Deciding Whether an Op Amp Needs a Bias Compensation Resistor

Adding a resistor to the noninverting input of an inverting amplifier is a familiar way to reduce bias-current error. It is useful only when the two input currents have the appropriate relationship. Copying the resistor into every amplifier circuit can add noise and offset without providing the expected cancellation.

Calculate the two input drops

For a conventional inverting stage, the resistance seen by the inverting input with the independent source set to zero includes the input and feedback resistances in parallel. Equal resistance at the other input can cancel much of the error when both bias currents are well matched and flow in the same direction. Analog Devices explains the limitation for internally compensated and FET inputs in Don’t Use the Resistor I Told You to Use.

As a hypothetical comparison, two 100-nA currents differing by 2 nA can benefit from balanced 10-kilohm paths. Two uncertain currents individually near zero but capable of opposite polarity do not provide the same cancellation. Calculate output error using signed currents and the actual topology instead of multiplying one typical bias-current value by a resistance.

Inspect the complete resistance network

The sensor’s output resistance, protection resistor, and reference-source impedance are part of the calculation. In a multiplexed design, the resistance may change with channel selection. In a programmable gain stage, one fixed compensation resistor may be appropriate for one setting and poor for another. Bias current and offset current can also vary with common-mode voltage and temperature.

Verify before fitting the resistor

Provide an optional footprint when the uncertainty justifies a prototype comparison. Measure offset with a defined source impedance at several temperatures and input bias levels. Comparing populated and shorted options is useful only if the input voltage and thermal settling are controlled. A room-temperature improvement should not conceal a worse hot limit.

Use resistor noise scaling to assess the cost of extra resistance and chopper input impedance balance for switching-input effects that require a different analysis. Preserve the calculation with the selected amplifier part number so a later substitution triggers a review of the input architecture.