On Sat, 28 Jun 2008 21:52:29 -0700 (PDT), Paul <energymover@[EMAIL PROTECTED]
>
wrote:
>On Jun 26, 1:53 am, Rene Tschaggelar <n...@[EMAIL PROTECTED]
> wrote:
>> Paul wrote:
>>
>> > Thanks! As 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 trading 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
To prevent thermoelecric voltages, keep all pins at the same
temparature. But also all surrounding resistors etc. A cooling airflow
gives temperature differences. And resistors and opamps that get warm
may give some effects.
Pieter
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