[volt-nuts] Offset voltage temperature drift in rail-to-rail opamps

m k m1k3k1 at hotmail.com
Wed Aug 2 17:52:41 EDT 2017


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 at febo.com> on behalf of Charles Steinmetz <csteinmetz at yandex.com>
Sent: 02 August 2017 03:29
To: volt-nuts at 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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