STM32-E407 Noise on analog inputs

[Kolfschoten]

Hi all,

I have been using the E407 for various projects but I always run into problems with the analog inputs. Even when I connect a separate clean power supply to the Aref and take care about the Agnd, I still get a lot of noise on the ADC measurements. In some projects I had to switch to the Olimexino because the noise on the analog inputs rendered the E407 useless.
Has anyone had similar problems and how could I solve this?

Any help would be greatly appreciated.

[LubOlimex]

Hey,

Have you stumbled upon this document: http://www.st.com/web/en/resource/technical/document/application_note/CD00211314.pdf ?

Best regards,
Lub/OLIMEX

[Kolfschoten]

Thanks for the reaction. I had studied the document you mentioned before and I have implemented some of the advice from the document but without result.
I have looked at the Vref compared to Agnd with a scope and found a strange noise. Unfortunately I am not able to find any clock or component which is causing this noise. Even when disconnecting the switching power supplies (U8 and U9 on the schematic) and disconnecting the Ethernet controller, the noise is still there. It only disappears when the micro is not running (DUH). A colleague of mine is using the Discovery F4 which has the STM32F407VGT6 instead of the STM32F407ZGT6 and he gets only 1LSB noise. (we are using the same settings for the ADC)

I am out of ideas on where to look next. If any of you have any leads it would be much appreciated.

[LubOlimex]

I suppose that you are aware that most of the ADC inputs are used by other peripherals? The following ADC signals free-to-use and available at the ANALOG connector/header (make sure to test with these ones exactly):

A0 - ADC123_IN10
A1 - ADC3_IN4
A2 - ADC3_IN5
A3 - ADC3_IN6
A4 - ADC3_IN7
A5 - ADC3_IN8

At what clock are you running the ADC? Did you try to turn off the PLL and run the ADC at clock of the quartz crystal?

Best regards,
Lub/OLIMEX