WO1998025349A1

WO1998025349A1 - Slope analog-to-digital converter with ramp initiated prior to counter

Info

Publication number: WO1998025349A1
Application number: PCT/US1997/021326
Authority: WO
Inventors: James B. Nolan; Brian Dellacroce
Original assignee: Microchip Technology Incorporated
Priority date: 1996-12-03
Filing date: 1997-11-19
Publication date: 1998-06-11
Also published as: TW370741B

Abstract

A slope analog-to-digital converter (10) performs a conversion of the instantaneous value of a variable analog input voltage to a digital value with reduced conversion errors by starting the ramp charging prior to initiation of the counter/time/register (30). The charging capacitor (12) is first discharged to a level slightly below analog ground by a discharge reference circuit (23). The charging source (11) is then connected to the capacitor (12) to initiate the ramp voltage. When the ramp voltage passes through the ground level reference as sensed by a start count comparator (26) accumulation of counting pulses is begun. The capacitor voltage is ramped up until the level of the ramp voltage exceeds that of the analog input as sensed by a stop count comparator (27). The accumulated count constitutes a digital value corresponding to the value of the analog input. After the cumulative count is obtained the capacitor is again discharged to a level below ground in preparation for the next conversion.

Description

SLOPE ANALOG-TO-DIGITAL CONVERTER WITH RAMP INITIATED PRIOR TO COUNTER

Background of the Invention

The present invention relates generally to analog-to-digital (A/D) converter devices, and more particularly to slope A/D converters which are

especially suitable for use in conjunction with microcontrollers.

Microcontrollers are microprocessors which are specially implemented

with primary and peripheral functions and capabilities to enable control of certain specified parameters of an external system. In performing these functions, it is

frequently necessary to detect when a controlled parameter has reached a certain

value relative to a predetermined maximum or minimum value. For example, the controlled parameter may be the temperature within an enclosure such as a room, or

an oven. In any event, the controlled parameter is generally of analog form and is converted to an electrical signal by means of an appropriate transducer — a

thermoelectric transducer such as a thermistor in the case of temperature. For

purposes of analysis and use by the microcontroller in its control functions,

however, it is necessary to convert the analog signal to digital form. Typically, this

is done by means of an A/D converter.

One type of such a converter is a slope A/D converter. In its simplest

form, the slope A/D converter operates by starting a timer at time (t) = 0, which

then commences counting at a predetermined fixed frequency while a capacitor in the converter circuit is being charged by a constant current source. At the moment

that the capacitor reaches a charge (i.e., is charged) to a voltage that just exceeds the unknown sampled analog input voltage to the circuit, a comparator having the

unknown input voltage and the capacitor voltage as inputs changes state, to cause the

timer to stop counting. The digital (converted) value of the sampled input voltage is

represented by the count of the timer from the moment of time that the charging of

the capacitor began (at t = 0), up to the moment of time that the comparator changed state.

Such a calculation or determination of the value of the unknown sampled

analog input voltage is actually only an approximation, because it assumes that at

time t = 0, at which the timer count is zero, the voltage on the capacitor was zero. This assumption is frequently erroneous. One significant reason for the error is a delay in the time the charging of the capacitor commences, attributable to the "turn- on" time of the constant current source. Another significant but more subtle reason

is that a residual voltage usually remains on the capacitor from the previous or even

earlier charging cycle because of dielectric absorption inherent in the capacitor,

despite the fact that the capacitor is short-circuited to ground before each new

charging cycle (i.e., each new A/D conversion) is commenced. These factors lead

to errors in the starting time of the count and in the time at which the capacitor

charge reaches the instantaneous voltage of the analog input (and, thus, the ending

time of the count), resulting in conversion errors.

In at least one previously-available slope A/D converter, slope reference

voltages are incorporated in an attempt to improve the conversion accuracy. In that instance, a pair of reference voltages is included in the circuit, one high

and

the other low (S_REFLo)_> f°^r calculating the actual slope of the charging capacitor voltage. By knowing the actual slope, its intercept with the X-axis can be

determined for use in a correction of the conversion value. In particular, the X-

intercept corresponds to the count when the voltage across the capacitor is zero (as

an offset), and this value may be used to compensate for the subsequent A/D conversion results.

