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I'm adding an input bias current compensation network to the positive op-amp input of my band-pass multiple-feedback filter.

mfb filter

I'm setting the source impedance for the positive input pin equal to that of the negative pin.

Should I be matching the impedance at DC or at the frequency of interest?

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Bias currents are DC and will produce a DC offset voltage if input resistances are non-zero so, matching the DC resistance in both inputs is the normal thing to do (should the offsets be problematic).

Be careful though; the bias current on its own applies to just a single input so, look for what is known as the offset current - this is the current that you cannot hope to equalize with matched input resistances. Reason: the difference in bias currents for the two inputs (the bit you can do nothing about) is called "offset current".

Having said that, if your op-amp is a FET input type then you will probably be wasting your time. It really only applies to op-amps with BJTs at their inputs.

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I believe that input bias current and offset current are specified at DC. So match the two inputs at DC.

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