big noises on custom S32K142 board

[maksym_leski]

Hi everyone,

I am facing problem with my custom S32 board, which is based on S32K142 uC (it was designed for K144 but due to lack of processors on the market I was forced to buy K142). Board is meant to control actuators and measure their extension with ADC. Unfortunatelly the noises that ADC is receiving are very high. After disconnecting (dissoldering) everything from the board, and leaving it with simple RC filter I have 3% noises, which is too high. The very same code run on the S32K144-EVB with the same ADC0 channel 10 (PTC2) looks much better (screens below)

When connecting osciloscope directly to processor pin I'm receiving something that looks like 24 kHz saw

My question is:

1) Am I doing something wrong with software? I'm using continous mode ADC_PAL (tried also ADC_config driver and bare processor code, with the very same effect)

2) Designer of the board claims it is problem with processor (FS32K142MNVLL)

3) did anyone faced the simillar problem?

Thanks for any help

Best regards,

Max

EDIT:

I've checked the same noises on EVB at osciloscope and found out that there is also 83,3kHz saw on PTC2 pin configured as ADC0 CH10 input

[maksym_leski]

Thank you for your reply.

I found that problem is probably in the filter. As you can see on photos, the blue line is signal from potentiometer directly on the sensor, and the yellow lane is on capacitor connected to the processor pin. I have no idea where it comes from but there is a 83kHz sinus. This probably causes big noises on the ADC results

[Robin_Shen]

The core clock of PS32K144HFT0VLLT(S32K144EVB) is 80MHz, but your FS32K142MNVLL is 64MHz.
You can not use the code from S32K144, you need to modify the clock configuration in order to fit the 64MHz FS32K142MNVLL.

8.3kHz seems to be the ADC conversion frequency(sampling capacitance charged).
Will the frequency of saw changes when you modify ADC conversion frequency? If so you may need to redesign the external RC component.(please refer AN4373)

[Robin_Shen]

Hi maksym_leski,

Please check your RC circuit to ensure that the value is within the range specified in "Table 40. 12-bit ADC operating conditions".

For more detail about the saw, please refer: AN4373 Cookbook for SAR ADC Measurements

Best Regards,
Robin
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[Joey_van_Hummel]

Without schematics or board layouts, this is hard to say. My first guess is voltage drops over a high-impedance path while the internal sampling capacitor is charging.

We have multiple boards with S32K142, K144 and K146, all running without problems. ADC is usually stable within +/- 1 LSB.