But this method has certain drawbacks. First, the hardware needed for an adequate circuit must include means for generating the two slope reference

voltages, as well as for obtaining the ratio S_REFL0/(S_REFHI - S_REFL0), which is critical to the determination, and for calibration of the ratio and maintaining it constant to

attain the necessary accuracy. This requirement of additional hardware for generating the references, as well as the circuitry and calibration apparatus to

accommodate precise operation, can add considerable expense to the device and thus

adversely affect its competitiveness.

Another disadvantage of this prior technique is that the slope references

must be converted separately from the analog input, which reduces the accuracy of

the conversion and decreases the throughput of the conversion process. Since

optimizing the speed of the calculation is preeminent, the slope of the capacitor

voltage should be calculated only periodically, rather than with each conversion of the input signal. But conversion errors arise in part because of a desire for rapid

conversion, which is frustrated by often-repeated slope calculations.

The dielectric absorption of the capacitor is a function of the initial

voltage across the capacitor. The amount of time required to complete discharge the

capacitor must be adjusted according to this initial capacitor voltage. Two adjacent (i.e., successive) conversion cycles typically involve analog input signals of different magnitude. The first conversion cycle charges the capacitor to a certain voltage

which should be substantially identical to that of the analog input - say, 1 volt, for

the sake of example - to cause the timer to stop its count, and the capacitor is then discharged in preparation for the next conversion cycle. The second conversion cycle then charges the capacitor to, say, an analog input signal value of 3 volts,

which again stops the timer count, and initiates another discharge of the capacitor.

If both of these conversion cycles have the same discharge time, the value of the

residual charge on the capacitor upon completion of the discharge portion of the cycle will not be the same at the commencement of the next respective conversion

cycle, resulting in errors in the two (or more) analog-to-digital conversions. That

is, the slope calculation is inaccurate from conversion cycle to conversion cycle.

To minimize such errors, the capacitor should be discharged to the same

residual voltage at the end of each conversion cycle (actually, ≥ V_REP, where V_REF is positive reference potential), and the slope should be re-calculated for each

conversion. But this means a slowing of the conversion throughput, which is

disadvantageous for reasons mentioned above.

Another drawback of calculations using reference voltages S_REFHI and

S_REFLO i^{s m}at considerable mathematical manipulation must be performed to arrive at

an offset count (C_0FFSET). Referring to the Cartesian coordinate X-Y plot of FIG. 1,

the equation for a line is y = mx + b, where y is the value measured along the Y- axis, x is the value measured along the X-axis, m is a multiplier, and b is a constant

representative of the displacement of the value y from the origin (0) along the Y- axis. In the instant situation, however, the equation for determining the X-intercept

(C_OFFSET) is more complex, albeit of the same form, as follows:

Co_FFSET ⁼ CJJBPLO - K_REF

(1)

where C_0FFSET is the offset count (X-intercept), K_REP is the ratio S_REFL0/(S_REFHι -

S_REFL0), and C_REPH, and C_REFL0 are the A/D conversion results (in counts) for S_REPH,

and S_REFL0 • Subsequently, the sampled input voltage N_IN is calculated as:

* _IN ~ ( _IN " C_0FFSET) ^• K_BG/(C_BG - C_0FFSET), (2) where C_IN is the input count, K_BG is the bandgap voltage, and C_BG is the bandgap

voltage count.

It should be apparent from equations (1) and (2) that implementing a

determination of slope voltage becomes quite unwieldy, especially when using

assembly language, as is typically the case with a microcontroller. The ideal slope,

shown by the solid line on the plot, is equal to I X = I/C (where I is a constant

current), while the dashed line indicates the actual same slope with slope

displacement attributable to dielectric absorption. This corresponds to non-zero X

and Y intercepts. The X-intercept is the offset count (C_0FFSET) that must be used to

adjust the analog input count value as shown by equations (1) and (2). In an

implementation of a slope A/D conversion for determining the input voltage

according to the prior art of FIG. 1, then, the conversion and solving of resulting

equations require substantial program memory and consume a significant number of

computation cycles.

Accordingly, it is a principal aim of the present invention to provide a

new and improved slope A/D converter, which is of relatively simple configuration and which reduces the conversion errors encountered with slope A/D converters of the prior art.

