Fast recovery electrode amplifier

A fast recovery audio amplifier for use in electrophysiology and other neurological tests and techniques relating to nerve and muscle functions. The fast recovery amplifier recovers from overload in less than one millisecond, thus enabling its use in clinical electrophysiology to record time lapses from stimulus to response of less than several milliseconds. The amplifier achieves fast recovery primarily because it eliminates input and interstage coupling capacitors so that the feed-forward path from electrode to output is strictly a DC amplifier.

BACKGROUND OF THE INVENTION 
Clinical nerve conduction studies have been used for many years as a 
diagnostic technique in the field of neurology. As a part of these 
diagnostic techniques, electromyograms (EMGs) are produced in response to 
nerve stimulation by electrical means. By use of EMGs, nerve condition 
velocities can be measured and analyzed as an aid in the diagnosis and 
treatment of various neurological disorders. For example, the blink reflex 
can be recorded using surface electrodes placed on the orbicularis oculi 
muscle after stimulation of the trigeminal nerve at the supraorbital 
foramen. In measuring the blink reflex using electrophysiological 
techniques, the active recording electrode is generally located 
approximately two centimeters away from the cathode of the stimulating 
electrodes. Another example of the use of electrophysiological techniques 
in measuring nerve conduction is the measurement of sensory, nerve and 
muscle action by stimulation of the median nerve at multiple points along 
its course between the palm and distal portion of the forearm. Generally, 
this technique involves measurements in increments of one centimeter, and 
the minimal distance between the stimulating and recording electrode is 
usually two to three centimeters. With this minimal distance, the time 
lapse from stimulus to response (latency) is usually less than two 
milliseconds. 
The foregoing techniques are described in detail in various technical 
papers, especially those involving the work of Dr. Jun Kimura. Some of Dr. 
Kimura's work is described in the Journal of the Neurological Sciences, 
1978, 38:1-10; in the Archives of Neurology, April 1977, 34:246-249; in 
the Electroencephalography and Clinical Neurophysiology, 1978, 45:789-792. 
The latter publication was co-authored by me and describes some aspects of 
my invention as set forth hereinafter. 
Most electrode amplifiers used in electrophysiology recover from an 
overloading input to near quiescent baseline in five to ten milliseconds 
or more depending on the amplifier design and the amount of overload. 
Thus, in situations such as those referred to in the foregoing examples 
where the stimulus is coincident with an electrical event of sufficient 
magnitude to cause an overloading artifact, it is not possible to 
accurately record responses of latency less than several milliseconds. 
Thus, the accuracy and utility of EMGs using prior art electrode 
amplifiers is somewhat limited. 
To a certain degree, the deficiencies of prior art electrode amplifiers can 
be partially avoided by using mechanical stimulation or by attempting to 
improve stimulus isolation. Also, modification of an AC amplifier and use 
of a compensator to offset the artifact can also be used to improve the 
accuracy of the recorded responses. Most electrode amplifiers consist of 
several stages in order to achieve the required gain, and the stages are 
AC-coupled to block DC operating levels. Usually, one or more of the 
coupling capacitors is variable to adjust the low frequency cutoff. In the 
quiescent state, there is a charge on each of the coupling capacitors 
according to the DC voltage difference between the stages. If an 
overloading input saturates the first stage, the charge on the coupling 
capacitor to stage two will change. When the overload is gone and the 
first stage returns to normal, this voltage difference, amplified by the 
succeeding stages, results in a large or sometimes saturated deflection at 
the amplifier output. This offset than decays at a rate set by the linear 
coupling time-constant and thus accounts for a long recovery time. 
Attempts have been made to reduce the recovery time of an amplifier by 
shortening the coupling time constants and/or reducing the voltage gain. 
However, this results in a loss of low frequency response or a loss in 
sensitivity or both. Thus, attempts to reduce the recovery time of prior 
art amplifiers to the point where they meet all the specifications 
required for electrophysiological recording have not met with success. 
There is therefore a need for a relatively simple and inexpensive amplifier 
which recovers from overload in less then one millisecond without user 
adjustment, while still meeting the specifications required in clinical 
electrophysiology. 
SUMMARY OF THE INVENTION 
According to the invention, a fast recovery amplifier can be designed by 
eliminating the input and interstage coupling capaciters so that the 
feed-forward path from electrodes to output is strictly a DC amplifier. 
Then, in order to establish a low frequency cutoff and to eliminate DC 
electrode offset, an integrator feedback loop subtracts an offset signal 
from the stage one input. A circuit detects if the stage one output 
exceeds limits near saturation in either direction and, via a control 
circuit, opens the feedback loop. A capacitor, the charge on which 
represents the average offset value to keep the amplifier at baseline, 
holds this charge until the overload is gone. When the loop is reclosed, 
the capacitor restores the amplifier output to baseline. 
