On Sat, 21 Jun 2008 10:01:51 -0700 (PDT), Paul <energymover@[EMAIL PROTECTED]
>
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.
>
>
>The LMC660A has a typical voltage offset of 1mV and bias current of
>2fA, but that depends what type of op-amp circuit.
No. Those are characteristics of the chip.
The actual error voltage that a *circuit* generates is made up of
several contributors. One is the input offset voltage of the chip
itself. An additional error is any voltage drops created in external
resistors by the opamp bias currents. The errors are generally assumed
to add, because we can maybe know the polarity of the bias current
from the datasheet (but often we don't even know that) and we never
know the polarity of the offset, unless we measure it on one real
opamp.
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 don't think that's right. The Spice model shouldn't be that stupid.
Sounds like an interpretation issue.
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).
No, it's a property of the chip, not the circuit it's used in.
John
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