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ADS1120: ADS1120 not reading back data

Paul Dye
Prodigy 180 points
Part Number: ADS1120

Hi,

We have a PCB which uses an ADS1120.

Data doesn't seem to be written to the registers and on close inspection the SPI logic levels are unexpected particularly when compared to a working SPI bus.

It looks like the ADS1120 chip is faulty because we have removed the 47R series resistor and the MOSI line level is now correct.

We have already replaced the chip once.

Is this a known fault?

Is there something we could be doing to damage the part?

See images below….

Image - Bad read

Image - good read

Image - bad write

Image - good write

  • 0
    TI__Guru** 113765 points

    Hi Paul,

    Do you have a schematic that you can share?  Where are you probing the signals?  It looks like there is some driver contention.  Are you holding CS low continuously on the ADS1120?  Do you have any other devices connected to the SPI bus?  Are both the analog and digital supplies active as both must be active for communication to work properly?

    If I were to make a guess, there is more than one device active on the SPI bus at the same time.  Or it could be that you have DIN and DOUT incorrectly connected to your micro.  MISO should connect to DOUT and MOSI should connect to DIN.

    Best regards,

    Bob B 

  • 0 in reply to Bob Benjamin
    Prodigy 180 points

    Hi Bob,

    Thanks for the reply.

    The signals were probed on the microprocessor side of the 47R resistors.

    CS is being toggled. There is only 1 thing on the bus.

    The odd thing is that another sample with the identical layout works fine.

  • 0 in reply to Paul Dye
    TI__Guru** 113765 points

    Hi Paul,

    I should have looked more closely at the scope shots. DIN and DOUT are shorted together on the shots that show bad reads and writes. I would check with a microscope or good magnifying glass for a solder short some where between these traces from the resistors to the device pins of the ADS1120. There could also be a short near a via or an improper short between traces on the PCB. I've see cases where there are small shorts between adjacent traces due to improper etching on the PCB.

    Best regards,
    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 180 points

    Hi Bob,

    Thank you very much for your response, you were right there was short somewhere underneath the microprocessor!

    Really appreciate your help.

    Regards

    Paul