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USB comms induced ADC noise: reasonable expectation?

Question asked by oise.niko on Jul 5, 2014
Latest reply on Jul 6, 2014 by oise.niko

I am working on a data acquisition board that sends an ADC value to a host PC over USB. The system suffers from a large amount of noise in the ADC readings. This noise exists also on the internal ADC reference.

- Only USB and ADC functions are enabled, no other peripherals or MCU function.
- 4 layer board, 2 unbroken inner ground planes
- decoupling very near CPU power pins: 4x 47 nF, 3x 4.7nF C0G
- stm32 package does not export ADC REF+ or REF-. AVDD filtered via 47 uH inductor (resonance 1 MHz) + 47 nF.
- CPU core clock is 72 MHz, reduce to 48 MHz makes no difference.
- noise only visible (on oscilloscope) during active USB comms. Noise 'blips' on VDD precisely synchronised with USB data frames (1 ms). With USB not active, noise is approx. 5 mVp-p.
- External power supply for AVDD makes no difference.

My question is, is it reasonable to expect at least 10 bit accuracy with simultaneous USB comms? Or should I just use an external ADC if I plan on using USB?