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Thread: Measuring Analogue Voltages w/o Sharing Grounds
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face BY : Richard Prosser email (remove spam text)



As an addendum to the differential A/D and instrumentation amplifiers, and
depending on the precision you require, a standard differential amplifier
using 1,2 or three opamps could be used. As the "common" ground connection
is taken as being a low impedance source, the single op-amp config may well
be adequate. In this configuration run the "common" side to the inverting
input and the "signal" side to the non-inverting input. Common mode error (&
l.f. noise rejection) will  be mainly determined by resistor matching so us
1% or better if you need useful accuracy. This will limit any current loops
to the values determined by your input resistors.
If you need better isolation a double op-amp config will give you high input
Z but somewhat limited common mode range. 3 op-amp will give you best
performance overall - but at the expense of realestate.
Suggested opamp - OP177 for low DC drift & offset (I think a quad pack
version is also available now)


Richard P

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