GB1247492A

GB1247492A - Improvements in apparatus for monitoring the impedance between electrodes

Info

Publication number: GB1247492A
Application number: GB39294/70A
Authority: GB
Inventors: Douglas Edward Maynard
Original assignee: National Development Corp
Current assignee: National Development Corp
Priority date: 1967-10-31
Filing date: 1967-10-31
Publication date: 1971-09-22

Abstract

1,247,492. Monitoring inter-electrode impedance. NATIONAL RESEARCH DEVELOPMENT CORP. Oct. 29, 1968 [Oct. 31, 1967; Feb. 19, 1968], Nos. 39293/70 and 39294/70. Divided out of 1, 243, 786. Headings G1N and G1U. Apparatus for monitoring the impedance between electrodes 10, 11, and particularly for detecting open-circuit therebetween, comprises a capacitor 24 across the electrodes, an oscillator 21 supplying a voltage to the capacitor and to a resistor 25 representing inter-electrode impedance, and a phase comparison circuit which compares the phase of a signal from the oscillator with that across the capacitor. A frequency selective amplifier 30 separates the signals from the oscillator 21 and posses them via dipping amplifier 31 to a first input of AND-gate 32. The second input of the gate is fed with signals from the amplifier 21 via clipping amplifier 33 and a phase inverting circuit so that, provided the impedance of resistance 25 is low compared with that of the capacitor at the frequency of the oscillator 21, the two inputs to the gate will be in antiphase and there will be zero voltage from the smoothing network 34. However an open circuit between the electrodes produces gate input signals in phase quadrature and the network produces an output of V volts. Further the bias of the dipping amplifier 31 is adjusted so that, in the absence of an input signal to it, caused by circuit failure in amplifier 13 it causes the first input to the gate to be "on". Thus the gate passes the whole of the signal from the dipping amplifier 33, yielding on output of 2V from the smoothing network. In a modification a 90 degree phase shift is incorporated in amplifier 30. The apparatus is applied to monitoring the impedance between skin-contacting electrodes providing signals to an EEG as described in Specification 1,243,786 to which reference is made, particularly for determining whether the electrodes have become detached from the patient.