[sdiy] Paralleling lots of ADC inputs

[ASSI]

On Monday, April 30, 2018 5:14:51 PM CEST Tom Wiltshire wrote:
> 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?

The datasheet should have some details what kind of signal conditioning the 
particular ADC is expecting, but it's usually a bad idea to just tie together 
multiple ADC inputs unless you can make sure they don't sample at the same 
time.  Most ADC inputs present a time-dependent impedance that can be quite 
low at the sampling point.  Also if capacitive sampling is used (very likely 
these days), the paralleled input capacitances will influence each other and 
degrade the ADC performance significantly.  At the minimum and only if your 
actual source is already reasonably low-impedance I'd put an isolation network 
in front of each input (RC lowpass is the most simple one).  An actual buffer 
amplifier is better, but you might need an auto-zero buffer for your 
application to get rid of offset voltages.


Regards,
Achim.
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