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Mon Jan 10 18:31:21 EST 2011

Analog control voltages

Something Gert mentioned made me think.  What about analog control
signals?

In the current design, the calibration is part of a complete control
loop spanning digital PWM to (digitally measured) frequency output,
with analog circuitry in the middle.

In this scheme there is no way for for analog controls
voltages/currents to benifit from the digital
compensation/calibration.  Could we get to something like 1V/octave in
a predictable way?  Or say a less wasteful 250mV to keep things within
the 5V supply.

Currently the compensation is about:

  - Temp comp, which is voltage SCALE

  - Component mismatch, which is voltage OFFSET

The base line seems to be that OFFSET compensation is easy to share
with analog modulation: Digitally set base offset + add analog signals
before they go into the transistor.

The SCALE compensation is another story.  Bringing voltage scale
compensation into the analog domain will require some extra
components.  ( More multipliers, or some high frequency choppers. )

The question is then: is it worth it?  Or should analog control
signals simply be digitized and run through the digital compensation?
Or, in the case of control knobs for instance, do we really care that
much about a little bit of scale offset?  Meaning, can analog control
inputs go uncompensated?


Conclusion: it's not worth it.  Leave room for hacks (i.e. current
inputs) but design for minimal components when all controls are
digitally compensated.



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