A linearly reading micro-electrode resistance meter

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Electroencephalography and Clin ical Neurophysiology Elsevier Publishing C o m p a n y , A m s t e r d a m - .Pri~atcd in The Netherlands

TECHNICAL CONTRIBUTION A LINEARLY READING MICRO-ELECTRODERESISTANCEMETER H U G H J. SPENCER

Department of Physiology, University of Manitoba, Winnipeg, Manitoba (Canada) (Accepted for publication: April 22, 1971)

CIRCUIT D E S C R I P T I O N An N-channel depletion mode junction field effect transistor (FET) wired as a constant current source is used to supply the measuring current. The current source configuration used here is insensitive to changes in both load resistance, supply voltage and temperature, provided the voltage drop across the load resistor (in this case the micro-electrode) is at least 2 V less than the supply voltage. With a metering current of between 10 to 20 nA this condition is fulfilled for maximal electrode resistances of 200 Mfl given a 6 V supply, and 500 Mr2 with 12 V.

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The magnitude of the current flow is determined by the gate-source cutoff voltage (Vgs(co))of the FETs used and the value of the external gate-source resistor. For the FET types suggested here, the 100 Mi'2 resistor gives a constant current flow of approximately 10 nA; the exact value of this resistor is unimportant in this application. This resistor provides negative voltage feedback to the gate which enhances the characteristic FET constant current behavior considerably. A second FET in the source follower configuration is used as a high input resistance voltmeter which measures the voltage developed across the micro-electrode without causing circuit loading. Since FET source followers exhibit an

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~- 3Mol KCI chlorldedwire Fig. I. Circuit diagram of the micro-electrode resistance meter. Resistance values are in ohms and the resistors can be ½ watt carbon composition types. Although it is advisable to use a high stability resistor as the current controlling resistor, it is not essential. The two trimmer potentiometers are cermet or carbon types. The circuit can be constructed in any suitable manner, but care must be taken to prevent contamination of the input and current source circuitry with saline or KC1 solutions, as this would result in erratic and erroneous readings.

Electroenceph. clin. Neurophysiol., 1971, 31:518 519

519

MICRO-ELECTRODE RESISTANCE METER output voltage offset, the meter is connected between the follower output and a 5 k ~ zero potentiometer to permit zeroing the meter with the input short circuited (0 ~). A 50 k ~ calibration potentiometer in series with the 100/JA meter, is used to set the meter to full scale with a 100 M ~ precision resistor across the input. Larger resistors may be used if desired, depending on the resistance range to be covered, Once calibrated initially the instrument should require no further calibration, Accuracy is limited only by the accuracy of the resistance used to calibcate full scale deflection on the meter, and the meter accuracy. However, it is advisable to use Ag/AgC1 electrodes to make connection to the micro-electrode and the KC1 bath into which the micro-electrode dips. This will reduce the development of voltage offsets which would s u m with the voltage developed across the micro-electrode by the constant current source and so produce erroneous readings, When not in use it is good practice to short the Ag/AgC1 electrodes together and leave them in the KC1 solution to prevent offset voltages developing, Any suitable battery m a y be used to power the circuit, but mercury batteries are recommended for their long shelf life characteristics, as the total current drain of the instrument will be small (4 mA) and intermittent,

SUMMARY A nanoampere constant current source forces current through the micro-electrode. The voltage so developed is measured by a simple, high input resistance field effect transistor voltmeter and is directly proportional to the microelectrode resistance. Since a low (10 nA) current is used, electrode polarization and thermal damage is prevented. Electrodes with resistances in excess of 500 M ~ can be measured. RESUME A P P A R E I L DE M E S U R E DE R E S I S T A N C E DE M I C R O - E L E C T R O D E S EN L E C T U R E D I R E C T E U n e source de courant constant en nanoamp6res fait passer le courant au travers de la micro-61ectrode. Le voltage ainsi obtenu est mesur6 par un voltm6tre simple (transistor effet de champ) h r6sistance d'entr6e 61ev6e et directement proportionnel h la r6sistance de la micro-61ectrode. Etant donn6 q u ' o n utilise un courant faible (10 nA), la polarisation de l'61ectrode et la d6t6rioration thermique sont 6vit~es. II est possible de mesurer des 61ectrodes avec des r6sistances d6passant 500 M ~ .

Reference : SPENCER, H. J. A linearly reading micro-electrode resistance meter. Electropnceph. clin. Neurophysiol., 1971, 31 : 518 519.