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ADS1278 one channel not working properly

TAO DENG
Prodigy 40 points
Other Parts Discussed in Thread: PGA281, ADS1278

The chip ADS1278 was used in our product for data aqquisition. Since the prototye design stage(about 20pcs), there are several ICs functioning abnormally, mostly one channel not working properly. The collected data of this channel is of full scale 3V(VREF 3v), test condition is floating differential input(each pin is 10MΩ resistor paralleled to GND) of pre-amplifier. Normally the data is of small amplitude around ground level. Pre-amplifier IS is PGA281.

  • 0
    TI__Mastermind 42795 points
    Hello,
    I am moving this post to the precision data converters forum for you.
  • 0 in reply to Eric Wright
    TI__Guru 67206 points
    Hello Tao,

    Could you please share a schematic of your test circuit setup and a scope capture of your results? Also, does this happen to the same channel on every device that you test?

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 40 points

    Hello Ryan Andrews,

    Thanks for your reply. The schematic is as follows(ADS1278 and PGA281), REFP of ADS1278 is connected from IC ADR443BR(3v voltage ref), not presented in the schematic. ADS1278 is controlled by FPGA chip.

    Please take a note that  Vref of PGA281 is with another 2.5V voltage ref. Possibly It may cause bad result. The captured waveform(dc coupled, 1V/vertical, 250us/horizontal) at input + of first channel of ADS1278 is as below, same as othe seven channels.

    The problem is only with 3 or 4 boards, not all prototype. The 3v full scale amplitude of abnormal channel is not captured by scope, but read out from FPGA program(output to PC through RS232). The simple block diagram as below:

    Best Regards,

    Tao DENG

  • 0 in reply to TAO DENG
    TI__Guru 67206 points
    Hello Tao,

    The VOCM pin of the PGA281 sets the output common-mode voltage. This means that the output signals on both VOP and VON will be centered around VOCM, which is 2.5V in your application. Therefore, I expect AIN1P to look like the scope capture you are showing.

    I'm not sure I understand the problem. For the channels that are "failing," what is the measured output voltage of the PGA281 and what is the output of the ADS1278 for those channels?

    Best Regards,
  • 0 in reply to TAO DENG
    TI__Guru 71526 points
    Tao,
    Did you resolve this issue?
    ~Leonard