[sdiy] Differential 12dB/oct filter

[Tom Wiltshire]

On 27 Apr 2016, at 16:44, "Richie Burnett" <rburnett at richieburnett.co.uk> wrote:

>> 
>> 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.)

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? I suppose the ADC is referenced to the same signal, so perhaps it doesn't. But still it seems a bit wild.

Regards,
Tom