ADC readings 'sticking' anomaly

[Sahiel]

I'm using the STM32H7B3.

ADC configuration:

Sample time : ~2us

LL_ADC_SAMPLINGTIME_64CYCLES_5  

Calibrations: 

HAL_ADCEx_LinearCalibration_FactorLoad(adc2)

HAL_ADCEx_Calibration_Start(adc2, ADC_CALIB_OFFSET, ADC_SINGLE_ENDED)

Observations:

The ADC counts seem to be stuck at values which are highlighted in the picture above at around 0.31 to 0.35 V and 0.62V to 0.66V

The ADC > DR also seems to be stuck and reads 255, 511 at the two voltage ranges mentioned above

Input signal source:

Analog Input voltage range is 0V - 5V in YELLOW converted to 3.3V scale which is also the Vref

Using only RED boxed channel that is connected to the ADC pin on the mcu 

Things that I tried:

• Increasing the sample time to ~11us
• Tried different calibration combinations HAL_ADCEx_Calibration_Start(adc2, ADC_CALIB_OFFSET_LINEARITY, ADC_SINGLE_ENDED)
• Ignored the DMA buffer values and only observed the ADC > DR which also seems to directly jump from 255 to 288 and 511 to 544 

Nothing changes and it's repeatable 

Any suggestions would be helpful

Thank you!

[Sahiel · Accepted solution]

This issue was resolved when the F_adc which was 35Mhz was reduced to 6.25Mhz

[TDK]

@Sahiel please pick a solution and mark it as solved.

[waclawek.jan]

> This issue was resolved when the F_adc which was 35Mhz was reduced to 10Mhz

According to STM32H7B3 datasheet, the maximum ADC clock frequency is 50MHz, but ADC_CR.BOOST has to be set accordingly.

@Sahiel , how was BOOST set with Fadc=35MHz, when the problem was observed - was it 0b11?

JW