UM2217 ADC HAL description not coincide with RM0433 Reference manual in ADC conversion time.

[PStor]

UM2217 ADC HAL description not coincide with RM0433 Reference manual in ADC conversion time.

On page 54 of UM2217 there is the following:

“Sampling time value to be set for the selected channel. Unit: ADC clock cycles Conversion time is the

addition of sampling time and processing time (12.5 ADC clock cycles at ADC resolution 12 bits, 10.5 cycles at 10 bits, 8.5 cycles at 8 bits, 6.5 cycles at 6 bits)�?

On page  926 RM 0433 there is the following:

“The total conversion time is calculated as follows:

TCONV = Sampling time + 7.5 ADC clock cycles

Example:

With Fadc_ker_ck = 24 MHz and a sampling time of 1.5 ADC clock cycles (14-bit mode):

TCONV = (1.5 + 7.5) ADC clock cycles = 9 ADC clock cycles = 0.375 μs (14 bit mode for

fast channels)�?.

From the example in RM 0433 ADC conversion time is one halve of digit capacity.

What conversion time is correct?

   

[Singh.Harjit]

​The analog to digital conversion process involves two steps:

1) Sampling the input signal - this step connects the external source to an internal capacitor so that charge from the external source can be transferred to the internal capacitor.

2) Conversion - the internal capacitor is disconnected from the external source and the analog to digital conversion is performed.

In the example you give above, the sampling time is 1.5 ADC clocks and the conversion time is 7.5 clocks.

NOTE: it is super important to get the sampling time right. To select this, you have to do some analysis to figure out how long it takes the external source impedance to charge the internal capacitor. Most ST datasheets have a table and/or give you the equation to determine this. Search for Rain in the datasheet. Table 185 in DS12110 (datasheet for one of the STM32H7 parts), has the sampling time for different source impedances and resolutions.

So, to do the conversion, you have to set two parameters - the sampling time and the ADC resolution.