[sdiy] Mixed-signal problems (dsPIC digital delay project)

Neil Johnson neil.johnson71 at gmail.com
Fri Jan 27 17:26:54 CET 2017


> (3) Running the chip with the DAC disabled too didn't help. Without the DAC turned on, the inputs to the differential amp are basically floating outputs from the dsPIC, so there was more noise (20mV) like that than with it outputting zeros.
> You say that this kills most of the noise for you, so I'm a little suspicious that I'm not seeing the same thing. Did you need to do anything else?

When you disable the DAC put a small resistor across the pins,
otherwise they will float as you found out.

Also, you use Vref for the wrong purposes in many places all over your
schematics.  While that may not be the main contributor of noise, it
doesn't help doing daft things.

Another test you can do - hold the MCU in reset and measure the noise,
also with the small resistor across the DAC outputs.


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