I posted a similar thread earlier but I think it got confusing. I now want to boil it to a single entity.
I have an opamp in non inverting mode. It requires three voltages. V+, V- and the signal. There can be circumstances when the signal is applied and the power voltages are zero. From PICs, this can be dangerous because power to a pin will turn the microcontroller on and it could apply more current than the GPIO pin could handle.
Is there a danger of too much power entering the inputs when the opamp is unpowered and cannot respond to make that current "zero". Typically, The opamp will be powered by about -10v and +40v and the voltage on the input side should be about 4V max.

 

It’s bad form to apply a voltage input to an unpowered op-amp but you might get away with it if you use an input resistor to limit current. The data sheet for the opamp might even address the issue. It would be better to avoid it.

 

It's going to depend on the specific opamp. Read the data sheet. But, in general, it is bad design.

 

Hi fmrPIC, as wayneh says a good measure is to always have some reasonable sized value input resistors into you Opamp. But you can also look at the schematic diagram of whats inside the opamp, that some manufacturers supply in the data sheet.

For example with some low leakage opamps, if the inputs go to the gates of two Jfets, then they are pretty much isolated from the rest of the IC and baring a tiny leakage current you should be OK (?) But maybe someone else can confirm this?

An example of where this is useful is for battery voltage monitoring, where the inputs are connected across the battery cell, but you really don't want to have the opamp powered up all the time.

 

In the days of super beta inputs to OpAmp, and in general jfet inputs, you
can get outright failure, punch thru for example, or worse partial damage, as I
recall noise performance would be significantly affected. Compounding this is
no shortage of datasheets that instruct to limit current, but no value given.
Leaves one to wonder what is the threshold of current that starts to degrade noise,
leakage, performance.

http://www.righto.com/2016/12/inside-lm108-op-amp-superbeta.html

Regards, Dana.