On Jun 26, 1:53=A0am, Rene Tschaggelar <n...@[EMAIL PROTECTED]
> wrote:
> Paul wrote:
>
> > Thanks! =A0As you said the output offset can always be corrected, but
> > it's great to know that a 2mV op-amp chip such as the INA116PA can
> > apply DC voltages as low as a few nanovolts on the input device
> > without adding shunt resistors. Of course one can always add a shunt
> > resistor to lower the input voltage across the DUT, something I knew
> > about, but of course that has obvious effects of decreasing the DUT's
> > effective input voltage to the op-amp.
>
> > I'm wondering if there are any op-amps or perhaps a BiFET amp circuit
> > that could achieve a few nanovolts across say a 200K ohm device while
> > consuming no more than a few microwatts. The idea is that such a
> > microwatt amp would have considerably less input thermoelectric
> > effects. Thermoelectric effects can generate a half dozen or more
> > microvolts on the DUT unless carefully balanced with dummy resistors.
> > I believe Linear Tech has some microwatt op-amps, but nothing near
> > 25fA bias current.
>
> Paul,
> a thermoelectric effect means you get a voltage
> from a temperature difference in case different
> metal combinations are involved. They act as
> input offset voltage, independent on the bias
> current.
> These thermoelectric effects are in the microvolt
> per Kelvin region. and thus are only to be
> considered in high DC-gain applications.
>
> While FET Input opamps have far lower bias currents,
> they don't achieve the low input offset voltage
> common to bipolar input OpAmps.
>
> There are Fet input opAmps that get rid of the
> input offset voltage by =A0trading bandwidth against
> the chopper feature.
>
> Rene
Hi,
I'll try to clarify:
I am referring to the input voltage on the *DUT* caused by the op-amp,
and therefore if the bias current through the DUT is decreased then
the offset voltage on the DUT will be less-- ohms law.
The op-amps I am working with have offsets around 0.5uV to a few uV.
Therefore thermoelectric effects should be considered. As far as I
know instrumentation op-amp appear to have to least thermoelectric
effects since both input pins go to the same polarity on both op-amps,
the + pin, but there are still thermoelectric effects since both op-
amps are not 100% identical. Other circuits such as the inverter
require dummy resistors and such to help reduce the thermoelectric
voltages on the DUT.
My interest in BiFET's is to design a low power amp circuit with low
bias current.
Thanks,
Paul
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