ELECTROMAGNETICALLY SHIELDED NARROW BAND ELECTROENCEPHALOGRAPHIC AMPLIFIER
CROSS REFERENCE TO RELATED
This application is related to the following concurrently filed copending U.S. patent applications, all assigned to the assignee of the present invention: Electroencephalographic (EEG) Cap by Sherwin having U.S. Ser. No. 727,031; Evoked Potential Autorefractometry System by Bernard, Roth, Mohan, Sherwin and Zomp having U.S. Ser. No. 727,032; Low Noise EEG Probe [5 Wiring System by Sherwin having U.S. Ser. No. 727,060; Subcaratinaceous EEG Probe by Sherwin and Mohan; and Shielded, Self-preparing Electrode Suitable For Electroencephalographic Mapping having, U.S. Ser. No. 727,058. 20
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to an amplifier for amplifying a pair of signals indicating electrical 2j potential of skin at two locations and more particularly to an electroencephalographic amplifier having a narrow band which is suitable for cortical evoked potential studies which use an input signal that matches the narrow band of the amplifier and to use with an evoked 30 potential autorefractometry system.
2. Description of the Related Art
One type of electroencephalographic (EEG) study utilizes an input signal having a specific frequency which causes a cortical evoked potential that can be 35 measured using EEG probes or electrodes on the scalp of the subject under study. Another type of EEG study relies on the prior identification of a specific frequency, such as alpha wave activity which increases immediately prior to a subject dozing off. Presently, such stud- 40 ies are conducted using an EEG amplifier which is capable of detecting a wide range of frequencies from direct current to several thousand hertz. In order to provide the capability to amplify signals with such a wide range of frequencies, conventional EEG amplifi- 45 ers utilize chopper stabilization and cost ten to fifteen thousand dollars. However, despite the high cost and elaborate circuitry of such amplifiers, they do not eliminate all noise present in the environment in which they are used. This is particularly a problem in commerical 50 environments when shielding from environmental electrical noise is normally not provided, such as in an optician's offie.
One example of such a conventional EEG amplifier is Model 611 manufactured by Beckman. This amplifier 55 uses chopper stabilization circuitry and provides the flexibility described above. However, many types of EEG studies utilize two or more probes for sensing skin potential of the scalp in addition to a reference lead attached, for example, to an ear lobe. These types of 60 studies display the difference between the potential of the two probes attached to the scalp. In order to perform such studies with conventional amplifiers, such as Beckman Model 611, one of the scalp probes is used as the reference lead and the other scalp probe is con- 65 nected to the input to the amplifier. As a result, fluctuations in electrical potential sensed by the first scalp probe cause the internal ground of the input portion of
the amplifier to fluctuate. As result, input impedance is limited to three to five kilohms.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a low cost, low noise EEG amplifier.
Another object of the present invention is to provide an EEG amplifier which provides true differential amplification.
A further object of the present invention is to provide an EEG amplifier which eliminates virtually all extraneous signals.
Another object of the present invention is to provide an EEG amplifier with high input impedance for use with virtually any type of EEG probe or electrode.
A further object of the present invention is to provide an EEG amplifier suitable for a commercial environment.
Another object of the present invention is to provide an EEG amplifier suitable for an evoked potential autorefractometry system.
The above objects are attained by providing an amplifier comprising an input for receiving first and second input signals indicating electrical potential of skin at two locations and first and second impedance elements for providing a constant impedance to the first and second input signals and for outputting first and second internal signals, the constant impedance provided by each of the first and second impedance elements being substantially equal. A differential amplifier subtracts the first internal signal from the second internal signal to produce a third internal signal which is amplified and filtered by a filter to produce a narrow band output signal. Batteries provide an internal power source for the amplifier which is shielded by a shield from external electrical and magnetic signals.
These objects, together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like reference numerals refer to like parts throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded view of an amplifier according to the present invention; and
FIG. 2 is a circuit diagram of the electrical components in the amplifier illustrated in FIG. 1.
DESCRIPTION OF THE PREFERRED
An EEG amplifier according to the present invention is capable of extremely low noise operation due to the physical construction of the amplifier. As illustrated in FIG. 1, a metal enclosure 10 comprises a metal top 10a and a metal bottom Wb which surround all of the electronic components 12 on a circuit board 14 when the top 10a is attached to the bottom 10b by screws 16. The circuit board 14 is a ground plane circuit board which includes a copper film 18 on one side thereof. To eliminate noise from power lines, batteries 20 provide power for the amplifier. An input jack 21 and an output jack 22 are located on the top 10a and connected to the electronic components 12.
The connections of the electronic components 12 for one embodiment of an amplifier according to the present invention are illustrated schematically in FIG. 2.