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ADS8860: ADS8860 / ADS8861 Not releasing SDO on power-up

Chris Ponder
Prodigy 120 points
Part Number: ADS8860

I have a design with three TI ADC’s two ADS8861IDGS and one ADS8860IDGS as we have two differential channels and one single-ended. All three share their SCLK and SDO lines and have switchable ConvST and DIN lines. The problem I have is that on power-up all three devices are outputting on SDO, this means that we have to connect all three devices and run through a sampling single cycle, at which point they all release the SDO line. This behaviour is very frustrating and is not stated in the datasheet.

I have included the schematic below. VRef is 5V, AVDD is 5V and DVDD is 3V3. On power-up ConvST is pulled to GND and SDO & DIN (nCS) are pulled to DVDD, all with 10k resistors. Would anyone be able to take a look at this and tell me if I should be doing something differently to make the devices power up with SDO high impedance?

  • 0
    TI__Expert 3175 points
    Hi Chris,

    I'm looking into this and have a few questions/requests:

    1) Can you verify that AVDD in your system is 5 V? The absolute max for these devices is 4 V, with a recommended operating voltage max of 3.6V. Please test the startup with 3.3 or 3.6V AVDD (you may have to replace the current units in case they are damaged).

    2) Double checking: SDO and DIN are pulled high while CONVST is pulled low (pull up resistor is only shown for SDO)?

    3) What communication mode are you using, 4 wire CS mode or daisy chain?

    4) Can you please send a scope capture of the data lines (any DIN and CONVST as long as it is from the same device will suffice) upon power-on?

    5) Finally, can you please send a scope capture of the AVDD and DVDD supplies as they are powering up (and with AVDD <= 3.6V)?
  • 0 in reply to Evan Sawyer
    Prodigy 120 points
    Thanks Evan, I think your point 1) is probably the crux of the problem, no idea how I missed that, thank you. SDO and DIN are both pulled high, the DIN pullup is on the next schematic up. The intention was to use 4-wire CS, but we have ended in 3-wire mode, as 4-wire wasn't working properly. Most likely due to 5V on AVDD. I will make the change to 3V3, change the chips and then do some testing before troubling you any more.