Circuit Function
circuit is designed to convert battery voltages between 3.2 V to 4.2 V into telemetry of -5 V to +5 V.
Problem Statement
Output pin no.8 of the LM124 IC showed 10 mV oscillations of very low frequency in ambient condition. Also, change of 500 mV in output voltage (measured at pin no.8) was observed under temperature change of 50ºC from ambient to cold. A similar observation was made when temperature was changed from ambient to hot. An exhaustive series of experiments on the circuit intended to find the cause of change in output voltage with temperature ruled out the role of other components like resistors or zener diode. Subsequently replacement of LM124 with new one solved the problem.
Suspect
AVO switch (AVO meters are ‘Motwane’ make Multi-meters) are used in the test set up as shown in above figure to measure current drawn by the circuit. It also isolates circuit from transient during power supply switch on. However, AVO switches used in the test set-up might stress the device as described by following cases, rendering it prone to degrade over a period of time.
CASE I
Since simultaneous switching on of ±Vcc (±15V)through AVO switches is practically difficult to achieve, switch on of +Vcc AVO first and –Vcc AVO open circuit results into non-nominal current of 3mA through input pin of op-amp to ground irrespective of existence of 6 microampere current mirror internal to LM124 .
CASE II
Also, switch on of –Vcc AVO first and + Vcc AVO open circuit might result in reverse biasing of base emitter junctions in internal transistors present on input side of the op-amp. Following voltages were observed at various junctions in the circuit assuming input to the circuit was applied before supply voltages (±Vcc).
Inference
Improper switching on/off method of op-amp through AVO switches might have stressed the LM124 by applying reverse biased across base emitter junctions of internal transistors and/ or by allowing non-nominal current through input pin of LM124 to ground. Sustainment of such stress condition while testing this circuit over the period of two months could have degraded some of LM124 parameters like input offset current and/or input offset voltage resulting into their variation in temperature beyond limits mentioned in the datasheet.
Can switch on of +15V alone with -15 line open circuit or vice versa stress the op-amp? Can such condition lead to degradation of op-amp parameters and eventually cause these parameters to change with temperature?