internal bias current compensation-op amp

[lasteem1]

I see many experienced analog designers that impedance match the non-inverting and the inverting pins when an op-amp is internally compensated. Do most of you guys do this too? If not, what are Rules of Thumb for handling op-amps that compensate bias currents internally?

 

[crutschow]

The rule of thumb is, how much offset does the input bias current generate in the input impedance and can you tolerate that in your circuit. If that value is too high then you need to add the equivalent resistance on the other input so that the offset equals the offset current (difference in bias current between the two inputs) through the input impedance, not the bias current.

 

[lasteem1]

The rule of thumb is, how much offset does the input bias current generate in the input impedance and can you tolerate that in your circuit. If that value is too high then you need to add the equivalent resistance on the other input so that the offset equals the offset current (difference in bias current between the two inputs) through the input impedance, not the bias current.

Usually when op-amps are internally compensated for bias currents their residual bias currents aren't equal and, in fact, they can be opposite in polarity because of slight mis-matches in the transistors or current sources. When this is the case I don't think you EVER want to have the equivalent impedance on the other input. However, if you have to add some divider network to add a DC bias(say to find a difference between an input and a known DC value) then what is the ROT for those resistors(and associated decoupling cap)? Do you make the impedance as small as possible?