Test Op Amps Via Simple Input Offset Voltage Measurement

Measuring this one vital parameter (Input Offset Voltage) provides a simple means of determining if the DUT (Device Under Test) is either good or bad. The circuit is very simple, requiring only four garden variety components and selector switch (or protoboard) to accommodate devices with multiple sections.

Schematic

LM324 datasheet

Voltage follower connection

Each of the op amps to be tested is wired as a voltage follower (output connected to the inverting input). Virtually all garden variety op amps are stable in the voltage follower connection. The gain is unity (or 1) so that the voltage at the output is supposed to be exactly the same as the voltage on the non-inverting input. Any error reflects the input offset voltage, and can be easily measured with a good DVM.

DVM

My 15year old Radio Shack 22-174B DVM handles it well, reading only 0.1mV with the leads shorted (see photo) –ideally it should read zero mV –your DVM may do better or perhaps it even has 10uV resolution.

Input offset measurements with a random LM324 out of my junkbox

Pin 1: 0mV
Pin 7: 0.4mV
Pin 8: 1.2mV
Pin 14: 1mV

Specification: ±7mV max
My observation is that this is a good device with all sections measuring well below the ±7mV maximum.

Implications

For the input offset voltage to read within the limits, implies that both the op amp input and output circuit structures are working properly and have not been damaged via electrostatic discharge, or has not been fried by fault current etc.

Certainly, there are far more parameters that may be measured such as input bias current, input offset current, slew rate, output current limit, output saturation voltages (high & low), quiescent power supply current etc., but to measure all parameters would take all day unless one has an automated test fixture…and already has it programmed…

Conclusion

The input offset voltage measurement is a very good thermometer for determining device health.

Photos