DFSDM decimated frequency

[LeonidP]

Hi

 

I have a simple question about DFSDM configuration.

 

I want to apply sinc5 with 32 samples length and therefore get 32 times decimation. Btw, I use internal ADC1/ADC2 in dual mode. STM32H725.

Thus, for a given ADC frequency F I expect to get decimated frequency Fd = F/32. Here I assume that FOSR = 32 and IOSR = 1. To my opinion there should be no dependency of sinc order X.

But I see quite different results. To be more accurate, I use one channel/filter block with FOSR = 1, and another one with a combination of FOSR and X to be investigated.

 

I compared decimated frequencies of the filters and experimentally got the following relation between F and Fd (i.e. input frequency and decimated frequency):

Fd = F/((FOSR+1)*X+0.85) for all FOSR > 1. If FOSR = 1, the input frequency seems to be unchanged.

Just to stress: I expected Fd = F/FOSR. Perhaps, Fd = F/(FOSR+1). But why is it multiplied by X and got additional 0.85?

[LeonidP]

I got it. Fast mode should be enabled