Summary of the Invention

In a preferred embodiment of the invention, an enhanced slope A/D

converter uses two references, one of which is a positive voltage indicative of a predetermined bandgap voltage, nominally 1.20V, equal to the positive reference potential, and the other of which is analog ground. The second reference is actually a sample of the instantaneous level of the analog input signal to the second

comparator, with respect to analog ground acting as the comparator reference input.

A capacitor in the slope A/D converter is charged via a switching circuit to a first imprecise voltage level which is slightly below the level of electrical

ground. The capacitor is then charged by a constant current source to a second

voltage level which is above ground and is predetermined to be higher than any

sample of the instantaneous level of an analog input signal to be converted to a digital value. A first comparator is responsive to the capacitor reaching a level of

charge just exceeding ground level, to trigger commencement of a timed count by a timer/register. A second comparator is responsive to the capacitor reaching a

charge level representative of the analog input voltage which exceeds ground level but is normally less than or equal to the bandgap voltage, to trigger a cessation of

the timed count. The register portion of the timer/register stores the digital

conversion value equivalent to the count reached upon cessation, which is

representative of the digital value of the instantaneous level of the sampled analog input signal. The capacitor is then discharged to intialize the converter for the next analog-to-digital conversion.

Such a slope A/D converter is not as susceptible to conversion errors attributable to the turn-on delay of the constant current source, because the source is

already operating ~ having charged the capacitor from the below-ground level up to

ground level before the count is commenced — ; or attributable to a residual charge on the capacitor, because any such charge is immaterial to either the commencement

or cessation of the count since those points are determined by the capacitor voltage reaching ground level and reaching the instantaneous level of the sampled analog

input signal.

Accordingly, it is a more specific objective of the present invention to

provide a slope A/D converter with substantial improvement in conversion accuracy by avoiding a delay in operation of the charging means for the measuring capacitor,

and by eliminating the effect of a residual charge on the capacitor between

conversions.

Yet another aim of the invention is to provide a microcontroller for use

with a slope A/D converter to control the value of a parameter which is applied as a

representative input to the converter. Brief Description of the Drawings

The above and still further aims, objectives, features, aspects and attendant advantages of the present invention will become apparent from a consideration of the following detailed description of a presently contemplated best

mode of practicing the invention, by reference to a preferred embodiment and method thereof, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a plot of a plane Cartesian coordinate system wherein

positions of points are determined from abscissa and ordinate parallel to X- and Y-

axes, respectively, for explanation of a prior art X-intercept calculation of slope A/D

conversion, described above;

FIG. 2 is a block diagram of a preferred embodiment of a slope A/D

converter according to the present invention;

FIG. 3 is a graph illustrating the operation of the converter of FIG. 2;

FIG. 4 is a more detailed schematic diagram of the slope A/D converter

of FIG. 2;

FIG. 5 is a plot of a plane Cartesian coordinate system similar to that of

FIG. 1, but illustrating the improved effect obtained with the device and method of

the present invention;

FIG. 6 is an alternative embodiment of the slope A/D converter of FIG.

2; and

FIG. 7 is a simplified block diagram showing a slope A/D converter in

conjunction with a microcontroller according to the invention. Detailed Description of the Presently Preferred Embodiment and Method

A preferred embodiment of a slope A/D converter 10 according to the

present invention is shown in block diagrammatic form in FIG. 2. The circuit 10

includes a constant current source 11, a capacitor 12, a plurality of switches (which

may be transistors or other electronic devices, despite the conventional schematic

single-throw representation in the Figure) 15, 17, 18, and 20, a discharge reference

voltage source 23, a pair of comparators 26 and 27, and an A/D timer and capture

register (sometimes referred to herein as a counter /timer) 30.

Constant current source 11 has an output port connected to one pole of

switch 18, to a first (negative) input of comparator 26, and to a first (negative) input

of comparator 27. The arm of switch 18 is connected to the arm of switch 17, the

latter having a pole connected to electrical ground. It will be understood that each the switches may be and preferably is a metal-oxide-silicon field effect transistor

(MOSFET) with source, drain, and gate electrodes, and an electrical path which is

open or closed through the source-drain electrodes depending on the relative

voltages on those and the gate electrode. The arms of switches 17 and 18, in

addition to being connected to each other, are connected to one terminal (plate) of

capacitor 12. The other terminal of the capacitor is connected to a pole of switch 15

and a pole of switch 20. The arm of switch 15 is connected to discharge reference

voltage source 23, and the arm of switch 20 is connected to ground.

