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Trouble with ADS8556 EVM in hardware parallel mode

Asaf Carmeli
Prodigy 260 points
Other Parts Discussed in Thread: ADS8556

Hi,

For a project of mine, i'm trying to connect the ADS8556EVM to a Raspberry Pi. 

Since my needs are simple (~1Ksps) and to keep the design simpler, i decided to use Parallel mode / hardware mode. 

The voltages i used are 5V from the Pi as the analog supply and HVDD, 3.3V as the VIO (BVDD?) and ground as the VDDS (since i'm not measuring negative voltage).

I got all the signaling going on, after asserting the CONV i see the hold rising and falling, then i assert CS and toggle RD for each parallel word. 

My issue is this - regardless of the input voltage in any of the channels (which gets to the ADS8556 chip itself, i checked), i read back consistent garbage on the parallel word. 

If the configuration is set to word, x2 range, i see only bits 8 and 9 toggle (seems random to me). 

If i change to byte mode, i see more bits toggling, also randomly. 

Range will also change the resulting output, but again - will have no correlation to the input voltage (be it ground, 1V DC, 3.3V DC, etc...).

Please help. If i need to, i can integrate serial mode. I'd like to remain in hardware mode though, since i don't require any special configuration.

Thanks in advance,

Asaf.

  • 0
    TI__Guru**** 180831 points
    Hi Asaf,

    Unfortunately you cannot simply tie AVss on the ADS8556 to ground, you have to have at least a -5V supply (see page 3 of the datasheet). Try changing you power rails and let us know if that clears up the conversion troubles you are seeing.