ADC sampling frequency questions

[NicRoberts]

Doing the set up in MX
I have four ADC channels on APB2 with PCLK2 at 84 MHz sampling 4 microphones.

The recommended audio sampling rate stretch from 44.1 kHz up to 96 kHz, so initially I'm happy anywhere in that range for now. 

I have set the clock pre-scaler to divide PCLK2 / 4 so the ADC clock is at 21 MHz

I have 12 bit resolution which takes 15 ADC cycles but then in the "rank" section of the parameter settings there is a pull down for sampling time for each channel. I'm guessing that the value I pass in the "rank" section is what I would use to get the final sampling rate.

e.g. if I chose 84 cycles for each channel then I would have a sample rate of 250 kHz per channel giving a sample rate for all conversions of 62.5 kHz?

Or do I add the sample time to the resolution time e.g. 84+15 giving a sample rate of ~212 kHz per channel?

[TDK · Accepted solution]

The total time to convert a channel is sampling time + conversion time.

I believe 12-bit resolution requires 12 cycles to convert, not 15. So your total time would be 84 + 12 cycles per sample.

[NicRoberts]

Thanks, MX is saying "15 ADC clock cycles" for 12 bits which is an annoying number for sure.

[TDK]

If you want a specific sampling time, it's easier to have them triggered from a timer rather than in continuous mode. Set up the conversions to complete at least as fast as the timer triggers.

[AScha.3]

See rm...ADC :

So ADC needs 12 ckls for conversion, + time for sampling ; 

min. 12+3 = 15, max. 12+480 = 492 clks.

At your chosen 21 MHz you get : 0.71 us ... 23.4 us conversion time.

So just select the sampling time that comes close to the conversion time you want.

(or adjust the adc clock in allowed range to get other conversion times.)