[sdiy] Digital VCA - Choosing an ADC

[Tom Wiltshire]

On 28 Aug 2011, at 07:50, Matthew Smith wrote:

> From what has been said, it looks like I can go down one of two routes:
> 
> 1) With a single-ended ADC. Most of the affordable, > 2 channels, ones I've looked at are single-ended. To handle a signal that goes below ground, I would need to ADD the magnitude of the negative-going signal (+2.5V) to get the 0..5V level required.

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.

> This part allows me to go up to 1Msps, so I would probably sample at 100ksps, rather than 50ksps - would this take care of the harmonics those analogue ramps are creating?

Probably not. A ramp's 2nd harmonic is a 1/2 the level of the fundamental - e.g. roughly 6dB down. The 4th harmonic is at 1/4, another 6dB down (you can see where I'm going with this). So you have to get out to the 256th harmonic before the level is even 48dB down. If you assumed that -48dB was ok, 50000/256 = 195Hz maximum frequency before the 256th harmonic starts to alias. *Not* everso high!

All that said, if you put a few caps in to roll-off the top end (say, above 20KHz) and sample at 100KHz, it'll come out ok. The better the filtering, the better, but you choose how far you want to go.

HTH,
Tom