The second (positive) input of comparator 26 is connected to ground,

and the second (positive) input of comparator 27 is connected to receive the analog

input signal whose instantaneous value has been sampled (for example, by a sample- and-hold circuit, not shown) and is to be converted to a digital value by the

converter circuit 10. The outputs of the two comparators are coupled to respective

start (commence) and stop (cease) inputs of counter /timer 30.

In operation of the slope A/D converter 10 of FIG. 2, switches 15 and

17 are closed initially, while at the same time switches 18 and 20 are open.

Consequently, capacitor 12 is then connected between ground and the discharge

reference voltage of source 23. Sufficient time is allotted to allow the capacitor to

charge (or discharge) so that the level of charge on the capacitor reaches the

imprecise and non-critical value of the reference voltage, which may range from

about 0.3 v to 0.5 v below the level of electrical ground, for example. This

imprecise and non-critical value is a key point in the context of the present invention, in that it is only necessary to discharge the capacitor slightly below ground. Simultaneous with this charging (or discharging) of the capacitor, the A/D

counter /timer 30 is reset to zero. During these events, comparators 26 and 27 are

held inactive.

When the charge on capacitor 12 reaches the discharge reference voltage

level, switches 15 and 17 are opened, and, an instant of time later, switches 18 and

20 are closed. The timing of the switch transitions is such that switches 15 and 17

are never closed at the same time as switches 18 and 20 are closed. Now the

capacitor begins charging by virtue of current flow from constant current source 11,

commencing from the level of the discharge reference voltage of source 23 just

below ground to a predetermined voltage level exceeding the highest level

anticipated for any instantaneous sample voltage of the analog input signal to comparator 27.

When the charge on capacitor 12 reaches or barely exceeds the level of

electrical ground (0), comparator 26 responds by transitioning low to cause A/D

counter/timer 30 to commence its count. Capacitor 12 continues charging and, when its voltage level reaches or barely exceeds the sampled voltage level of the

analog input signal to comparator 27, the latter comparator responds by transitioning

low to cause A/D counter /timer 30 to cease counting.

The A/D result may now simply be read — or otherwise taken and used,

for example as an input to a microcontroller with which the converter is associated -

- from the capture register(s) of A/D counter /timer 30. An accurate digital value

representative of the analog input to the converter is obtained without need for

additional program memory or a multiplicity of computation cycles as have

characterized prior art converters. In the circuit of the present invention, the start of the A/D conversion is always triggered at the zero (ground)-crossing point. It is immaterial whether dielectric absorption maintains some residual charge on the

capacitor, because the transitioning of comparator 26 and the commencement of the

count occurs at the precise time that the voltage level on the capacitor crosses

ground level, and that operation takes place for each conversion.

Further, the conversion is independent of any delay that may be

experienced in turning on the constant current source, because that source is already

operating upon (and indeed, is ultimately responsible for) occurrence of the

comparator transition to commence the count. In addition, conversion throughput is increased by virtue of a faster computation of the digital conversion value than has been achievable by prior art techniques. After the A/D conversion result is read

from the capture register 30, capacitor 12 is discharged by closing switches 17 and

20 while 15 and 18 are left open. The overall operating sequence is then repeated

for the next conversion.

This operation is further illustrated by reference to FIGS. 3, 4, and 5.

FIG. 3, which is a timing diagram of voltage (Y-axis) versus time (X-axis) at

various points in the sequence of operation of the slope A/D converter of FIG. 2,

illustrates two exemplary cycles of conversions of the analog input voltage. The

only difference between the two cycles is in the charge duration. Initially, the

DSCHG signal (see FIG. 4, a more detailed diagram of the converter) is active, and

the CHG signal is inactive. The CAPl node of capacitor 12 is grounded, and the

CAP2 node of the capacitor is allowed to charge to the discharge reference voltage (i.e. , a level at least slightly below analog ground, 0.5 v in this example) through

switches 15 and 17. Next, the DSCHG signal is deasserted (once allowing the

capacitor to fully charge), and a short time later the CHG signal is asserted which

grounds the CAP2 capacitor node through switch 20 and connects the CAPl

capacitor node to the constant current source through switch 18 (not shown in FIG.

