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Comment by kragen | Hacker News Reader

How do you compensate for Vos with feedback?

You probably don't, but your opamp might. https://www.analog.com/en/resources/technical-articles/to-ch...

Technically you probably could do it externally in most cases but it would require a bunch of extra stuff, and be a pain, so usually it's best to use the stuff built into the amplifier itself.

Oh, sure. You could totally build a chopper op-amp out of two discrete op-amps. I'd never thought about actually doing that...

I've seen designs that use a Chopper Op-Amp to actually auto-zero a power-OpAmp, effectively transferring the low Vos characteristics to a different OpAmp with completely different characteristics.

If the op-amp has a stable Vos I feel like maybe you could zero it by hand with a trimpot? You just need a button to short the inputs together while you're trimming it.

This is necessary because Vos is often a voltage dependent inaccuracy. So as your circuit functions it's Vos is changing up and down.

Yes, if the opamp doesn't have a stable Vos, you probably can't do a very good job of zeroing it with just a trimpot.

Of course National Semiconductor had a chip for this, the LMC669 Auto-Zero.

https://www.electronicsurplus.com/national-semiconductor-cor...

That's only rated good to 5 μV, according to the datasheet. You'd probably be better off with a non-auto-zero LM324B, whose worst-case Vos is rated at 3 μV IIRC. Of course it didn't exist at the time (01989) which is why National made the LMC669.

But the potential advantage that building a chopper (or auto-zero) out of ordinary opamps would be that you don't need to source, order, and await specialized chips. A long-discontinued bolt-on auto-zero for a regular opamp has almost all the disadvantages of just buying an auto-zero opamp.

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