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input bias current return path vs. resistor noise

Question asked by chriz on Jun 26, 2018
Latest reply on Jun 27, 2018 by chriz

Hallo,

 

after going through some application notes for instrumentation measurement I need some expert advice for an analog frontend.

 

I have to measure very precise a DC voltage across a floating 10k resistor in two cases

 

a) U < 200nV

b) U = 10V

 

The setup is in a stable lab environment and I am not constrained otherwise.

 

As I understand it, the bias resistors should provide a symmetric path to ground form each opamp input.

Also, it should be much larger than my 10k source, maybe 5Meg?

But low enough no to introduce too much noise.

 

Which is the lowest resistor value I can use to keep thermal noise low and in combination with which opamp?

 

Alternative:

Could I get away with unsymmetrical bias resistors?

Can I connect only one opamp input via a low value resistor to gnd and accept the additional offset voltage created at my DUT?

For example use low input bias current devices like the AD8220B in case a) and the ADA4530-1 in case b) ?

 

Thank you for support.

 

Kind regards,

Chris

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