Why do instrumentation op amps have low offset voltage?

[treez]

The following states that insturmentation op amps have low DC offset voltage..........

https://en.wikipedia.org/wiki/Instrumentation_amplifier

.....but why is this?......surely if the three op-amps individually have a high offset voltage , then the instrumentation op amp will also have a high offset voltage?

...why should the IN Amp configuration give low offset?

 

[mtwieg]

For the three-amp model, offset voltage in the two input amplifiers will cancel out due to the high CMRR of the input stage. Only the difference in offset between the two input amps (call it the offset offset voltage) will have any effect on output offset. The third amp is typically designed to have a very low offset voltage, and if it doesn't then to overall amp will have a high offset voltage.