High Offset with AD8065/66. - diyAudio
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Old 5th November 2015, 05:15 AM   #1
ammel68 is offline ammel68  United States
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Default High Offset with AD8065/66.

Every time I try using the AD8065/66 within the feedback loop of another op-amp or buffer, I get .5V or more offset at the outputs.
If I use it alone the offset is quite low.

Being this a FET part with low input bias currents, the high offset is a complete mystery to me.

Can someone explain why the this particular chip isn't happy inside a global feedback loop?

Thanks...
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Old 5th November 2015, 05:35 AM   #2
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Its a rather high bandwidth part - perhaps your 'offset' is in fact oscillation. To use it with a buffer, that buffer would need to contribute negligible phase shift itself.

Take a look at the OL gain/phase plot. The zero crossing is around 65MHz where there's a phase margin of ~50deg. Then consider say the BUF634's phase response at the same frequency - its about 50deg at 25oC, getting worse at higher temps. A recipe for oscillation at unity gain for sure. That's with the BUF634 in wideband mode, in low Iq mode there's absolutely no hope of stability except at impractically high gain.
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Old 5th November 2015, 09:27 AM   #3
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...or when draining away some open-loop gain, for that matter.
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Old 5th November 2015, 05:57 PM   #4
ammel68 is offline ammel68  United States
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Interesting...I had no idea about the phase shift issue.

I have found most, but certainly not all, FET op-amps work okay inside a feedback loop.

I typically use the 8065/66 with a gain of 3.

Thank you for the replies.
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Old 5th November 2015, 08:00 PM   #5
Bonsai is offline Bonsai  England
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Can you post a circuit. Should be an easy one to fix.
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Old 6th November 2015, 03:39 AM   #6
ammel68 is offline ammel68  United States
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Quote:
Originally Posted by Bonsai View Post
Can you post a circuit. Should be an easy one to fix.
Sure, I was trying to use it in place of the AD744 in this circuit.
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File Type: png Jung-744-811-Schematic.png (51.3 KB, 44 views)
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Old 6th November 2015, 03:46 AM   #7
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Can't see an OL transresistance/phase plot for the AD811 so it won't be an easy one to predict.
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Old 6th November 2015, 05:13 AM   #8
ammel68 is offline ammel68  United States
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What exactly are you trying to predict?

I've tried the AD8065 in place of the 744 and there is considerable offset at the outputs.
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Old 6th November 2015, 05:17 AM   #9
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Whether the combination is likely to be stable, and if so, over what range of gains.
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Old 6th November 2015, 07:15 AM   #10
Bonsai is offline Bonsai  England
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Put a small Capp - say 22 pF between pins 2 and 5.

Secondly, make sure you have Rs fitted close to U2.

All your feedbzck connections need to be really short with composite amps like this - any stray capacitance from the - inputs to ground or the rails will cause problems.

To get your circuit going, I would consider starting off with the AD744 as shown in your circuit.
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Last edited by Bonsai; 6th November 2015 at 07:25 AM.
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