ~ given by ; 15 16 ~ > T tan ~2T . Equation (9) 18 Fortunately, for a "brick-wall" type loss characteristic filter, 19 one can compensate for the frequency warping. The compensation process used for a high-pass filter is illustrated in Figure 3.
~21 Once the loss characteristic of the prewarped filter is determined ~2 (upper left of Figure 3), an analog filter is designed to meet the 23 transformed loss characteristic. The transfer function of the 24 digital filter H(z) is then obtained by making the algebraic substitution of Eq. (l); i.e., 26 ~
27 H(z) = H(5)¦ `
28 / ) 11 ~ (l + z-l)~ Equation (lO) 29 In summary, the desigr, of the high pass filter can be performed in the following steps:
~ : .
.>:
::: :
. 11-"': . . ' ' . : -I`
~ 3 1 1. The specified passband and stopband frequencies ~ p and ~ s 2 I are translated into the corresponding values Q p and Q 3 using 3 ¦ the relation Q = t2/T) tan (~2T).
5 ¦ 2. ~n analog filter is d~signed from the transformed band-limit 6 ¦ frequencies.
8 ¦ 3. The z domain transfer function is obtained by applying the 9 ¦ following transformation:
10 l .'
- .
, . . ~ i . .
. . , , .
. , .
l`\
;' ~ S~3 1 Combining Eqs. (6b) and (7), we get 4 Hul(z) Vi (Z) [Z (1- ~)][z2 _ z(2- ~) + 1 - ql + ~l ~2]
51 Equation (8) 61 which is the transfer function of a third-order high-pass filter.
7¦ In the actual design of a filter using the principle of 81 the invention, the conformal mapping from the s-plane to the 9 z-plane is the bilinear transformation defined by Eq. (l). Since 0 the entire j Q axis of the s-plane is mapped onto the unit circle in the z-plane, the aliasing error inherent in other design methods 12 using an analog model is eliminated. However, there is a nonlinear 13 relationship between the an~alog frequency Q and the discrete-time 14 frequency ~ given by ; 15 16 ~ > T tan ~2T . Equation (9) 18 Fortunately, for a "brick-wall" type loss characteristic filter, 19 one can compensate for the frequency warping. The compensation process used for a high-pass filter is illustrated in Figure 3.
~21 Once the loss characteristic of the prewarped filter is determined ~2 (upper left of Figure 3), an analog filter is designed to meet the 23 transformed loss characteristic. The transfer function of the 24 digital filter H(z) is then obtained by making the algebraic substitution of Eq. (l); i.e., 26 ~
27 H(z) = H(5)¦ `
28 / ) 11 ~ (l + z-l)~ Equation (lO) 29 In summary, the desigr, of the high pass filter can be performed in the following steps:
~ : .
.>:
::: :
. 11-"': . . ' ' . : -I`
~ 3 1 1. The specified passband and stopband frequencies ~ p and ~ s 2 I are translated into the corresponding values Q p and Q 3 using 3 ¦ the relation Q = t2/T) tan (~2T).
5 ¦ 2. ~n analog filter is d~signed from the transformed band-limit 6 ¦ frequencies.
8 ¦ 3. The z domain transfer function is obtained by applying the 9 ¦ following transformation:
10 l .'
11 I s ~~~~~ T z+1 1~ I .
13 ¦ to the s domain transfer function.
lS ¦ 4. The capacitor ratios ~1~ c~2, 13, ~4, and 15 ar~ obtained by 16 ¦ equatins the corresponding coefficients of the "z" power in 17 ¦ equation 8 and the derived transfer function.
1 The foregoing procedure may be illustrated by ass~ming 19 the design of a particular high-pass switched-capacitor filter satisfying pr~selected loss specifications using a typical sempling 21 rate (e.g. 16kHz~. These specifications are indicated by the 22 shaded areas in Fig. 4.
~3 For a third-order high-pass analog filter function ~4 designed to meet the loss specifications of Figure 4 by applying a frequency transformation to a low-pass prototype function, the 26 resulting high-pass transfer function is given by:
28 H(s) = - 5 207s3 2 Equation (11) ` (A + 1.59s)(A -~ 1.59As ~ 3.28s ) ' ~!g .' , . .
` 3 where A = 1200~ . Thenl H(z) -is obtained using the bilinear ,: . . .
~ ` -12- -; I , .
` !l l ., 1¦ transformation which here becomes: .
2 I . ..
3 I s ~ (32000) Z+l .
4 I . . .
5 ¦ This gives:
~ 1 ` 3 7 I Hlz) =0.877 (z-l) Equation (12) 81(z-0.862)(z -1.876z + 0.892) 9¦ By equating the coefficients of the corresponding degrees of z 0 in Eqs. (12) and (8) and solving for the unknown ql, the following 1¦ capacitor ratios are obtained:
121 . ` .
