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...
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...
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.
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.
Sure, I was trying to use it in place of the AD744 in this circuit.
Attachments
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.
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.
Last edited:
Did you mean R5 instead of Rs?
The circuit above works fine, with quite low offset, using the AD744 and a number of other FET op-amps.
There is definitely something funky going on with the AD8065, though.
I remember trying to use the 8066 in the feedback loop of a TPA6120 and offset was high in that circuit as well.
The 744 is driving the 811 buffer through a compensation pin. This compensation pin is something very specific to the 744, and there is no reason at all to expect pin 5 from any other amplifier on the planet to work that way.
One could replace the 811 stage with something else, but I would not expect to be able to treat pin 5 from any other chip like pin 5 from a 744.
One could replace the 811 stage with something else, but I would not expect to be able to treat pin 5 from any other chip like pin 5 from a 744.
Exactly!
I think AD makes (or used to make) one other amp that could be used this way.
This circuit is unique for the AD744 - the AD744 output stage is not used at all.
mlloyd1
I think AD makes (or used to make) one other amp that could be used this way.
This circuit is unique for the AD744 - the AD744 output stage is not used at all.
mlloyd1
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.