[sdiy] Differential 12dB/oct filter

[Richie Burnett]

> 12-bit audio isn't too bad (after all, 12-bit/32KHz was an expensive 
> studio rack mount delay processor in the middle 1980s!!) and the 16-bit 
> output gives me headroom for processing.

Sounds fair enough to me.

> It's funny, I haven't had such noise problems with it when doing 
> oscillator designs, so I think perhaps some of my noise is coming from the 
> ADC or the signal going into it. I should probably do everything I can to 
> ensure that the input is smooth and clean before worrying about the 
> output.

Have you tried outputting a clean pre-calculated 16-bit sinewave from a 
lookup table to the DAC.  That should show it's true performance without any 
degradation due to the ADC.

> Can you recommend how I should generate a low noise 1.65V reference for 
> the input biasing?

Just use a resistive divider from your +3V3 analogue power rail to your 
analogue ground rail.  Either bias the ADC input pin with this and AC couple 
your signal into the mid-point of the divider.  _Or_ bias an amplifier / 
filter to this quiescent operating point, and then tie the op-amps output 
directly to the ADC input.  (The ADC input likes a stiff voltage source 
because it takes a bite of charge out of the input when it does it's sample 
and hold thing at the start of each conversion.)

> Yes. Matching capacitors in such a filter might be the killer here. The 
> whole point of the differential output is to kill the common-mode noise, 
> so if I screw that up, I might as well go back to square one.

You can actually get away with a single capacitor across the inputs of the 
diff amplifier. So thats:

dsPIC DAC output pins, first-stage filter resistors, first-stage filter cap 
across the differential audio signal, then the usual 4 resistors and 2 
capacitors of your differential "amplifilter" circuit you posted earlier.

>> If your ultimate aim is a single-ended audio output
>>
>> I might be tempted to just follow the diff amp with a Sallen-Key or MFB 
>> filter though.  You have more control over the pole placements, no 
>> loading effects, less R's and C's, no common-mode degradation due to 
>> capacitor tolerances, but the extra cost of another op-amp :-(

> Yes. I've done this as well. But it's only a 12dB filter, so it's not a 
> huge effect.

I take it you don't want to just stick a stereo 24-bit CODEC like the CS4270 
on the DCI port?  That would give you superior ADC/DAC performance, stereo 
In/Out and take care of all the anti-aliasing and anti-imaging filtering for 
you.

-Richie,