I saw it in a data sheet of an Analog Devices MOSFET-input precision op amp. Has anyone tried to use such an op amp, so you can tell me if
1 µV voltage offset is true?
Yes, in a chopper or zero drift opamp. They use various switching architectures that can theoretically cancel all offset and drift. The switching comes with some downsides however - usually residual voltage and current noise. However they keep improving.
See Opa189 for a recently released example with impressive specs.
I understand that a chopper keeps flipping the input DC signal so that an AC signal is obtained and amplified.
But to do it with microvolt signals requires using noise-free switches. The only relatively noise-free method of
switching that comes to my mind is using photoresistors and proper light sources. But I know that photoresistors
are slow - 10s of milliseconds.
So how is it done?
I understand that a chopper keeps flipping the input DC signal so that an AC signal is obtained and amplified.
But to do it with microvolt signals requires using noise-free switches.
Don't mix different things:
* offset is DC
* noise is AC
Low offset doesn't mean low noise.The one has nothing to do with the other.
And generally chopper stabilized Opamps are more noisy than standard Opamps....especially the older ones.
In former this was a problem, but modern ones greatly improved in this.