Instrumentation opamp?

Some numbers would help. The OPA227 and 277 have low offset.

George H.

Tabby schrieb:

Hello,

you should know before how low the offset should be and how fast is necessary. Some numbers, voltage in µV and max. frequency or output swing.

Bye

Sounds like a cmos chopamp. Unless you need really low noise, too.

AD8628 sort of thing. In real life, expect bigger offsets than the datasheet says, less than 10 uV but up there.

John

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The above really are the specs, its basically a case of the faster and more accurately it can perform the better. Cost is almost no object.

NT

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

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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?

George H.

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Wow! 20uV at ~1GHz.

George H.

swing.

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.

Thanks

NT

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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.

George H.

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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.

thanks,

NT

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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?)

George H.