[sdiy] Precision voltage offset

[harry]

Using the rails for a precision voltage reference is a big mistake...
Only use them if you do not care about accuracy (sometimes you don't)

The inductance and resistance of the rails means that often, the current draw
from a load (can u say LED ?) will cause a significant voltage drop... which
kills
the precision.  Often in DIY work this shows its ugly head as modulation in a
VCO
that you didn't want.

Reference IC's are a better way to go. Often Zeners, and other shunt
regulators... are
better in stability than garden variety series regulators like the 78xx series.

I like the LM4040- your voltage here... they are pretty reasonable.  But watch
out for temperature changes...they are quite good but there are better ways.

Usually the power supply voltage for the op-amp supply is much less critical
than the
reference. Opamps have quite good power supply rejection ratio...

H^) harry

media at mail1.nai.net wrote:

> >Instead of a voltage divider and selector switch to the summing input, you
> >could have a single appropriately sized resistor and trimmer connected to
> >your reference voltage. They key is having a close match (or trimming) your
> >R-in and R-feedback so you don't compromise the accuracy of your keyboard
> >CV.  The only thing I would change here is having a fixed reference as
> >opposed to using the power supply rails like I did (me bad).
>
> Really??  I would think the rails would be rather stable.  Is the
> fluctuation audible??
>
> >So, one LT1013, and handful of resistors and one trimmer and you are set.
> >There is a lengthy and detailed document explaining how the circuit works
> >that will help you select just what you need.
> >http://www.wiseguysynth.com/larry/jlh-822/822_theory.htm
>
> That could have saved me some typing!!

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