Paul wrote:
> 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.
>
>
>
>
> Here's my main concern. If I build the INA116PA for DC application,
> which is an internal Instrumentation op-amp chip (3 op-amps), and the
> impedance of my DUT is 200 Kohms, then what bias currents could a good
> EE such as yourself expect? I mean, for a 200K ohm DUT input source we
> cannot have both 0.5mV offset and 3fA bias on the DUT. I think V=I*R
> applies, so if the bias current is 3fA then V = 3fA * 200Kohms = 0.6
> nV.
>
> Thanks,
> Paul
Incorrect, one *can* have eve 3mV offset and 3fA bias.
Learn what an ideal op-amp is, then learn about each of the various
real-life error components.
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