[sdiy] Digital VCA - Choosing an ADC

[Olivier Gillet]

> I second what Olivier said: Use an inverting op-amp to sort out the gain of the input and add an offset so that the incoming signal's ground becomes 2.5V. Yes, the offset is positive at the output of the op-amp, so you'll have to find a negative voltage to use for the offset - but you've got -15V flying around in there, right?! Use that with a gain of 1/6th and you've got -2.5V.

I don't think you'll even need a negative rail.

To get something AC-coupled is easy:

Input to -ve input of the op-amp through cap and 2R resistor
R feeback resistor
Vcc / 2 to +ve input of the op-amp

You get a -0.5 gain which shifts the signal by 2.5V, so it's good for
a -5V..5V bipolar audio input.

To get something DC coupled is a bit more complicated, as a first
guess I'd say connect the 5V rail to -ve input through a 2R resistor.

The only big problem I see with that is that you have no guarantee
that a 0V input will fall exactly on word 2048 on the DAC -- if your
mid-rail is not spot on. This might be a problem for a VCA - a 0V
input could be translated into 2049 by the DAC, so you wouldn't
totally silence the signal! This kind of trimming could be done in
software, at no hardware cost.

Olivier