I have a sensitive circuit that requires a DC offset null to be maintained where uVs of offset can be bad. I'm looking for a DC feedback solution to keep the DC offset preferably below 1uV, nV accuracy would be nice.
I know of no analog solution for this. Can anyone recommend me what is probably going to be a digital solution?
Thanks in advance.

 

Anyone?

 

Getting below microvolt offsets is very difficult due to the thermoelectric voltages of common wiring such as shown below.
Perhaps one of these zero-drift op amps will work for you.

 

What if I kept the DC servo component leads very short and the current tracks very thick? 1uv offset is sufficient.

 

I vaguely recall seeing an ap note, quite possibly from Linear Tech (which would mean probably written by Jim Williams) where a zero drift amplifier was used to drive offset trim of another very fast amp in closed-loop fashion.

To manage thermoelectric potentials, keep the numbers of generating junctions "symmetric" as far as possible and keep everything isothermal - use air shields, insulation, etc. Keep any heat generating devices well away. There are certainly ap notes from years ago, definitely from Linear Tech & I think also Analog Devices, that discuss these issues. I've seen circuitry where waving a bit of air at it with small piece of paper would result in large transient error.

Low offset tolerance can often mean extremely low tolerance for leakage current over the surface of circuit boards. This has been made considerably harder to deal with with SM components (hard to hang a solder joint in free air or use a teflon-insulated turret lug). Boards must be extemely well cleaned (so none of that impossible-to-clean "no-clean" flux) and may need to be conformally coated after a thorough baking if cleaned with water, which is invariably required for complete cleaning.

Don't use glass-packaged diodes in critical locations, unless the circuit is kept in the dark (which it usually isn't while you're doing bench work on it) - pretty much every semiconductor junction will play photodiode when you don't want it to.

Expect a bit ("a bit" he says - who does he think he's kidding?) of pain and suffering.

Note the cadmium-tin solder in the table crutschow posted. I have no idea if it can still be found, especially since the RoHS push, but it can sometimes be very helpful.

 

Keep in mind AC offset transients are fine for my application. I will have several amps of AC transients. The load of the circuit is like 0.001 ohms so I only need the DC level voltage to be consistent so i'm not dumping DC current into it.

 

Thanks for the advice ebp, I still need a recommendation on a mechanism to get me down to that 1uv resolution though. Digital is not my strongsuit.