From: Winfield Hill
Subject: Re: Zeroing differential amplifiers (AoE)....
Date: 23 Dec 2002 04:58:52 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
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
> is not critical, but I wish to provide some compensation for DC
> offsets in the amp output automatically.
I'm not sure what differential amplifier configuration you're
considering. The purpose of the circuit on page 393 is to zero
out any dc input when the auto-zero logic signal is present.
If your input stage (replacing U1) is a differential amplifier,
the auto-zero opamp (U2) will zero any dc offset resulting from
the input stage, without any regard to the nature of that stage.
Usually in differential amplifiers it's important to maintain a
high CMRR at high frequencies. This requires both symmetrical
circuitry and generally a high-frequency CMRR trim capability.
Certainly one would not want to disturb these circuits with a
extra input paths, but there is an easy way to add an auto-zero
input signal, which is at the differential amplifier's output
reference node. For example, consider figure 7.27, just return
the two grounded resistor paths to the output of your autozero
integrator amplifier, or to an attenuated, buffered version of
the integrator output.
One other option for you to consider, zero-offset error chopper
amplifiers, discussed on pages 415 to 420.