How to Null the output of a diff amp please? - diyAudio
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Old 8th June 2015, 07:46 PM   #1
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Default How to Null the output of a diff amp please?

Hello,

Hopefully an easy question for someone

I wish to utilize a standard configuration instrumentation differential amplifier thus:
Click the image to open in full size.

I would like to know how such an amplifier can be null'd?

i.e. if inputs are shorted together (by pressing SW2 'Test') then how can the circuit be modified to be able to ensure the output sits at 0V?

I was thinking maybe instead of connecting R24 to GND, connect it instead to an adjustable reference which nominally sits at 0V.

I have also seen this which intrigues me:
Click the image to open in full size.

I like the way the second stage amplification is implemented, maybe replacing the two 1M resistors with a trimmer and reference connected to GND.

Anyway ideas, corrections, suggestions greatly appreciated!

Cheers,

Andy
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Old 9th June 2015, 02:07 AM   #2
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.
To give you the idea, Google for:

Application Note 31 Op Amp Circuit Collection

Look on pages 6 and 7.

An absolute null is probably impossible because of component and temperature variations. But single digits is achievable.

Or, of course, use an output capacitor.
.

Last edited by bentsnake; 9th June 2015 at 02:13 AM.
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Old 9th June 2015, 02:23 AM   #3
DUG is offline DUG  Canada
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Introduce a low impedance/low resistance controlled voltage where R24 connects to ground.

See jpg...courtesy of TI from one of their data sheets.
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Old 9th June 2015, 07:55 AM   #4
AndrewT is offline AndrewT  Scotland
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there are a couple of THAT ap notes that mention this.
I think Whitlock also does a servo for the diff amp.

Jung maybe?
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Old 9th June 2015, 08:00 AM   #5
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Cheers chaps,

@AndrewT, have you any useful links to what you mention please?

Also note that I am measuring things in the DC domain... The input to the diff amp is a stable measurement that if anything will only change with altering temperature of the devices being measured (compared).

Thanks again,

Andy
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Old 9th June 2015, 09:05 AM   #6
AndrewT is offline AndrewT  Scotland
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Chopper type.
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Old 9th June 2015, 07:48 PM   #7
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A chopper... mmmm, maybe?

However I only want to achieve a differential V gain of 1000 preferably, thus converting mV difference (up to 10mV abs max) at the two inputs to a corresponding V (with reference to my elusive GND) at the output. Using a chopper maybe a sledgehammer to crack a nut perhaps? I am no expert when it comes to opamps.

Thanks again,

Andy
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Old 10th June 2015, 02:12 AM   #8
DUG is offline DUG  Canada
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Some low offset opamps:

Linear - Amplifiers - Instrumentation, OP Amps, Buffer Amps | Integrated Circuits (ICs) | DigiKey

Some almost two orders of magnitude better than LT1058.
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