[sdiy] Paralleling lots of ADC inputs

[rburnett at richieburnett.co.uk]

The ADC inputs are theoretically high-impedance so they don't need any 
particular buffering...

_However_  Each ADC does draw a little bit of current when it's internal 
sample-and-hold capacitor is connected to take a measure of the input 
voltage.  That's why they usually suggest that the source impedance for 
the ADC is something low.  I usually get around this by driving ADC 
inputs via a passive low-pass RC filter, where the output capacitor of 
this RC filter also provides enough local stored charge that the bite 
taken out by the ADC's comparatively tiny sampling capacitor is very 
small.

So the passive RC lowpass filter serves three purposes:

1. It provides some (or all) of the anti-alias filtering for the ADC
2. It provides a nice low impedance to the ADC's sample-and-hold 
circuit.
3. The resistive part of the filter limits current flow into the ADC if 
the circuit's analogue input is over-driven.

Of course you can beef up the anti-alias filtering with a more 
aggressive (higher order) active filter like MFB or Sallen-key if the 
particular application demands it, but I still usually make the last 
stage passive where it feeds the ADC.

-Richie,


On 2018-04-30 16:14, Tom Wiltshire wrote:
> Has anyone ever tried paralleling up lots of uP ADC inputs?
> 
> I’ve got several dsPIC processors running simultaneously producing
> audio (I was asking about the comms on this board the other day - I’ll
> show you when it’s a bit further along) and I want to add vibrato. The
> obvious way for me to get the same vibrato on each individual
> processor is to use an ADC input to vary the pitch. This has some
> advantages, like I can use whatever waveshape I like (and I don’t have
> to decide now) and I can provide a pitch modulation input. The
> downside is that noise can get in, but I can filter the ADC results
> heavily to minimise that, so I’m not too worried.
> 
> So - I need to tie together several ADC inputs. Can I just tie them
> together? Should I drive the tied-together-inputs with an op-amp
> follower or something to make sure I’ve got a nice low impedance
> output? Or do the inputs themselves need buffers in front of them
> (e.g. tied-together-signal -> opamp buffer -> ADC input). Has anyone
> tried anything like this?
> 
> Any experience you can offer would be appreciated.
> 
> Thanks,
> Tom
> 
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> Synth & Stompbox DIY
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