An alternative and improved system for decreasing the recovery time of an 
electrode amplifier is to insert a "sample-and-hold" circuit in the path 
between the remote preamp and the main amplifier. The connection of the 
feedback loop is such that the sample and hold circuit opens the feedback 
loop in response to a central circuit. In this system, one of the control 
possibilities is to detect overload of the preamp with about 50 
microseconds prediction. The feedback loop is then opened either on 
overload or predicated overload and reclosed only on actual recovery of 
the preamp signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The fast recovery amplifier of the invention is designed primarily for the 
performance of diagnostic tests of nerve and muscle functions in the 
clinical practice of neurology. The fast recovery amplifier is designed 
for use with standard well-known storage oscilloscopes such as those 
marketed by Tektronix, Inc. Oscilloscope main frames of this type are 
presently available as part of some commercial EMG machines. 
The fast recovery amplifier of FIG. 1 incorporates the principles of the 
invention in a two-stage amplifier with the input of the first stage 
amplifier 10 (the preamp) being connected to positive and negative 
recording electrodes 12. The first stage output is connected to the input 
of the second stage amplifier 14 with no interstage coupling capacitor 
included. Amplifier 14 has provision for gain selection. 
To achieve fast recovery, a feed-back loop 16 interconnects the first stage 
output and input. The feedback loop includes a switch 18 and a variable 
capacitor 20. 
A control circuit 22 connected to the output of the first stage amplifier 
10 detects whether or not the first stage output exceeds limits near 
saturation in either direction. Through a limit detector 24 and control 
26, control circuit 22 opens or closes the feedback loop circuit 16. Thus, 
when feedback loop circuit 16 is closed by action of the control circuit 
22, circuit 16 will subtract an offset signal from the input of the first 
stage amplifier 10. This is shown schematically as a subtraction in the 
negative lead of electrodes 12. Feedback loop circuit 16 thus sets a 
single low-frequency breakpoint at 
EQU f=(K/2.pi.RC) 
where K is the fixed gain from the first stage offset input to the first 
stage output, C is the value of capacitor 20 and R is the value of 
resistor 28 in the feedback loop circuit 16. Thus, by varying capacitor 
20, the low frequency cutoff is changed. The higher the value of capacitor 
20, the lower the low-frequency cutoff and the quicker the recovery of the 
amplifier 10 from overload. This is in contrast to prior art amplifiers of 
this type where the opposite is true. To prevent the delay of recovery by 
the gain of succeeding stages, the high-frequency break point must be set 
in or after the last stage amplifier 14 as shown by variable capacitor 29. 
In operation, when the control circuit 22 detects an output from the first 
stage amplifier 10 that exceeds limits near saturation in either 
direction, switch 18 is operated to open the feedback loop circuit 16. The 
charge on capacitor 20, representing the average offset value needed to 
keep the amplifier 10 at baseline, holds this charge until the limit 
detector 24 detects that the overload is gone or a fixed time duration is 
exceeded and then switch 18 is operated by control 26 to reclose the 
feedback loop circuit 16 and restore the amplifier 10 to baseline. 
FIG. 3 sets forth a detailed schematic that accomplishes the functions of 
the system of FIG. 1. Those skilled in the art will recognize that other 
circuits can be employed to achieve the same result. 
Where the fast recovery amplifier circuit of the invention is used in 
connection with clinical electrophysiology, the output signal from 
amplifier 10 and the control 26 function to open and close the switch 18 
in feedback loop 16. The control scheme preferably used is an open time of 
feedback loop circuit 16 of a predetermined amount, such as 1.2 
milliseconds. However, other control functions besides fixed time can be 
utilized. For example, the duration of the overload can serve as the 
control function, as more fully explained in connection with the 
embodiment of FIG. 2. Also, when used in clinical electrophysiology, the 
first stage amplifier 10 and associated circuitry is preferably contained 
in a remote "preamp" box which can be placed near the electrode site. The 
circuit of FIG. 3 to the left of the broken line is the portion of the 
circuit that would normally be included in the preamp box. 
In FIG. 2 there is illustrated a second embodiment of a fast recovery 
amplifier suitable for use in clinical electrophysiology. 
In the second embodiment, the first stage amplifier 30 has its input lines 
connected to the positive and negative leads of the recording electrodes 
32. Similar to the first embodiment of FIG. 1, there is a feedback loop 
circuit 34 that subtracts an offset signal from the input of the first 
stage amplifier 30, this being shown schematically as a subtraction in the 
negative electrode lead. A feedback loop circuit 34 includes a resistor 36 
and a variable capacitor 38 that functions in a manner similar to 
capacitor 20 of the embodiment of FIG. 1. 
The second embodiment of FIG. 2, however, includes a sample-and-hold 
circuit 40 that performs the function of switch 18 of the first 
embodiment. In other words, the sample-and-hold circuit 40 opens the 
feedback loop circuit 34 on overload and recloses it and restores the 
amplifier 30 to baseline in accordance with the control portion of the 
circuit. 
However, instead of the fixed open time control of the embodiment of FIG. 
