It compensates the drift to zero. At a price though. See the datasheet.

Which drift is this? Is it the output voltage, when the diff voltage at 
the inputs is 0?
Which costs?
Thx

The offset drift is zero. The cost is the chopping noise and that the 
copping frequency should be avoided in the signal spectrum. The chopping 
frequency is in the order of kHz.

Sorry, but what is chopping noise?

Chopping noise is produced by Chopper-Op-Amps like ICL7650 due to their 
working principle.
"Normal" OP-Amps are in the mean time as good as Choppers, see 
AD8571/AD8572/AD8574 for example.

Arno

But the AD8571/AD8572/AD8574 are no "normal" Op-Amps they are chopper 
stabilized Amplifiers

Best regards

Anja

Pardon, i read only the first sentences of the datasheet, where they 
pointed out a huge difference between standard choppers and this AD874x 
amps.

Arno

Hi, Thomas,

there may be several technical solutions to avoid drift.

(Earlier chopper amplifyer modulated the DC-diffence voltage at the 
inputs into AC, amplified the AC driftless and modulated this back to 
DC.

I'm using the ICL7650.
This amplifier of ingenious design is "chopper stabilized OpAmp". It 
uses a chopper to sense the difference voltage at the inputs of the 
OpAmp.
The OpAmp itself does not use chopping.

When the OpAmp is in a closed loop circuit, the difference should be 
zero. But there is the offset voltage.

The chopper amplifier detects this offset and uses the biasing circuits 
to minimize the difference.

One of the penalties occurs upon saturation of the OpAmp, caused by 
overloading input signals.
In this case the chopper amplifier detects a severe difference voltage, 
tries to minimize it - and produces a severe inbalance in the bias 
circuit. After this event the chopper stabilized OpAmp uses some time to 
find its stebility again.

You'll find further information in the data sheet.

Ciao
Wolfgang Horn