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how to route codec (ADC/DAC), analog pins not all on one side
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loki42:
The inputs need to be high impedance. I've got for an op amp with low current noise (though this seems really hard to work out from some data sheets... ) If I ran into the op amp + input and used dual supply voltages would that mean lower noise? The op amp is 0.6  fA/√Hz and the Johnson noise would be 127 nV / √Hz for the resistor I think? I think that means that the resistor noise would dominate?   
capt bullshot:
If the resistor is n series with the input and dominaters the node impedance, yes, I'd agree.
Anyway, high impedance input doesn't neccessarily mean there must be a high value resistor placed in series with the signal input, it's the node impedance connected to that input that is relevant.

It's a bit fishing in the dark without schematic and application relevant information (e.g. what is the signal source and its requirements / properties). Can't tell if dual supply helps without context.
Many applications have been successfully solved before, and once you've found such often it's adviseable to follow that path.
loki42:
The resistor isn't in series.  It's biasing from the VCOM. I simulated it LT spice and the noise contribution from the resistor  appears quite low. 
capt bullshot:
For analysis of the root cause of the noise, it might be helpful if you're able to feed the ADC output samples into all kind of visual display (long and short term waveform display,  spectrum display / FFT). This could help you to judge if the noise originates from a white noise source, or is rather correlated to digital noise of your system.


--- Quote ---The resistor isn't in series.  It's biasing from the VCOM. I simulated it LT spice and the noise contribution from the resistor  appears quite low. 
--- End quote ---
Should be OK then.
thm_w:

--- Quote from: loki42 on October 24, 2019, 12:31:10 am ---Do you mean the acid traps stuff doesn't matter? I've seen a few people say it hasn't  been an issue for a decade or two.

What would be ideal for the 3v3 cap wise?  Oh and there are planes where ever there are power traces so I've made that trace big now to make that more obvious, as @capt bullshot suggested.

I'm not sure if I need to use anything more snazzy than an LM358 to buffer VCOM. I given it's buffering the reference any noise in there will couple directly but I'm not sure if the level with no gain is an issue.

--- End quote ---

Sorry for being confusing:
- Yes acid traps stuff does not matter, at all.
- 3 3v3 caps is fine, I was responding to single via that mrjoda pointed out is not "ideal", but probably going to be OK.

VCOM, I don't know enough to comment.
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