Operational Auto Zero Amplifiers with in the loop adjustment

In the loop adjustment offers the advantage that the adjustment is done exactly at the same operating point as the measurement. Therefore common mode errors are avoided.
The circuit has a drawback: It can not be used for comparators.

The Basic Circuits

The most simple circuit looks like this:

Fig. 1: Auto 0 amplifier with in the loop adjustment

The amplifier consists of two identical stages OP1 and OP2.
Before starting the adjustment of the signal path via OP1 the auxiliary amplifier OP2 must be adjusted. Therefore switches S2 and S4 are closed while S1 and S3 are open. The output signal of OP2 is fed back to the trimming input adj thus adjusting OP2.
When S2 and S4 open the adjustment voltage is stored in C2.
To adjust OP1 switches S1 and S3 are closed. The differential input voltage of an ideal amplifier OP1 with unlimited gain would be 0V. Since there is an offset the voltage present at the input of OP2 is not 0V. OP2 now trimms OP1 via it's adjust pin adj until the differential voltage between the feedback tap and Vin approaches 0V. After opening S3 and S1 the adjustment of OP1 is stored in C1.
Now the cycle restarts again with adjusting OP1 at a slightly different common mode voltage (due to the offset of OP1 being trimmed away). After repeating the cycle several times both amplifiers OP1 and OP2 have been adjusted using identical operating points. Therefore systematic errors caused by the limited common mode rejection of OP1 and OP2 are canceled too (provided OP1 and OP2 use the same dircuit and layout).