Hi,
we need to amplify a 10mV to 100mV sense voltage developed over a shunt resistance on a 12V rail into a 3,3V range of an ADC. Precision is not of paramount importance and this being a low-volume project individual tuning is acceptable.
An obvious first though was using a OpAmp specified for up to12V supply voltage and configuring it as a differential amplifier with gain around 27. I am aware that the tolerances of the divider networks are of importance. Also, I assume that an instrumentation amplifier would probably be the better choice, but it is deemed to costly.
Now: most OpAmps are specified for their inputs to only slightly exceed their supply which means that I would need to power the device from the 12V rail, which however is not very well regulated. Also this would usually mean that the output could exceed the 3.3V limit of the ADC in case of very high current (and hence sense voltage). Then I found the LM321 which seemingly allows a full 32V on its inputs unrelated to the supply voltage. Is that really the case?
Put simply:
1. Does the LM321 function properly as a differential amplifier when powered by 3.3V and 12V on the inputs?
2. Are there other (cheaper) devices that do provide such functionality?
3. When powering from 12V rails is inevitable, how to safely limit the output to 3.3V possibly with another OpAmp?
Thanks
Arne