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ADS8685: Shared SPI Interface

Brian Gosselin
Expert 4745 points
Part Number: ADS8685

Hi Team,

My customer desires to use the ADS8685 in a shared SPI interface mode for multiple devices, but not using the daisy-chain topology.  It appears that ADS8685 is compatible with standard SPI interface and I see that it has a /CS input.  My thought is you could share the CLK, SCLK, DIN, and DOUT from each ADS8685 device with independent /CS for each ADS8685 device as you normally would for SPI.

Can you confirm my understanding?

Best Regards,

Brian Gosselin
TI Analog FAE

  • 0
    TI__Expert 3462 points

    Hi Brian,

    Depending on how the customer will need to acquire the data from the multiple ADS8685 ADCs, the digital topology will differ. Below I explain three different scenarios:

    1. If all ADS8685 ADCs will be sampled at the same point in time with the same settings, then the SCLK, CS, and SDI can be tied together. This would require separate SDO lines from each ADC.

    2. If the ADS8685 ADCs may or may not be sampled at the same point in time then a separate CS will be required for each ADC as well as separate SDO lines so that if they are sampled at the same point in time then the output data from each can be collected separately. SCLK and SDI can still be common to all ADCs.

    3. If the ADS8685 ADCs will not be sampled at the same point in time then SCLK and SDI can be common to all ADCs with separate CS and SDO signals to allow the one selected ADC to be sampled at a time. The SDO lines are not recommended to be connected together since the output state is not guaranteed to be high-impedance when the ADC is not operating.

    One other concern that comes up when driving multiple ADCs with the same SCLK, SDI, or CS is to make sure the digital interface (typically FPGA) is able to drive all of the ADCs at once. Adding more and more ADCs will increase the total trace length and trace capacitance so it may be necessary to use a digital buffer to ensure correct digital signaling to all of the ADCs.

    Please reach out if you have any follow-up questions.

    Thank you,

    Reed Kaczmarek