13 I ql = ~.124 a2 = 0.129 15 I ~3 5 = 0.877 . .
~ 161 ~4 = 0.138 .
17 I - .
18 I The actual resulting loss characteristic of the resul ing filter 19 ¦ of thîs example is shown by the curved line in Figure 4.
~20 ¦ From the foregoing, it should be apparent that the :
:~ ~121 ¦ present invention provides a third-order high-pass switched-22 capacitor filter section having significant performance advantages. . ~:
231 In production, it is readily adaptable for implementation as an 24 I integrated circuit device, and a design technique is provided for 25I obtaining the element values of the filter. .Since the design is `26¦ based on the bilinear z-transform, it does not cause any dis.tortion ..
271 that is inherent in some other design methods based on an analog 28¦ model. Also, sinoe the design is carried out in the z-domain, the ~2g¦ resulting loss can be programmed by varying the clocking frequency.
~ol Higher-order filters can be o~ ained by the tandem connection of second-order sections follow.ed by one or more first~order pole-zero ,.
~ : `~ -13-,;' 1 ¦ sections. The filter is immune to DC offset signals present at the 2 ¦ input, and the internal DC offsets are not amplified so much as 3 ¦ to cause dynamic range problems.
4 ¦ To those skilled in the art to which this invention re-¦ lates, many changes in construction and widely differing embodiment 6 I and applications of the invention will suggest themselves without 7 ¦ departing from the spiri. and scope of the invention. The dis- .
8 ¦ closures and the description herein are purely illustrative and are ~ not irterd-~ to b~ ny ~e l~-~Li~g .` 20 . :
~ 21 ~. .
'221 .
Z3 .
' 24 .
26 - .
27 .
~28 ~
29 . .
3o .
`~'` . ` `
13 ¦ to the s domain transfer function.
lS ¦ 4. The capacitor ratios ~1~ c~2, 13, ~4, and 15 ar~ obtained by 16 ¦ equatins the corresponding coefficients of the "z" power in 17 ¦ equation 8 and the derived transfer function.
1 The foregoing procedure may be illustrated by ass~ming 19 the design of a particular high-pass switched-capacitor filter satisfying pr~selected loss specifications using a typical sempling 21 rate (e.g. 16kHz~. These specifications are indicated by the 22 shaded areas in Fig. 4.
~3 For a third-order high-pass analog filter function ~4 designed to meet the loss specifications of Figure 4 by applying a frequency transformation to a low-pass prototype function, the 26 resulting high-pass transfer function is given by:
28 H(s) = - 5 207s3 2 Equation (11) ` (A + 1.59s)(A -~ 1.59As ~ 3.28s ) ' ~!g .' , . .
` 3 where A = 1200~ . Thenl H(z) -is obtained using the bilinear ,: . . .
~ ` -12- -; I , .
` !l l ., 1¦ transformation which here becomes: .
2 I . ..
3 I s ~ (32000) Z+l .
4 I . . .
5 ¦ This gives:
~ 1 ` 3 7 I Hlz) =0.877 (z-l) Equation (12) 81(z-0.862)(z -1.876z + 0.892) 9¦ By equating the coefficients of the corresponding degrees of z 0 in Eqs. (12) and (8) and solving for the unknown ql, the following 1¦ capacitor ratios are obtained:
121 . ` .
13 I ql = ~.124 a2 = 0.129 15 I ~3 5 = 0.877 . .
~ 161 ~4 = 0.138 .
17 I - .
18 I The actual resulting loss characteristic of the resul ing filter 19 ¦ of thîs example is shown by the curved line in Figure 4.
~20 ¦ From the foregoing, it should be apparent that the :
:~ ~121 ¦ present invention provides a third-order high-pass switched-22 capacitor filter section having significant performance advantages. . ~:
231 In production, it is readily adaptable for implementation as an 24 I integrated circuit device, and a design technique is provided for 25I obtaining the element values of the filter. .Since the design is `26¦ based on the bilinear z-transform, it does not cause any dis.tortion ..
271 that is inherent in some other design methods based on an analog 28¦ model. Also, sinoe the design is carried out in the z-domain, the ~2g¦ resulting loss can be programmed by varying the clocking frequency.
~ol Higher-order filters can be o~ ained by the tandem connection of second-order sections follow.ed by one or more first~order pole-zero ,.
~ : `~ -13-,;' 1 ¦ sections. The filter is immune to DC offset signals present at the 2 ¦ input, and the internal DC offsets are not amplified so much as 3 ¦ to cause dynamic range problems.
4 ¦ To those skilled in the art to which this invention re-¦ lates, many changes in construction and widely differing embodiment 6 I and applications of the invention will suggest themselves without 7 ¦ departing from the spiri. and scope of the invention. The dis- .
