[sdiy] LAG circuit de-tuning VCO question

[Dave Kendall]

Hi Tim.

On Nov 5, 2011, at 19:10, Tim Parkhurst wrote:

>>> What op-amps are you using?
>> TL072s. With the new caps, they seem to be OK, but maybe I should 
>> consider LT1013s or similar?
>> I need to bite the bullet and get a load for CV summing jobs....
>>
>
> I think the advantage of using the LT1013 is that is has very a low
> offset voltage. Here's what I don't understand about Low Offset op
> amps: In a CV application, isn't it a farily easy thing to just add a
> trimmer to be able to zero out any offset? Of course, using a low
> offset op-amp is probably cheaper and easier than adding a bunch of
> trimmers to the CV chain but still, it's possible, to dial it out,
> right? Also, Lets say you have a CV summer built from a dual op amp.
> The first op amp is wired up as an inverting summer and will introduce
> a certain offset voltage. Now the second op amp is wired up as an
> inverter follower. Wouldn't that second op amp introduce an equal (and
> opposite) offset that would very nearly cancel out the first amp's
> offset? Of course, this is assuming that both amps in the package have
> identical offset voltages, but I would think a modern dual op amp
> would be pretty good in that respect. It should certainly get you
> close, right?

That makes sense. So far I've been using the dual inverting 
configuration a lot, and always for summers. IIRC, it was a comment by 
Paul Schreiber on the list or somewhere a while back recommending the 
LT1013 that got me thinking, in that they are low offset *and* low 
drift. The second factor could become important for critical VCO usage.
>
> Oh, and I know the problem has been solved with the right type of cap,
> but adding a guard ring to ground around the 'hot' terminal of the
> hold cap is said to help reduce cap leakage. And finally, make sure to
> clean off any flux on the circuit, especially around the hold cap.

Thanks Tim - will do. At the moment I'm using a pre-existing circuit, 
so cannot easily add a guard ring, but fortunately the hot pin is a 
good 5-6mm away from any other traces, so that should help I guess.

cheers,
Dave