From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Zeroing differential amplifiers (AoE)....
Date: Tue, 24 Dec 2002 03:50:13 GMT
NNTP-Posting-Date: Mon, 23 Dec 2002 19:50:13 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Jay Andersson wrote:
> I am trying to zero a differential amplifier using a technique similar to
> that in AoE Chapter 7, where an integrator controlled by a FET switch
> provides an offset voltage back to the amp inputs.
> I am having trouble with how this effects the CMRR of the amp - is there a
> standard technique for this that is simple and does not require very high
> precision resitors etc. - my application os not critical, but I wish to
> provide some compensation for DC offsets in the amp output automatically.
> I would like some advice as I'm a bit stuck...
> Best regards,
> Jay Andersson
Chopper stabilized op-amps are inverting and have the non-inverting
side referred to ground.
They are not useable for differential applications, and by definition
CMRR is not relevant.
If you want to use a chopper stabalized op amp for differential work,
then one ugly way is to float the supplies.
A nicer way is to use two, each as an inverter (for + and - inputs),
the outputs driving a third amplifier (this scheme is called an
The offset voltage drift is fixed by the chopper scheme, and the CMRR
of the third amp is improved by the gain of the drivers - assuming
There are so many off the shelf op-amps now that have untrimmed
microvolt input offset voltages, that the low bandwidth and higher noise
of a chopper system is almost unacceptable.