4). Because the capacitor polarity has been reversed, the CAPl node is -0.5V with

respect to ground at this time. Subsequently, the CAPl capacitor node begins

charging from the constant current source.

When the capacitor voltage reaches or slightly exceeds analog ground,

the comparator 26 trips low, which starts the A/D timer used in the slope

conversion, as indicated by the NSTART signal in FIG. 3. The capacitor continues to charge while the timer is running, until the capacitor voltage reaches or slightly

exceeds the analog input level. At that time, comparator 27 switches low, stopping

the A/D timer as indicated by the NSTOP signal in FIG. 3. The capacitor may

continue to be charged until the DSCHG signal is re-activated or the current source

reaches its compliance limit, whichever occurs first, but without adding to the

content of the A/D timer. The user then reads or otherwise uses the contents of the

A/D timer for the result of the conversion. At this point the capacitor is discharged

(to a voltage level at least slightly below the analog ground level), and the sequence is repeated for each conversion cycle.

In FIGS. 3 and 4, A_IN is the analog input voltage to be converted to a

digital value, and V_REP is the discharge reference voltage. FIG. 5 is similar to the

Cartesian coordinate plot of FIG. 1 except that it now represents the results obtained

with the embodiment of FIGS. 2 and 4. The zero count always occurs at zero volts,

thereby substantially eliminating the error attributable to offset, and the dielectric

absorption effects and turn-on delay effects are minimized.

It will be observed that the slope A/D conversion method of the

invention converts an instantaneous analog value measurement to an accurate digital

representation in a manner akin to initially storing a packet of energy of magnitude just below analog zero in a substantially depleted energy storage means, gradually adding a second packet of energy to the energy storage means to increase its stored

energy to a level not only exceeding the predetermined reference energy level but

ultimately exceeding the highest value which is anticipated for the analog input, and

counting the increments of time starting with achievement of analog zero level in the energy storage means and ending with achievement of the then-current level of the instantaneous analog input in the energy storage means, as a digital value representative of that instantaneous analog input level. In response to cessation of the timed count, the energy storage means is again depleted in preparation for a new

analog-to-digital conversion.

An alternative embodiment of a slope A/D converter 10' according to

the present invention is shown in block diagrammatic form in FIG. 6. In this

alternative embodiment, means are provided for converting the bandgap reference

simultaneously with converting the analog input. As in the embodiment of FIG. 2,

the circuit 10' includes a constant current source 11', a capacitor 12' , a plurality of

switches 15' , 17', 18', and 20' , a discharge reference voltage source 23' , a pair of

comparators 26' and 27', and an A/D timer/capture register 30'.

The embodiment of FIG. 6, however, additionally includes a second

A/D timer/capture register 40, and a comparator 42. The operation of this

alternative embodiment is substantially identical to that of the preferred slope A/D

converter embodiment of FIG. 2, except that the count of A/D timer/counter 40 is

commenced simultaneously with the count of A/D timer /counter 30' owing to the

parallel connection of the same (start count) input of each of the two to the output of

comparator 26' . Additionally, the count of timer/counter 40 is stopped when the

charge on capacitor 12' at one input of comparator 42 barely exceeds the bandgap

reference voltage applied as the other input of the latter comparator.

FIG. 7 illustrates the use of the slope A/D converter of FIG. 2 (or of

FIG. 6) of the present invention in or in conjunction with a microcontroller 50 which is used to control one or more parameters of an external system, such as the

temperature within an enclosure. The enclosure temperature is monitored and converted to the analog input voltage to the A/D converter, and the A/D conversion is used to allow the microcontroller to control the operation of a heating or cooling system to accurately maintain the enclosure temperature at a preset value.

Although certain preferred embodiments and methods are described herein, persons skilled in the art will recognize from the foregoing description that

variations and modifications of the described embodiments and methods may be

made without departing from the true spirit and scope of the invention. For

example, the constant current source may be replaced by any device capable of

delivering charge to the capacitor at a constant rate, to assure that the count, or the

timed interval itself, is a digital value accurately representing the then-current analog

input voltage. Also, although a capacitor is the preferred energy storage means, it could be replaced with some other device for storing electrical energy that will not

suffer substantial leakage during the period the time interval is measured.

