Offset voltage temperature drift in rail-to-rail opamps
Hi,
I am currently looking at the LT1677/1678/1679 family of opamps, which
have an increadibly low tempco for the input offset voltage (0.4µV/°C typ).
Ie they are not that far of from chopper/auto-zero opamps (only a factor
of 20-50), but with a quite a bit lower noise. And more imporantly,
they do not have the noise peak in the 0.1-10MHz region these zero-drift
opamps are plagued with.
But, when the input voltage comes close within the upper or lower rail,
the input offset voltage specs get much worse. Which is ok, as I am
not so much interested in the absolute offset as its drift.
But is my assumption correct, that with worse offset spec, close to the
rails, also the offset voltage drift specs get worse?
Another question I have is, whether I am just fooling myself with
the noise spec. If I am mainly interested in the noise performance
in the range between 0.1Hz and 100Hz (maybe even just up to 10Hz),
are the LT167x a good choice? Or should I rather stick to the LTC2057?
(Yes, there is still the option of using an LT1677 and an LT2057 to
compensate its offset. But I don't want to go that way due to the
increase of complexity...at least not yet)
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
Attila wrote:
I am currently looking at the LT1677/1678/1679 family of opamps, which
have an increadibly low tempco for the input offset voltage (0.4µV/°C typ).
* * *
But, when the input voltage comes close within the upper or lower rail,
the input offset voltage specs get much worse. * * *
is my assumption correct, that with worse offset spec, close to the
rails, also the offset voltage drift specs get worse?
That is almost certainly true, but it is not specified by the
manufacturer. You would have to test for yourself to know for sure.
Note that, in addition to the offset voltage changes, the input current
and input offset currents also change with the common-mode voltage.
Both are due to the rail-to-rail input architecture, and are common to
RRI opamps.
Compare this to the AD8675, which is RRO but not RRI. Its input offset
tempco (typical) is even better than the LT1677 (0.2uV/C), as well as
its input noise (2.8nV/sqrtHz with a very low 5Hz 1/f corner frequency),
but without the input common mode effects of the 1677.
If RRI is important in your application, you may be able to do without
an actual RRI opamp by running the opamp on higher supply voltages than
the rest of the circuit.
Best regards,
Charles
Hi,
Some years ago I designed in an RRI opamp that needed inputs down to ground and only a single rail was in use on that part of the product, the opamp certainly went very "soft" open loop gain leakage etc all getting much worse as a consequence of having the input tranistors/fets having to cope with rail to rail operation. I sort of regretted designing it in, but it was still a little bit better than the 358 in there before.
From: volt-nuts volt-nuts-bounces+m1k3k1=hotmail.com@febo.com on behalf of Charles Steinmetz csteinmetz@yandex.com
Sent: 02 August 2017 03:29
To: volt-nuts@febo.com
Subject: Re: [volt-nuts] Offset voltage temperature drift in rail-to-rail opamps
Attila wrote:
I am currently looking at the LT1677/1678/1679 family of opamps, which
have an increadibly low tempco for the input offset voltage (0.4µV/°C typ).
* * *
But, when the input voltage comes close within the upper or lower rail,
the input offset voltage specs get much worse. * * *
is my assumption correct, that with worse offset spec, close to the
rails, also the offset voltage drift specs get worse?
That is almost certainly true, but it is not specified by the
manufacturer. You would have to test for yourself to know for sure.
Note that, in addition to the offset voltage changes, the input current
and input offset currents also change with the common-mode voltage.
Both are due to the rail-to-rail input architecture, and are common to
RRI opamps.
Compare this to the AD8675, which is RRO but not RRI. Its input offset
tempco (typical) is even better than the LT1677 (0.2uV/C), as well as
its input noise (2.8nV/sqrtHz with a very low 5Hz 1/f corner frequency),
but without the input common mode effects of the 1677.
If RRI is important in your application, you may be able to do without
an actual RRI opamp by running the opamp on higher supply voltages than
the rest of the circuit.
Best regards,
Charles
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