ADC noise issue (pin PA0) STM32F746

[PRV]

ADC noise issue (pin PA0) STM32F746 microcontroller

Please explain the presence of increased noise at the PA0 pin of the STM32F746ZGT6 microcontroller in the ADC mode.

ADC parameters:

• Clock frequency - 13.5 MHz (with a prescaler 4)

• Sampling frequency (period) 100 ms

• Sampling period 56 cycles

Below are oscillograms taken from the NUCLEO-F746ZG development board.

ES0290 Rev 7 clause 2.2.2 - did not help

The same noises are observed on the prototype of the assembled device - everything is exactly like on the NUCLEO-F746ZG.

Below is a part of the analog input of the product prototype on STM32F746ZGT6 - pin PA0.

There is no ripple in the power supply of the operational amplifier (OP484), the D10 input is not connected anywhere.

Plans to use all analog inputs.

Recommend a solution.

[TDK]

> the D10 input is not connected anywhere

Measure a high (infinite) impedance source will have noise. What useful result are you expecting out of this circuit anyway?

If you don't want noise, ensure the input is sufficiently low impedance.

[PRV]

D10 is connected to ground through R47 | R53.

These analog circuits have other ADC inputs as well, but have no noise.

[TDK]

> D10 is connected to ground through R47 | R53.

Oh, you're right, I overlooked that.

What is the op-amp? Note that even "rail to rail" op-amps typically aren't rail to rail. Perhaps it has more noise when it is trying to drive the output to ground. I would separate it from the equation by tying the input to ground directly. Some op-amps are not stable in voltage follower mode.

You could measure the noise without the ADC on but with the pin still in analog mode to be sure the ADC is causing the issue here.

The ADC will introduce noise when it switches on the sampling capacitor. There is no way around that. But if your input is at GND, you shouldn't see 40mV of noise. due to this. It should also be very periodic, lining up with your sampling interval, rather than just random white noise. A capacitor on the input can help, although the voltage follower should be doing a decent job of keeping the voltage constant here.

[andrewg]

This is quite similar to what I've got here on stm32f401

https://community.st.com/s/question/0D53W000015Z0MwSAK/adc-noise-on-stm32f401

The next things I'm going to try is to place large caps on the power supply inputs to the board and even possibly run it on batteries to see if those made a difference.