On Sat, 21 Jun 2008 08:25:59 -0700 (PDT), Paul <energymover@[EMAIL PROTECTED]
>
wrote:
>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.
The offset voltage is *differential*. You can blame it on either pin,
or both pins... it doesn't matter who you blame, the result is the
same: offset voltage becomes measurement error.
>
>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?
The offset voltage error is a different thing from the input bias
current. They are unrelated [1]. You can of course generate a real,
external-to-the-opamp error voltage by dumping the bias current into
real external resistance, but that's a different matter entirely.
John
[1] Some opamps have low offsets and high bias currents, and some vice
versa. Chopper amps are low on both; cheap bipolars are high on both.
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