8 ¦ closures and the description herein are purely illustrative and are ~ not irterd-~ to b~ ny ~e l~-~Li~g .` 20 . :
~ 21 ~. .
'221 .
Z3 .
' 24 .
26 - .
27 .
~28 ~
29 . .
3o .
`~'` . ` `
Claims (8)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A high-pass sampled-data filter comprising a first integrating operational amplifier;
input means connected to said first operational amplifier for supplying a continuous signal voltage to be filtered, said input means having a first switch means adapted to be connected with and controlled by a first phase of a continuous two-phase alternating clock generator, a first switched capacitor connected to ground and to said input means, and a first gain multiplying capacitor in said input means between said first switched capaci-tor and the input to said first operational amplifier;
a bypass lead connected between the input and output of said first operational amplifier for providing a feedback thereto and containing an alternating second switch means connected to said clock generator with a second switched capacitor connected to ground and to said second switch means so that it is charged during a second clock phase and discharged as a feedback during each first clock phase;
a second integrating operational amplifier;
means for connecting the output of said first operational amplifier to an input of said second operational amplifier including a third switched capacitor and third switch means connected to said clock generator so that said third switched capacitor is charged during each said second clock phase and is discharged to said second operational amplifier during each said first clock phase;
feedback means connecting the output of said second opera-tional amplifier to said input means of said first operational amplifier including a fourth switch means connected to said clock generator, a fourth switched capacitor connected between said fourth switch means and ground, said fourth switch means being connected so that the output of said second operational amplifier is fed back to charge said fourth switched capacitor during each second clock phase and said fourth capacitor is discharged during each first clock phase;
a third integrating operational amplifier;
conductive means for connecting the output of the first operational amplifier to an input terminal of said third opera-tional amplifier including a fifth switch means connected to the second clock phase of said clock generator, a fifth switched capacitor connected to said conductive means and a second gain multiplying capacitor between the fifth switch means and said input terminal;
feedback means connected between the input and output of said third operational amplifier including a sixth switched capacitor and a sixth switch means connected thereto and to said clock generator so that said sixth capacitor is charged during each first phase clock cycle and is discharged to feedback during each second phase clock cycle.
input means connected to said first operational amplifier for supplying a continuous signal voltage to be filtered, said input means having a first switch means adapted to be connected with and controlled by a first phase of a continuous two-phase alternating clock generator, a first switched capacitor connected to ground and to said input means, and a first gain multiplying capacitor in said input means between said first switched capaci-tor and the input to said first operational amplifier;
a bypass lead connected between the input and output of said first operational amplifier for providing a feedback thereto and containing an alternating second switch means connected to said clock generator with a second switched capacitor connected to ground and to said second switch means so that it is charged during a second clock phase and discharged as a feedback during each first clock phase;
a second integrating operational amplifier;
means for connecting the output of said first operational amplifier to an input of said second operational amplifier including a third switched capacitor and third switch means connected to said clock generator so that said third switched capacitor is charged during each said second clock phase and is discharged to said second operational amplifier during each said first clock phase;
feedback means connecting the output of said second opera-tional amplifier to said input means of said first operational amplifier including a fourth switch means connected to said clock generator, a fourth switched capacitor connected between said fourth switch means and ground, said fourth switch means being connected so that the output of said second operational amplifier is fed back to charge said fourth switched capacitor during each second clock phase and said fourth capacitor is discharged during each first clock phase;
a third integrating operational amplifier;
conductive means for connecting the output of the first operational amplifier to an input terminal of said third opera-tional amplifier including a fifth switch means connected to the second clock phase of said clock generator, a fifth switched capacitor connected to said conductive means and a second gain multiplying capacitor between the fifth switch means and said input terminal;
feedback means connected between the input and output of said third operational amplifier including a sixth switched capacitor and a sixth switch means connected thereto and to said clock generator so that said sixth capacitor is charged during each first phase clock cycle and is discharged to feedback during each second phase clock cycle.
2. The filter as described in Claim 1 wherein all of said switch means comprise MOSFET devices whose gates are connected to either ? or ? phase of the clock generator.
3. The filter as described in Claim 1 wherein all of said switched capacitors have preselected values in related ratios to each other in order to provide a filter with predetermined characteristics.
