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ADS8688: daisy-chain only works with up to 8 ics

Eva Dtlj
Prodigy 30 points
Part Number: ADS8688

I have custom boards with 2 daisy-chained ADS8688 devices each, and I would like to daisy-chain 7 of these boards together. Which would be a daisy-chain of 14 ADS8688 devices in total. 

I am talking to the boards with a single SPI interface on an Arduino Due.

Everything works with 4 boards (8 ADS8688). But when I add another board (10 ADS8688) the analog data from the last 2 devices is wrong.
It looks like the analog value of the first pin on device 9 is copied to all the other pins of device 9 as well.
All the values of device 10 resemble each other as well, but I can't tell right now if they are tied to a specific input pin. 
(These values are very close to each other, but are not exactly the same.)

I am aware that more devices mean a higher load on the SPI lines. And that you recommend digital buffers. Which I haven't implemented yet. 
Before I attempt that, I wanted to ask if there are any other limits as to how many ADS8688 devices can be chained together?

  • 0
    TI__Mastermind 39075 points

    Hello Eva,

    Welcome to E2E community.

    Can you please let me know what your SCLK frequency is? and what's your sampling rate for all ADS8688s?

    Thanks.

    Best regards

    Dale

  • 0 in reply to Dale Li
    Prodigy 30 points
    Hello Dale,

    The SPI communication runs at 17MHz
    and I get to a sampling rate of ~1811Hz for 112 sensors (14x8)

    thanks,
    Eva
  • 0 in reply to Eva Dtlj
    TI__Mastermind 39075 points

    Hello Eva,

    I apologized for late response, and pasted days were holiday in US.

    Please notice that higher number ADS8688 devices than FOUR in daisy-chain will lead to timing errors as mentioned in datasheet(page 36). Also, your devices are not designed on same circuit boards, this may cause timing issues by the signal routing.

    Best regards

    Dale

  • 0 in reply to Dale Li
    Prodigy 30 points

    Hello Dale,

    Yes I noticed that in the datasheet. And it surprised me that I got 8 daisy-chained devices to work.
    Could you describe how those "digital timing errors" might manifest? Something obvious to look for?
    I haven't fully setup my prototype yet (attach all sensors) in order to look out for noise, but I will do so soon.

    If I would add digital buffers to the daisy-chain, how many would you recommend, to add at what interval?

    thanks,

    Eva

  • 0 in reply to Eva Dtlj
    TI__Mastermind 39075 points
    Hello Eva,
    The errors will be setup and hold time violations leading to incorrect data capture by the host controller.
    We do not have any performance data captured in this mode. It depends on the board routing and 4 was an approximate number for daisy chaining. In this case it could connect more devices in daisy chain. We also do not have data wrt where we should include digital buffers. These tests were not done.

    Thanks.

    Regards
    Dale