A time-locked photon isolated calibrator

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Electroencephalography and Clinical Neurophysiology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

TECHNICAL

CONTRIBUTION

A TIME-LOCKED PHOTON ISOLATED CALIBRATOR 1 ROBERTP. SCOBEY Department of Behavioral Biology, School of Medicine, University of Cal(/brnia, Davis, Cal([~ 95616 (U.S.A.) (Accepted for publication: November 15, 1971)

The calibration of single-ended biological amplifiers is simple, but the calibration of differential amplifiers by the injection of microvolt signals into the subject leads has previously required relatively costly instrumentation (Emde 1964). With the recent development of inexpensive gallium arsenide photo-emitting diodes and matching photodetectors, optical isolation of many electronic circuits has become greatly simplified. This communication reports on the development of an isolated 14-channel microvolt calibrator intended for use with EEG recorders and signal averaging devices. The photodiode coupled pair (also called photon coupled isolator) (Monsanto, MCD 1; Hewlett Packard, HP4320; and others) is well suited for the injection of square-wave microvolt signals into the input leads of differential input amplifiers. In the apparatus described here, stimulation circuitry at ground potential provides for activation of the phoSHIELD

to-emitting diode. A second element, a detector consisting of a solid state diode, is electrically isolated from the ground potential; it loads the input leads by only a few picofarads and many thousands of megohms. The current generated in the photon detector is roughly proportional to the current in the photo-emitting diode, thereby permitting remote control of the amplitude and duration of the calibration pulse and the rise and fall time of the electrical current at the detector is a fraction of a microsecond. If the current from the detector is 20/xA, a load resistor of 1 D will generate a 20 pV potential difference and can provide a low insertion resistance in series with the input leads of the amplifier. These parameters insure that the addition of the calibration device to the input leads of a differential amplifier produce an insignificant change in amplifier performance (Fig. 1).

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AMPL1 .4 Fig. 1. The photon isolated calibrator. A few milliamperes of current (I1) through the photo-emitting diode is sufficient to generate tens of microamperes through the resistor R 2. The receiving diode is well isolated from ground potential, and the receiving diode with the addition of a few ohms of resistance in the preparation leads produces a negligible disturbance to the normal operation of the biological differential amplifier. The calibration signal (V2) can be added to the biological signal while the leads are attached to the preparation. i Supported by a grant from the U.S. Public Health Service, National Institutes of Health, HD-04335.

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Fig. 2. A 14-channel microvolt calibrator.An initiationpulse to the calibrator trigger input (cal. trig. in.) initiates a delay (T~ + T 2 + T3) from the time of the initiation pulse to the time that the stimulus is presented to the subject. After the delay of T~ the calibration pulse is presented to the subject leads for the time T 2. The circuitry providing the time T 2 activates the logic amplifiers (LA) which provide current through the photo-emitting diodes. The adjustment of the current through the photo-emitting diode by R 1 permits the microvolt calibration signal across R~ to be adjusted to a desired value.

Electroenceph. clin. Neurophysiol., 1972, 32:568-570

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PHOTON ISOLATED CALIBRATOR

Fig. 3. Records of the calibration pulse with noise and after averaging. Two records are overplotted in the illustration. The

fine trace shows a 20 ~V calibration signal injected in the subject leads of an EEG amplifier mixed with electrical noise. The duration of the calibration signal was 20 msec and was delayed from the start of the record by 2 msec. These signals were then recorded on magnetic tape at 1 7/8 in./sec speed, and later 64 individual records were averaged. The calibration signal appears on each record, providing not only a clear voltage and time calibration, but also an estimation of the fidelity of reproduction through all the electronics and an estimate of the amplitude of the residual noise in the record after computation.

