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ADC problem with sampling frequency of 2MSPS

Question asked by smrtkai on Jan 27, 2016
Latest reply on Feb 2, 2016 by smrtkai

I do have some strange behavior with an ADC, that I cannot explain. I want an ADC to run with a frequency higher than 2 MHz and DMA. I am using an STM32F4 Discovery board, so the ADC's support 2,4MSPS.

My configuration is as follows:
ABP2: 86 MHz
ADC3-Prescaler: 4
Resolution: 6 Bit
Sampling Time: 3 Cycles

With this configuration, I expect a sampling frequency of ~2,4MHz (86/4/(6+3)).

I want to verify it by using a pin, that I toggle on the DMA callbacks (half-full and full). My DMA memory size is set to 160 values.

Unfortunately the pin does not toggle. When reducing the sampling frequency to ~1,2 MHz by setting the ADC prescaler to 8, I get an pulse signal at the pin with ~7,5kHz. This is axactly the freqeuncy I expected (7,5kHz*160samples = 1,2MSPS) 

So sampling with 1,2 MSPS works but with higher freqeuncies does not. Even setting the adc prescaler to 6, does not work. :-(

What am I missing? I would expect, that this s
hould work.