[sdiy] Summing op amp calculation fail

[Tom Wiltshire]

On 28 Sep 2009, at 19:03, Justin Owen wrote:

> Hello, time to come asking for help again I'm afraid.

Nice to see some real electronics on the list again.

What resistors are you using for the voltage divider that gives you  
the virtual ground? What cap? I think you might find that the amp is  
dragging the virtual ground around so that what you think is going in  
isn't actually the voltage it gets. Have you tested the voltage at  
the + amp input?

Another thing to try would be varying the resistance you use in your  
circuit. Have you tried exchanging all the 10Ks for 100Ks? It  
shouldn't change anything from a gain point of view, but it alters  
currents and loads in the circuit, and might be enough to make it go.

I don't have any deep theoretical understanding of these circuits,  
but I have plenty of experience playing with them, and virtual  
grounds always cause problems. Those problems miraculously disappear  
when you move to dual supply. It's  a pain, but there it is.

Try the circuit using a voltage follower/unity-gtain buffer op-amp on  
the virtual ground. This lowers the impedance of the virtual ground  
and makes it more difficult for the rest of the circuit to shift it  
about. If that works, you know where the problem is coming from.

Hope this helps,
Regards,
Tom