[sdiy] Differential 12dB/oct filter

[Richie Burnett]

> What you describe first is what I've got. Resistor divider across the 
> rails biasing the input,
> with the input signal capacitively coupled to it. Doesn't that just put 
> any noise on the 3V3
> rail straight into the input?

There shouldn't be any significant noise on your analogue 3V3 supply!

If you're using the same 3V3 supply for the digital Vss/Vdd then that's 
probably where the problem lies.  dsPICs draw quite large and sharp current 
pulses from the digital supply during times when they're active doing MACs 
etc, compared to the idle time in block processing.  I used separate LTC1763 
regulators for the analogue and digital 3V3 rails, to stop digital noise 
getting into the sensitive analogue side.

-Richie,