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< L Fig. 4. A schematic circuit diagram for a 14-channel calibrator. In addition to the basic components outlined in Fig. 2, the instrument provided laboratory interfacing. An incoming short duration pulse at the input CAL. TRIG. IN (A of timing diagram) started timer T Oto provide a standardized pulse (B) of 0.5 msec at the SYNC output. This pulse was recorded on magnetic tape to mark the beginning of a calibration sequence. Simultaneously, the timer T~ was triggered to begin a delay period (C) as described for Fig. 2. Timers To, T 1 and T3 have identical components. The longer duration of the calibration period (T2 of Fig. 2, D of timing diagram) was determined by a hybrid circuit of both integrated circuits (MC 846P) and discrete components (Q1, Q2, D113T, etc.). The termination of the second delay period T 3 triggered a fourth timer to provide a short pulse (F) at STIM. TRIG. OUT to begin the stimulus to the patient. The amplitude calibration of the device was facilitated by measurement of a signal at the photodiode (asterisks) which was 10 times the amplitude of the actual calibration signal. Adjustment of the 2K variable resistor in the remote box permitted independent amplitude calibration of each channel. All transistors were 2N5172.

Electroenceph. clin. Neurophysiol., 1972, 32:568-570

570 The amplitude stability of the calibration pulse is limited mainly by the dependence of the detector current output on temperature (approximately a 0.5 ~ decrease per degree C). For most laboratory use, the decrement in current output can be compensated by use of tungsten or nickel wire as the load resistor R 2. The resistor then has roughly the opposite temperature coefficient of the detector, resulting in a decrease in the dependence of calibration voltage on temperature by a factor of 5 or more. These principles have been used to design and construct a 20 #V calibrator with 14 different outputs, one for each input of a 14-channel EEG amplifier system (Fig. 2). The 14 photodiode coupled pairs were mounted in a remote box placed close to the patient leads in order to minimize the unbalance capacitance introduced by additional wiring. The wiring to the photo-emitting diode and the wiring of the amplifier leads to the photodetector were carefully shielded from one another. The photo-emitting diodes were driven by standard DTL logic devices (Motorola MC846P) which can drive up to four photo-emitting diodes. The duration of the "on" period for the diodes was controlled by a solid state timer T 2. The onset of the calibration pulse was delayed by a second timer T1, providing (in this instance) 2 msec of control record. After the 20 pV Calibration signal was injected into the patient leads, a delay of 2 msec before the onset of the stimulus was provided by timer T 3 to insure that the end of the calibration pulse would be clearly separated from any stimulus artifact. For durations of less than 5 msec, the Motorola monostable vibrator MC851P was satisfactory. The calibrator has proved to be especially useful for the calibration of microvolt visual evoked potentials for time and amplitude and for providing an estimation of the baseline stability of averaged evoked potentials. Fig. 3 illustrates with overplotting a record of the 20 pV calibration signal superimposed on 20 #V of white noise (thefine trace), together with the average of 64 such records (the heavy trace). In the system

R . P . SCOBEY described previously, the stimulus to the subject was given 2 msec after the end of the calibration signal, but the computation of the average was begun with the activation of the calibrator. In this way, each averaged evoked potential from the averaging device had a calibration pulse leading the biopotential. A schematic circuit diagram for a 14-channel calibrator is given in Fig. 4. SUMMARY The calibration of differential amplifiers by injection of a microvolt signal into the input leads during recording of biopotentials can be accomplished simply and inexpensively by means of recently developed photodiode pairs. An instrument for simultaneous calibration of 14 channels of evoked potential data is described. RESUME ETALONNEUR ISOLE DE PHOTON LIE AU TEMPS L'dtalonnage d'amplificateurs diff6rentiels par injection d'un signal d'un microvolt dans les chalnes d'entr6e au cours de l'enregistrement de bio-potentiels peut &re r6alis6e de faqon simple et 6conomique au moyen de paires de photodiodes r6cemment raises au point. Un instrument de calibration simultan6e de 14 chalnes donn6es de potentiels 6voqu6s est d6crit. The help of D. Howard in the construction and testing of this device is gratefully acknowledged.

REFERENCE EMDE, J. W. A time locked, low level calibrator. Electroenceph, clin. Neurophysiol., 1964, 16: 616-618.

Reference: SCOBI~Y,R. P. A time-locked photon isolated calibrator. Electroenceph. clin. Neurophysiol., 1972, 32: 568-570.