[sdiy] Multiplying DACs

[Olivier Gillet]

> If I a) bumped my resolution up to 12 bit (which would tie in with my
> EEPROM-based concept) to suit these DACs, can I actually frequency modulate
> just by applying a signal to the external reference pin? Or would I be
> amplitude modulating?

This would be amplitude modulation with modulator feedthrough, because
the output of your DAC swings between 0 and Vref, so it has a DC, and
the product of this DC and the modulator will be present.

http://img214.imageshack.us/img214/2739/imagecbg.png
first figure: modulating VREF
second figure: removing the DC from the DAC output (with a fixed VREF)
and feeding it into a VCA

Also, check your datasheet, I doubt the Vref can swing to negative
voltages (on the MCP492x it can't). I think there are better VCA and
ring-modulator than the one-quadrant multiplier you can get out of
messing with Vref.

Olivier