P
Paul
Guest
On Jun 26, 1:53 am, Rene Tschaggelar <n...@none.net> wrote:
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
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