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how to route codec (ADC/DAC), analog pins not all on one side

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mengfei:

--- Quote from: loki42 on October 24, 2019, 01:51:51 am ---The ones under U4 just become the ground plane, if you look at the image with the planes.

--- End quote ---

I meant the traces going inside U4 pins, why are they crooked? & some other  traces have weird routing too, did you use auto route?
   

loki42:
no but I did use the push / shove router and did move a few things. Is there any performance issue with this or is it aesthetic?

Should I look for a buffer with lower noise than the LM358 or will it be low enough to not be an issue buffering VCOM.  Should I worry about low frequency return paths passing under the oscillator? 

capt bullshot:

--- Quote from: loki42 on October 24, 2019, 12:31:10 am ---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 ---

Depends on your circuit. If VCOM isn't loaded (e.g. used like the example circuits), you don't need a buffer at all. If VCOM is loaded by anything that can cause current flowing into and out of the LM358 (any kind of AC), the result will be terrible. If you have a nice and steady DC load, you might get away with proper filtering.

The crooking mentioned is an aesthetic issue and surely has no impact on the performance.

Acid traps shouldn't be an issue with a modern fabricated PCB. Anyway, I do avoid acute angles, crooking etc. since it makes the layout look ugly to an experienced eye and it'll easily get judged as a pointless beginners work that wouldn't work anyway. If you do it for your own and don't showcase it and it works, aesthetics don't matter.

loki42:
I'm not sure how much load is unloaded, they are used to bias the op amps through 1 M resistors. It'll be around 25 uA total. I could get rid of that by switching to dual supplies.

capt bullshot:
1M resistors in the analog signal path isn't particular low noise anyway ... Biasing a "+" input of an OPAMP could be considered "no load", depends on the OPAMP, but the bias node itself would still have to be low impedance for AC. Your issue might not only be layout related.

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