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Original question:

ADS1299: Multiple ADS1299 in Daisy Chain

ADS1299: Daisy chain termination

Metin Ozbilen
Prodigy 20 points
Part Number: ADS1299

Hi,

We are desiging three daisy chained device, but want to use same PCB for single snd double chip designs. My question is instead of grounding daisy input if we connect a large enough pull down resistance do same operation. Can I still read the data? Is it possible to get next acquisition?

Thank you,

Metin

  • 0
    TI__Guru 57326 points

    Hello Metin,

    Thank you for your post.

    You are suggesting to use a pull-down resistor on all DAISYIN pins, while also connecting each DAISYIN to the next DOUT in the chain? I think that would work fine, as long as the pull-down is large enough.

    When another ADS1299 DOUT pin is trying to drive the DAISYIN pin of the next device in the chain, make sure the pull-down is large enough so as not to exceed the maximum digital I/O current in the data sheet (+/-10 uA). For a 3.3-V DVDD supply, this requires at least 330 kOhms. I would use 499k-1M to be on the conservative side.

    When there is not another ADS1299 device driving DAISYIN (i.e. the last device in the chain), the pull-down resistor can be replaced with a 0-ohm, if desired.

    Best regards,

  • 0 in reply to Ryan Andrews
    Prodigy 20 points

    Hi again,

    Thank you for your answer, if I still use same resistor on the last device in the chain, do I still get data right? Can I understand end of data by examine 0s? Can the daisy chain work replacing daisy in GND with pull-up resistor like you suggested?

    You answer is very critical for me? Change the way of my design? 

    Regards,

    Metin

  • 0 in reply to Metin Ozbilen
    TI__Guru 57326 points

    Hi Metin,

    I believe this will still work when you use a pull-down resistor on the DAISYIN pin on the last device in the chain. However, you do not need to rely on finding all 0s to know when the data read is complete. Instead, you should know exactly how many SCLK periods to send. You will have to read 9 words x 3 bytes/word x 3 devices = 81 bytes for 3 ADS1299 devices.

    Best regards,