Re: differential amplifier


ksoner wrote:
Hi;

Have you tried to design a difference amplifier with ordinary low cost
op-amps and resistors. do you have a succesful circuit to share with
high CMRR? I want to read a high DC voltage but DC voltage notr is not
connected to my power supply notr, they are isolated. thats what I need
to do. any other idea to read that voltage rather than a differnce
amplifier? the output should be linear of course from 0 to 400V. thanks
in advance...

Not easy. I assume you want to measure the voltage drop across
something like a one ohm resistor. The best way is to have like a V/F
converter running floating, then send the pulses down to ground level
with an optocoupler or capacitor, then back to voltage with a F/V
converter. You can get better than 1% accuracy and basically infinite
CMMR up to a few kilovolts.

There are also special chips for high-side current sensing and also
isolated op-amps, but they're a bit on the spendy side.

.

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