Accordingly, it is intended that the invention shall be limited only to the extent

required by the appended claims and the rules and principles of applicable law.

Claims

What is claimed is:

1. A slope analog-to-digital (A/D) converter, comprising:

capacitor means for storing voltage;

reference means for selectively placing said capacitor means at a preselected reference voltage level below electrical ground level;

constant current source means for selectively charging said capacitor means from said reference voltage level below ground level toward a predetermined

voltage level above ground level and exceeding an anticipated highest level of an analog input signal to the converter;

first comparator means responsive to said capacitor means first reaching

a charge at or slightly exceeding ground level during said selective charging for

initiating commencement of a timed count;

second comparator means responsive to said capacitor means first reaching a charge at or slightly exceeding the then-current actual level of said analog input signal to the converter but below said predetermined voltage level, for initiating cessation of the timed count; and

register means for storing a digital conversion value equivalent to the

count reached between commencement and cessation of the timed count, and thereby

constituting the digital value representative of the then-current actual level of said

analog input signal.

2. The slope A/D converter of claim 1, further including:

means responsive to cessation of the timed count for discharging said capacitor means in preparation for a new analog-to-digital conversion.

3. The slope A/D converter of claim 1, wherein said preselected

reference voltage level is imprecise.

4. A device for controlling a parameter of an external system, using

analog-to-digital (A/D) conversion of sampled values of the magnitude of a variable analog input voltage derived from the parameter to be controlled, comprising: a capacitor for storing voltage;

means for selectively delivering electrical charge to the capacitor at a

constant rate for storage of voltage in said capacitor according to the level of charge

delivered thereto over an interval of time; means for causing said charge delivering means to place a voltage on

said capacitor below a predetermined reference level, and for then increasing the

voltage stored on said capacitor until it exceeds the magnitude of the current sample

of the analog input voltage;

means for timing the interval between the point at which the increasing voltage on the capacitor reaches said predetermined reference level, and the point at

which said increasing voltage reaches the magnitude of said current sample of the

analog input voltage; and means for using said timed interval as a digital measure of the value of

the magnitude of said current sample of the analog input voltage.

5. The device of claim 4, further including:

means responsive to the cessation of said timed interval for discharging said capacitor in preparation for converting the next sampled value of the analog

input voltage.

6. The device of claim 5, wherein:

said predetermined reference level of the voltage stored on the capacitor is not a precisely regulated value but may lie anywhere in a predetermined range of

values without adversely affecting the accuracy of the A/D conversion of the current

sample of said analog input voltage.

7. The device of claim 6, further including:

means for using the timed interval to control said parameter of the

external system.

8. The device of claim 6, wherein:

said timing means includes means for counting fixed increments of time from said point at which the increasing voltage on the capacitor reaches said

predetermined reference level to said point at which the increasing voltage reaches the magnitude of said current sample of the analog input voltage, the cumulative count between the two points constituting said digital measure of the analog input voltage.

9. The device of claim 6, wherein:

said means for delivering electrical charge includes a constant current

source.

10. A method of converting an instantaneous value of a variable analog

input voltage to a digital value representative thereof, which comprises the steps of: discharging a capacitor until the level of the voltage stored therein is at

least slightly below the level of an electrical ground reference; charging the capacitor at a substantially constant rate toward a level of stored voltage that exceeds the magnitude of the anticipated highest value of the

analog input voltage; counting increments of time starting with the moment at which the level

of the voltage stored on the capacitor as a result of the charging reaches ground

reference level and ending with the moment at which the level of the voltage stored

on the capacitor as a result of the charging reaches the current magnitude of the

analog input voltage; and using the cumulative count as the digital value corresponding to the

value of the analog input voltage at the time the count ended.

11. The method of claim 10, including: performing the step of discharging said capacitor after reaching said

cumulative count, in preparation for the next analog-to-digital conversion of the

instantaneous value of the analog input voltage.

12. The method of claim 10, wherein:

the step of discharging the capacitor is performed without concern for

the precise level of the stored voltage therein relative to the level of electrical

ground, but only that the former is at least slightly less than the latter.