4. The filter as described in Claim 1 wherein said filter is a third order filter whose transfer function in the "z"
domain is expressed as:
wherein .alpha.1, .alpha.2, .alpha.3, .alpha.4, and .alpha.5 are preselected ratio factors for said capacitors in the circuit and. C1, C2 and C3 are values for the integrating capacitors for said first, second and third operational amplifiers; said first and second gain multiplying capacitors having values of .alpha.3C1 and .alpha.5C3, respectively, said first and fifth switched capacitor having a value of C, said second and fourth switched capacitors having a value of .alpha.1C1, said third switched capacitor having a value of .alpha.2C2, and said sixth switched capacitor having a value of .alpha.4C3.
domain is expressed as:
wherein .alpha.1, .alpha.2, .alpha.3, .alpha.4, and .alpha.5 are preselected ratio factors for said capacitors in the circuit and. C1, C2 and C3 are values for the integrating capacitors for said first, second and third operational amplifiers; said first and second gain multiplying capacitors having values of .alpha.3C1 and .alpha.5C3, respectively, said first and fifth switched capacitor having a value of C, said second and fourth switched capacitors having a value of .alpha.1C1, said third switched capacitor having a value of .alpha.2C2, and said sixth switched capacitor having a value of .alpha.4C3.
5. A sampled-data high pass filter comprising a first operational amplifier having its own feedback loop;
input means to said first operational amplifier;
a second operational amplifier;
means connecting the output of said first operational amplifier to the input of said second operational amplifier;
means for providing a feedback loop from the output of said second operational amplifier to said first operational amplifier;
a third operational amplifier having its own feedback loop;
means connecting the output of the first operational ampli-fier to the input of said third operational amplifier;
a plurality of switched capacitors in said feedback loops and said connecting means;
switch means operated by a continuous two-phase clock generator for controlling the charging and discharging of said switched capacitors so that said filter produces a predetermined filter characteristic with third order transfer function.
input means to said first operational amplifier;
a second operational amplifier;
means connecting the output of said first operational amplifier to the input of said second operational amplifier;
means for providing a feedback loop from the output of said second operational amplifier to said first operational amplifier;
a third operational amplifier having its own feedback loop;
means connecting the output of the first operational ampli-fier to the input of said third operational amplifier;
a plurality of switched capacitors in said feedback loops and said connecting means;
switch means operated by a continuous two-phase clock generator for controlling the charging and discharging of said switched capacitors so that said filter produces a predetermined filter characteristic with third order transfer function.
6. The filter as described in Claim 5 wherein said second operational amplifier is an integrator for integrating the output of said first operational amplifier before feeding it back to the input of said first operational amplifier.
7. The filter as described in Claim 5 including means for clocking said input means to said first operational amplifier and said output from said first operational amplifier to said third operational amplifier at different clock phases so that the analog feedthrough from said input to the output of said first operational amplifier is blocked.
8. The filter as described in Claim 5 wherein said third operational amplifier provides an additional pole-zero section to the transfer function of said filter, thereby making it a third order filter.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US940,717 | 1978-09-08 | ||
| US05/940,717 US4210872A (en) | 1978-09-08 | 1978-09-08 | High pass switched capacitor filter section |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1113553A true CA1113553A (en) | 1981-12-01 |
Family
ID=25475305
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA328,323A Expired CA1113553A (en) | 1978-09-08 | 1979-05-25 | High pass switched capacitor filter section |
Country Status (8)
| Country | Link |
|---|---|
| US (1) | US4210872A (en) |
| JP (1) | JPS5538796A (en) |
| CA (1) | CA1113553A (en) |
| DE (1) | DE2926900A1 (en) |
| FR (1) | FR2435856B1 (en) |
| GB (1) | GB2030409B (en) |
| IT (1) | IT1118800B (en) |
| NL (1) | NL184498C (en) |
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-
1978
- 1978-09-08 US US05/940,717 patent/US4210872A/en not_active Expired - Lifetime
-
1979
- 1979-05-25 CA CA328,323A patent/CA1113553A/en not_active Expired
- 1979-05-28 NL NLAANVRAGE7904161,A patent/NL184498C/en not_active IP Right Cessation
- 1979-06-01 GB GB7919111A patent/GB2030409B/en not_active Expired
- 1979-07-03 DE DE2926900A patent/DE2926900A1/en active Granted
- 1979-07-06 IT IT68417/79A patent/IT1118800B/en active
- 1979-09-06 FR FR7922331A patent/FR2435856B1/fr not_active Expired
- 1979-09-07 JP JP11434479A patent/JPS5538796A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| FR2435856A1 (en) | 1980-04-04 |
| US4210872A (en) | 1980-07-01 |
| NL184498C (en) | 1989-08-01 |
| DE2926900C2 (en) | 1989-06-15 |
| GB2030409B (en) | 1982-09-15 |
| NL184498B (en) | 1989-03-01 |
| GB2030409A (en) | 1980-04-02 |
| FR2435856B1 (en) | 1988-02-05 |
| NL7904161A (en) | 1980-03-11 |
| IT1118800B (en) | 1986-03-03 |
| DE2926900A1 (en) | 1980-03-20 |
| IT7968417A0 (en) | 1979-07-06 |
| JPS6336580B2 (en) | 1988-07-20 |
| JPS5538796A (en) | 1980-03-18 |
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