On Jun 21, 8:41 am, John Larkin
<jjlar...@[EMAIL PROTECTED]
> wrote:
> On Sat, 21 Jun 2008 08:25:59 -0700 (PDT), Paul <energymo...@[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.
The LMC660A has a typical voltage offset of 1mV and bias current of
2fA, but that depends what type of op-amp circuit. According to Spice
the input voltage offset for an inverting or differential circuit is
about what the Vos spec says, but for a non-inverting circuit it's a
few nanovolts on the "+" input pin. I'm wondering if the Vos in
datasheets is referring to a certain type of op-amp circuit such as
the inverting type (http://hyperphysics.phy-astr.gsu.edu/Hbase/
Electronic/opampvar.html#c2).
Regards,
Paul
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