[sdiy] LAG circuit de-tuning VCO question

[ASSI]

On Saturday 05 November 2011, Tim Parkhurst wrote:
> 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?

It all depends on how bad the offset drift is after the trim, but the 
general answer would be "not really".  That said, a trimmed opamp often 
performs better than an untrimmed one, but a "zero-offset, zero-drift" opamp 
will likely come out ahead, even assuming that a single trim would be 
sufficient and no cost for the trim operation itself. 

> 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?

No.  Offset is usually something that gets created by statistical device 
mismatch rather than incompetence of the layouter, so the offsets of 
multiple amps in the same package should show no correlation.  Their 
temperature drifts, however, should be similar.


Regards,
Achim.
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