First the background.
I’ve got a customer who wants to drive the inputs of an instrument from a separate signal generator. One of the inputs I designed correctly (series resistor and diodes to the supply rails.) but the other is just straight into an opamp input. (I didn’t expect someone would want to use an external source… but also the design is ~10 years old and I didn’t know as much back then.)
So when the instrument is powered up there is no problem. But if someone was to apply an external signal and the instrument was not turned on it would overdrive the opamp input. The opamp in question is an opa227, but I hope this question can be of broader scope. The typical opamp spec sheet says you can over drive by 0.7 volts over the supply rails. I set up a little test circuit (yeah a piece of white protoboard.) and sent in square waves from my sig. gen. (1- 20 Vp-p). Now exactly what happens depends on the power supply and the rest of the circuit. (The built in opamp protection circuit can power the supply rails.) But I think the worst case will be a dead short on the supply. So that’s what I did. It then looks like the built in protection circuit is a diode to the rail with a 10-15 ohm resistor in series. And at 20 Vp-p (10 V peak) I was seeing 5.2 Vp-p (2.6 V peak) at the opamp input. I let it run for several hours this way, and though I didn’t do any exhaustive testing, the opamp still seemed to work fine afterwards. (2.6V at 140 mA ~360mW of power dissipated)
So here’s the question: Can you over drive opamp inputs above the absolute maximum rating on the spec sheet? (I’m pretty sure we have all been prototyping something and turned off the power supply with the sig. gen. still connected.)