Can I simply hook up an opamp inputs, (either using a differential or in-amp topology that is/are also powered from a floating power supply), across a device under test that itself has exceeding voltages for the opamp (<50Vdc) ?
so idea is to build smth that uC's ADC can measure and eventually for PC to read (via serial).
The DUT can go as high as 50V and is only DC. Differentially I am not expecting anything higher than 2-5V across the inputs, worst case.
Thus I was wondering, How does a multimeter achieve differential measurement ? Without worrying about the voltage that the measurand is actually at, as long as the voltage between probes remain in the spec, I'm fine... (I do not want to build a multimeter (nor differential probes), but the functionality is the same...).
-> My understanding is that I should be able to hook up an independently powered opamp (dif-amp, in-amp ?) across another device's resistor, read out the voltage drop (by subtracting one input from another), without being connected to the ground with the Device Under Test. Right ?
I have tried to simulate different topologies but to no avail. At least Micro-Cap won't simulate floating power sources that well.. it seems.. I get errors and strange values.
a) If simulation won't allow me to detect floating power source, do I really need to connect the DUT and my measurement circuit to common GND at one point ? - Meaning the common mode voltage range should become an issue as both systems are tied to single reference... for me it doesn't add up.
That's why I want to have a floating measurement circuit, so I shouldn't worry about grounding (such as a voltmeter), but that's where simulation fails for me and I am unsure.
Another idea was to take two opamps, make them as buffers (maybe add an amplification stage as well) and just read each of them with separate ADC inputs and do the subtraction in Software - anyway my problem persists - Can an opamp measure these differences when being powered from floating power ?
I'm also referencing few topics, which should've lead me in a somewhat correct direction, but seems I'm missing something. I also started to think about possible grounding issues and reference points: