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AD797 offset null circuit
AD797 offset null circuit
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Old 21st November 2019, 10:04 AM   #1
gdan is offline gdan  Greece
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AD797 offset null circuit
Default AD797 offset null circuit

I have built a x100 Gain pre-amplifier using the AD797 opamp as shown below.
I have included a trimmer to null the output offfset as shown in fig. 55 of the datasheet.
The problem is that the output offset does not null but goes from +0.15V to +0.9V
Has someone else tried a similar offset null circuit with AD797 ?
Any suggestions ?
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Old 21st November 2019, 10:49 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Why not just fit a 220u capacitor in series with R34 as the DC is blocked with C45.
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Old 21st November 2019, 11:34 AM   #3
Mooly is offline Mooly  United Kingdom
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AD797 offset null circuit
I think the problem is that you are trying to trim offset errors caused by inappropriate circuit design/component values... remember the offset null adjustment is to null internal errors within the chip, not outside influenced design issues.

Your 30k input bias resistor is generating a relatively large DC voltage at the + input, the - input with its 10 ohm to ground generates essential no voltage at this node.

You either need to choose a more suitable opamp (very low input bias current) or redesign the circuit... or add coupling caps as Jon suggests.
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Old 21st November 2019, 01:03 PM   #4
Mark Tillotson is offline Mark Tillotson
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The 20k value for the nulling trimmer is for +/-15V supplies, you use +/-8V supply so 10k trimmer is required. Well that's my take, it presumably is a current injection nulling system.


However the large imbalance in dc resistances to the two inputs is probably the dominant factor.


BTW a gain of 100 in one opamp stage might lower your bandwidth and increase distortion too much, much better to limit gain in each stage to 10 or thereabouts. Two x10 stages cascaded outperforms one x100 stage is most respects except power consumption.


To improve noise performance a x20 followed by x5 stage is perhaps optimal here, since extra front-end gain usually prevents later stages from contributing significant noise.

Last edited by Mark Tillotson; 21st November 2019 at 01:11 PM.
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Old 21st November 2019, 01:13 PM   #5
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by JonSnell Electronic View Post
Why not just fit a 220u capacitor in series with R34 as the DC is blocked with C45.

Or add an output DC blocking cap in fact? The 10 ohm impedance of R34 is going to require a really big DC blocker value to avoid restricting LF gain.
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Old 21st November 2019, 01:45 PM   #6
Douglas Blake is offline Douglas Blake  Canada
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Quote:
Originally Posted by gdan View Post
I have built a x100 Gain pre-amplifier using the AD797 opamp as shown below.
I have included a trimmer to null the output offfset as shown in fig. 55 of the datasheet.
The problem is that the output offset does not null but goes from +0.15V to +0.9V
Has someone else tried a similar offset null circuit with AD797 ?
Any suggestions ?
Referencing your inverting input to ground with such a low value (10 ohms) is likely to unbalance the inputs in the op-amp somewhat. If your power supplies are not exactly equal you will end up with an output offset you can't null out.

Try upping the values of the parts in your feedback loop by, I would guess, a factor of at least 5 or maybe 10. You could also try adding a cap between R34 and ground so that the feedback chain can bias your input for you automatically.
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Old 21st November 2019, 02:20 PM   #7
gdan is offline gdan  Greece
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AD797 offset null circuit
Quote:
Originally Posted by JonSnell Electronic View Post
Why not just fit a 220u capacitor in series with R34 as the DC is blocked with C45.
I want to use this circuit for measurement of very low level noise at the low frequencies, so a capacitor of 16000 uF will be needed in series with 10 Ohm for a -3dB frequency at 1 Hz. Not very practical.
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Old 21st November 2019, 02:23 PM   #8
gdan is offline gdan  Greece
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AD797 offset null circuit
Quote:
Originally Posted by Mooly View Post
I think the problem is that you are trying to trim offset errors caused by inappropriate circuit design/component values... remember the offset null adjustment is to null internal errors within the chip, not outside influenced design issues.

Your 30k input bias resistor is generating a relatively large DC voltage at the + input, the - input with its 10 ohm to ground generates essential no voltage at this node.

You either need to choose a more suitable opamp (very low input bias current) or redesign the circuit... or add coupling caps as Jon suggests.
Good points. It will be good to reduce the value of R at input to 10k.
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Old 21st November 2019, 02:30 PM   #9
gdan is offline gdan  Greece
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AD797 offset null circuit
My concern came from the fact that even in the datasheet no information is given for the offset null of AD797.
Only figure 55 shows a recommended circuit without any reference on the text.
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Old 21st November 2019, 02:44 PM   #10
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Another idea is to add a preset between plus and minus rails, with the cursor to the top terminal of the 10R with a higher value resistor, say, 1KΩ.
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