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op-amp nV input offset voltage

Hi,

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. Although it has been my experience that for most op-amps the input offset voltage is due to the "-" input pin for the *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:

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Regards, Paul

Reply to
Paul
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The last time I checked, the offset voltage would be the difference between the (-) and (+) input with the Op-amp in (-) loop back mode.

So, if you were to put an op-amp in (-) loop back and lets say 5 volts on the (+) input, the (-) input should be offset no more than what the spec's state.

Or, I guess if you were using a +/- to common supply, you can simply tie the (+) to common with op-amp in (-) loop back. You should be seeing that offset factor at the (-)/output..

Maybe things have changed but that is what I go by..

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Reply to
Jamie

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