varying voltage on ADC impacting EXT?

[ljfischer]

This is a peculiar situation: I have a stm32f407 disco board, interfacing with a bunch of different external devices. Two in particular are an adc sampling voltage, and the ext driver being used to count pulses from a tach. Independently, these work perfectly. In fact, if the voltage being sampled by the ADC is reasonably constant, I have no issues. However, if the voltage being sampled is constantly changing, my tach pulses show lots of variation. I have hooked a scope to the tach pulse input line, and I see there is no change, irrespective of the voltage. I changed the tach input to a controlled 100hz HW pulse, and still see the same pattern. So there is some relationship between the ADC sampling a variant voltage, and the tach.

I know this could have many SW possibilities, but Im curious if anyone has any obvious things to chase. Incidentally, the problem seems to improve if I hook the scope up to the tach input line - but doesnt get perfect. I tried changing from the onboard pulldown resistor, to a hardwired 10K pulldown - no change...

[Giovanni]

Hi,

The impedance driving an ADC input determines the minimum sampling time, there are instructions about this in the STM32 Reference Manual. Lower impedances allow faster conversions.

Giovanni

[ljfischer]

The voltage value is not my issue. Im seeing some kind of weirdness with the tach/EXT. Only when the voltage being sampled on the ADC, is varying. If that voltage is steady, the tach/ext input counts correctly. Again, Im driving the tach with a fixed freq 100hz clock. There is no variation of that signal. I will debug, just wanted to know if anyone ever found a connection between ADC and the EXT drivers. Something silly Im missing...

[ljfischer]

Hmm. I think we can delete this thread. I removed some custom HW, and the problem went away. There is clearly a ground/noise issue, with my HW.