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

Regards,
Paul