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ADS1298: Daisy-Chain - RLD question

Brian Angiel
Expert 4070 points
Part Number: ADS1298

Hi team,

I'm looking at ADS1298 for an upcoming project. For this project, I want to daisy-chain two ADS1298 chips to reduce the signal count to control them. As described in section 9.4.2.2 (pages 57-58) of the datasheet, daisy-chaining requires that each chip be configured in the same way. It also says that the “RLD driver cannot be shared among the multiple devices.” Could you provide more detail on this statement?

Looking at Figure 45 on page 43, it shows the RLD (right-leg drive) connection for multiple devices. Device 1 is used to generate the RLD signal and the inputs from the other channels on the other devices are shared using RLDINV. The RLD op-amp on devices 2-N is powered down. If the devices are daisy-chained, would it be okay for the RLD op-amp on devices 2-N to be powered on? The RLDOUT, RLDREF, and RLDIN pins would not be used.

Thanks,

Brian

  • 0
    TI__Guru 66446 points

    Hi Brian,

    Thanks for your post.

    The intention behind Figure 45 is to illustrate a way to include electrode inputs from other ADS129x devices in deriving the common-mode cancellation signal. You would not want multiple RLD amplifier outputs actively driving the same electrode, so that is why it is discouraged to connect the RLDOUT pins from multiple ADS129X devices. The nodes "VA1-8" represent the 220k resistors at the output of each individual PGA amplifier (one on the positive output, one on the negative output). Effectively, the diagram below is just adding more signals in parallel to the summing node at the RLD amplifier inverting input (RLDINV) on Device 1.

    In the event that the RLDOUT, RLDREF, and RLDIN pins are not used on the daisy-chained devices, it would be fine to turn on those respective RLD amplifiers, but you're just wasting power at that point.

    Best regards,