[HBQRP] Who's an op amp expert?

[Sean Pepin]

On 02/18/2014 03:26 PM, Dale Botkin wrote:
> Trying to troubleshoot what should be a dead simple op amp circuit and 
> I could use a hand.

Hmm. I doubt I can help, but I have some suggestions.

R9 and R10 are pretty small. The only draw on the virtual ground at pin 
3 of U2A is the opamp's input bias current, which is rated in picoamps 
on this guy. You can safely use 100k for both. I would bypass this node 
with 1n, just to avoid any oddball stability issues. Doing this will 
help save power if that's a concern. None of this should affect your 
issue though.

Reduce C7 to 100n temporarily. It may be kind of large when considering 
the feedback network is made of very high value resistors. This is the 
closest thing to a relevant issue I can spot.

Reduce the value of the feedback network resistors to something like 15k 
and 22k. Output current can more easily charge C7 and this lowers the 
impedance of U2A pin 2, reducing the possibility for interference.

Add a small AC coupled load. The opamp is supposed to be able to source 
something like +/- 10mA, so it's no sissy. Something like 100n in series 
with U2A pin 1 and then 100k to ground.

Hope that helps. I have a hunch that it might be a forehead slapper 
though. I can't see anything that'd keep this from working.