One approach is to use a fast op amp with a chop-amp snooping its summing junction and tweaking out the offset. That's easier for an inverting application, but it can be done noninverting as well.
If you can live with 900 kHz GBW, look at the OPA378. I use a lot of those in geophysical instruments. Their noise is pretty nearly constant at 35 nV/sqrt(Hz) all the way down to DC, and it doesn't have a nasty noise peak at the chopping frequency.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
email: hobbs (atsign) electrooptical (period) net
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It's most helpful to put some numbers on things. Let's say you had a choice between 1nV of offset at 1kHz, 1uV at 1MHz, or 1mV at 1GHz. (Not that I can deliver any of those specs) What would be best?
Hi George. There has been some head scratching on this, but an answer is finally here. The ideal offset would be about 1uV, but upto 10uV could be accepted with a little reduction in performance.
1nV accuracy isnt needed, and would disappear among the other sources of error.
1mV would be non-functional.
Hi Tabby, (Did you get a frequency spec too?) Buy some opa277's (or equivalent OP-07's) If they don't work you should look into chopper opamps, as John and Phil suggested. (BTW they know much more about analog design than I do.)
Hey can't you just take out the DC offset in software? Short the input every once in a while and measure the offest.
Hi George, thanks. 07s at 75uV are out of spec, 277s don't quite manage it at 20uV, so it looks like chopper opamps. This will be the first iteration in development, there's no software involved at this stage, that can hopefully be done later in the process. There's no frequency spec yet, still just a case of what can this do.
Next question is where to get the opamps from, my usual suppliers (in England) dont do anything that accurate.
The 'good' grade of 277's has a typ. offset of 10 uV. I've never bought any nor measured the offset's of the cheaper grade that I have bought. Lot's of times there's some bigger offset in the signal chain, that swamps 10uV's. (bias currents though resistors, thermo- electric or just thermal stuff, a uV sounds hard if you don't know where to buy opamps?)
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