Fave dual chopamp?

Hi, all,

I'm a fan of the OPA2188 dual chopamp--32V, low flatband noise, no worries--but this time I'm using a 5V supply and need rail-to-rail output.

Chopamps are, well, individuals--they push current noise spikes out their inputs, and datasheets are notoriously coy about this.

Any faves?

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs
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We've used a lot of (37K!) AD8628 and they have worked well.

5 volts, RRIO, fairly cheap. I have seen a few uV of offset in some cases, specifically in a Sallen-Key filter, probably some charge injection thing.

I had to add a tiny trimpot to this board

formatting link

to tweak out the last few microvolts. Nuisance.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

The only chopamps I've used have been synthesized in software or firmware

-- turn off excitation, measure, turn on excitation, measure, subtract -- poof! an answer.

It takes the ADC offset out of the equation, which is nice if you're trying to find good, readily available ADCs, because it seems that all the monolithic ADCs have significant offset.

It also synchronizes the chopping noise with the measurement, meaning that you can make it go away.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

This is a thermopile front end for a fire detection application, so the only way to turn off the excitation is an extinguisher. ;)

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

Thanks, I'll try that one then.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

That's different. Although, if I were going into an ADC I'd consider chopping with an analog switch.

(And yes, I have a thing for end-to-end chopping when there's ADC's in the loop).

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Tim Wescott 
Control systems, embedded software and circuit design 
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Tim Wescott

I've used the ADA4528-2 but it is 5V total supply voltage iirc, not +/-5

The offset voltage was impressively low in my application.

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Chris Jones

Why would that be better than the sampler in the ADC? Just offset voltage?

Not a bad plan. My preference is to make the signal phase go from 0 to pi instead of on-off, because it saves SNR.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

Chopper wheel (mechanical) if front of the thermopile? Or some other sort of light blocking thing.

George H.

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George Herold

My preference, also! ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

Monolithic fast ADCs just seem to have crappy offset voltages. I suspect it's the comparator in the SAR rather than the sampler itself.

I suppose my tune might be different if I were designing a system that only needed to update slowly, and there was a decent-enough ADC for the job.

Me too, if that doesn't create more problems than it solves.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

Yeah, you can do that, but this has to go on a ginormo piece of heavy equipment in a very dirty environment with lots of shock and vibration.

Cheers

Phil Hobbs

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Phil Hobbs

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