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STM32L4: ADC generates 5 MHz noise, readings are not accurate

Question asked by communications.hmi on Jun 24, 2016
Latest reply on Jul 2, 2016 by baird.hal.001
I am using STM32L486RGT6 to read analog data. Programmed ADC1 in differential input mode reading from PC0 and PC1, single oversampled readings in an interrupt mode. PC0/PC1 are connected to an external OPA low impedance output.

When I start ADC, the PC0 input starts showing about 300 mV noise, in a form of consistent triangular wave at a frequency of 5 MHz. This happens only when I start the ADC. When I turn the ADC off, the noise disappears. The ADC readings themselves also are not accurate at all.

Checked other pins, including power supply pins and they are fine (no noise or oscillations). The board has very clean power supply lines at 3.3V, decoupling capacitors and ferrite beads. Also tried a different board with a different design, but the same problem.