1, the limit detection of the embodiment of FIG. 2 includes a circuit to 
predict possible overload of the amplifier 30 before it actually occurs 
and to reset the "open time" if the amplifier returns to baseline within 
predetermined limits. The predictive circuit determines at any moment if 
the output from amplifier 30 will overload within the next 50 
microseconds, for example, assuming that it maintains its current rate of 
change (slope). 
The sample-and-hold circuit 40 is normally in the "sample" mode in which 
the output signal from amplifier 30 passes through to the main or second 
stage amplifier 42. However, if the output signal from the first stage 
amplifier 30 exceeds limits near saturation in either direction, the 
sample-and-hold circuit 40 is switched to the "hold" mode. Overload 
detection in the output signal from amplifier 30 in both a positive and 
negative direction is accomplished using a single comparator 44 by full 
wave rectification of the output signal from amplifier 30 through 
rectifier 46. A second comparator 48 detects the return of the amplifier 
30 to normal limits and allows for the earlier cancellation of the "open 
time" interval established by timer 50. Of course, the open time interval 
could be set at any suitable time depending upon the application in which 
the circuit is used. In the first embodiment of FIG. 1, the open time 
interval was fixed and the output of amplifier 10 was saturated for the 
duration of the overload. In the embodiment of FIG. 2, overloads in the 
output signal from amplifier 30 less than the open time interval are 
suppressed, and on recovery the amplifier 30 is allowed to resume 
functioning almost immediately. This has been demonstrated to allow 
recovery times as short as 0.1 ms. 
If desired, and as indicated in FIG. 2, provision can be made to trigger a 
fast recovery sequence using external signals such as the sources of 
overloading artifacts. In addition, provision can be made to 
enable/disable the fast recovery feature or the recovery/reset feature. 
As in the embodiment of FIG. 1, the feedback loop circuit 34 establishes a 
low frequency cutoff and also eliminates DC electrode offset by 
subtracting an offset signal from the input of the first stage amplifier 
30. By varying the capacitor 38, the low frequency cutoff can be varied. 
In the hold mode of the sample-and-hold circuit 40 of FIG. 2, the output of 
the circuit 40 is a constant value equal to the output signal of the 
amplifier 30 at the time of switching to the hold mode, however, somewhat 
integrated over a small time period set by resistor 60 and capacitor 62. 
With the predictive detection of an overload in the output signal of 
amplifier 30, the main amplifier 42 and the feedback loop circuit 34 are 
never subjected to the overloaded output signal from amplifier 30, and 
thus in the final output the overload is largely suppressed. Again the 
high frequency break point is set after the last stage amplifier 42 by RC 
circuit 66, 68. 
I have shown in FIG. 4 an example of a detailed schematic of a circuit that 
will accomplish the functions of the system of FIG. 2. FIG. 4 shows all of 
the circuit except for the amplifier 30, the circuit for amplifier 30 
being the same as amplifier 10 of the first embodiment which is shown in 
detail in FIG. 3. 
From the description of the invention as set forth in the embodiments of 
FIGS. 1 and 2, it will be evident that a fast recovery amplifier has been 
produced by eliminating the input and interstage coupling capacitors found 
in the conventional electrode amplifiers. Thus, the feed forward path from 
the electrodes to the output is strictly a DC amplifier and recovers 
essentially immediately because it contains no significant capacitors to 
store charge. The fast recovery feature is essential to produce accurate 
and meaningful results in clinical electrophysiology. By using the fast 
recovery amplifier of the invention, the problem of stimulus artifact is 
greatly reduced when measuring nerve condition times over very short 
distances. In such situations where the minimal distance between the 
stimulating and recording electrode is two to three centimeters, a 
potential with a latency less than two milliseconds can be adequately 
recorded. The charts shown in FIG. 5 show a comparison between the fast 
recovery amplifier of the invention and conventional amplifiers. These 
charts were taken from tests in which the median nerve was stimulated at 
multiple points along its course between the palm and distal forearm using 
increments of 0.5 centimeters. Sensory nerve potentials were recorded 
using surface electrodes. The number in the left hand column of FIG. 5 
represents the distance in centimeters between the stimulating cathode and 
the recording electrode. The small arrow in each chart indicates the first 
response with clear onset of negative (upward) deflection from the 
baseline. 
I have thus designed a fast recovery amplifier that overcomes the problems 
of stimulus artifact thus permitting the detection and recording of short 
latency potentials. The fast recovery amplifier is easy to operate and 
more accurate than conventional means. It can be used in connection with 
digital readout for fast and accurate measurement of the response time, 
and provision can be made for connection to an external computer system 
for remote control and storage or analysis of data. 
Having thus described the invention in connection with preferred 
embodiments thereof, it will be evident to those skilled in the art that 
various revisions and modifications can be made to the invention without 
departing from the spirit and scope thereof. It is my intention, however, 
that all such revisions and modifications as are obvious to those skilled 
in the art will be included within the scope of the following claims.