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ADS1292R: 3 lead ECG input circuit - coupling and anti-aliasing

Piotr Ladnowski
Prodigy 10 points
Part Number: ADS1292R
Other Parts Discussed in Thread: ADS1292

Tool/software:

Hi TI team,

I was recently studying the reference design for the ADS1292R that you are sharing in this document (page 9):
https://www.ti.com/lit/ug/tidu875/tidu875.pdf?ts=1732700470600

This made me wonder whether you could tell me why is there a difference in copupling between channels? Both the first and the respiration inputs are AC coupled, while the second input is DC coupled. I did also notice that the anti-aliasing filtering is applied only to the 2nd input. What are design guidelines for choosing the coupling method and applying the aa filtering?

Looking forward to your reply,

Piotr Ładnowski

  • +1
    TI__Guru 66906 points

    Hello Piotr,

    Thank you for your post.

    The RESP_MODP/N pins are outputs which are transmitting an excitation signal to the body. This excitation signal is modulated by the slow changes in thoracic impedance due to patient respiration. This same signal is measured through the IN1P/N inputs and demodulated to reveal the respiration impedance waveform. Both the TX and RX paths are AC-coupled to prevent DC current from flowing as only the AC signal is of interest. Typically, an antialiasing LPF is not used here as any added impedance in the path will add noise to the sensitive measurement.

    ECG inputs can remain DC coupled. The common-mode voltage is typically established by the driven RL electrode such that the input signals are within range for the PGA on each channel. A single pole LPF can be added here with a cutoff ~ 10 kHz.

    For more details, please refer to the ADS1292 device data sheet.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 10 points

    Many thanks Ryan, this is just what I needed.