Hi,

As you know, the
Although it has been my experience that for most op-amps the input
offset voltage is due to the "-" input pin for the

I have a INA116PA Instrumentation op-amp where Ib typ = 3fA, Ib max 25fA, and Vos typ = 0.5mV. Now it seems to me in order for there to be 0.5mV on the input of this Instrumentation op-amp circuit with 3fA bias current that the DUT input impedance would have to be 0.50mV / 3.0fA = 170 Gohms. On the other hand, if the DUT input impedance is say 200 Kohms then would the input offset voltage be 3.0fA * 200Kohms = 0.6nV, disregarding thermoelectric effects?

INA116PA datasheet: http://focus.ti.com/lit/ds/symlink/ina116.pdf

Regards, Paul

As you know, the

***input***offset voltage is the voltage required across the op-amp's input terminals to drive the output voltage to zero.***most***part. For example, according to Spice the input offset voltage on the "+" input pin on a LMC660A op-amp for a non-inverting amp circuit is a few nanovolts, disregarding thermoelectric effects mind you, but a few millivolts on the "-" input pin. Although as you know the input signal is not applied to the "-" input pin for a non-inverting amp circuit, which means there's just a few nanovolts on the input of such a circuit if we disregard thermoelectric effects.I have a INA116PA Instrumentation op-amp where Ib typ = 3fA, Ib max 25fA, and Vos typ = 0.5mV. Now it seems to me in order for there to be 0.5mV on the input of this Instrumentation op-amp circuit with 3fA bias current that the DUT input impedance would have to be 0.50mV / 3.0fA = 170 Gohms. On the other hand, if the DUT input impedance is say 200 Kohms then would the input offset voltage be 3.0fA * 200Kohms = 0.6nV, disregarding thermoelectric effects?

INA116PA datasheet: http://focus.ti.com/lit/ds/symlink/ina116.pdf

Regards, Paul