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ADS1291: Electrode Configuration Mode Guidance

Tomisin Sanni
Prodigy 30 points
Part Number: ADS1291
Other Parts Discussed in Thread: ADS1292, TIDM-BIOSIGNMONITOR

Tool/software:

Hello,

I’m seeking advice on whether the ADS1291 IC will be suitable for my application and would appreciate some guidance on its implementation. My goal is to measure ECG signals using a single lead that can be reconfigured to work with either a 2-electrode setup (without RLD) or a 3-electrode setup (with RLD).

Since the AD8232 has two input channels, my plan is to feed the two electrodes into one channel (externally AC-coupled) while using the RLD to internally set the common-mode voltage. Then, I could feed the second channel with the same two electrodes but without any coupling, with the RLD configured as an output to drive the third electrode for the 3-electrode configuration.

Both configurations would never be used simultaneously, so I’m aiming for a reconfigurable system whereby I can switch the RLD function in software. Is this feasible with the ADS1291? If so, could you point me toward any resources or explain how I might achieve this setup?

Thank you for your help!

  • 0
    TI__Guru 66156 points

    Hello Tomisin,

    Thank you for your post and please excuse the delay during summer holidays.

    The ADS1291/2 does not have an internal provision to bias the electrode inputs to RLD. There would need to be an external pull-up resistor from each INxP/N pin to RLDOUT, which you could depopulate in the configuration which uses a dedicated RLD electrode.

    The ADS1292 does have an auxiliary input pair for applications which do not use the internal respiration feature. The IN3P/N inputs could be used as your AC-coupled inputs, each with a discrete pull-up to RLDOUT. Your IN1P/N and IN2P/N inputs could be used as DC-coupled inputs. The RLD output can then be connected to RLDIN and steered to any of those 4 DC coupled pins to be used as the RLD electrode (i.e. LA/RA measured on CH1, IN2P used as RLD).

    Does that sound like something which would work for your application?

    Regards,

    Ryan

  • 0
    Prodigy 30 points

    No worries at all Ryan, thanks for getting back to me.

    Ryan Andrews said:
    The ADS1291/2 does not have an internal provision to bias the electrode inputs to RLD. There would need to be an external pull-up resistor from each INxP/N pin to RLDOUT, which you could depopulate in the configuration which uses a dedicated RLD electrode.

    This is not viable unfortunately, as the system must be reconfigurable for either 2 or 3 electrode use on the fly. So would an analog switch be useful here to connect or disconnect these external bias resistors?

    Ryan Andrews said:
    The ADS1292 does have an auxiliary input pair for applications which do not use the internal respiration feature. The IN3P/N inputs could be used as your AC-coupled inputs, each with a discrete pull-up to RLDOUT. Your IN1P/N and IN2P/N inputs could be used as DC-coupled inputs. The RLD output can then be connected to RLDIN and steered to any of those 4 DC coupled pins to be used as the RLD electrode (i.e. LA/RA measured on CH1, IN2P used as RLD).

    This was slightly confusing at first but I think I understand what you mean now. I imagine it would also work by using IN1P and IN1N as AC-coupled inputs, then use IN2P, IN2N and IN3P as DC-coupled inputs. Therefore, I can steer the RLDOUT into IN3P (via RLDIN) to use as the RLD electrode?

    On the other hand, it looks like a similar idea is implemented in the TIDM-BIOSIGNMONITOR reference design. IN1P and IN1N are AC-coupled and mid-rail biased, allowing you to use only two electrodes, while IN2P and IN2N are DC-coupled. However, it looks like the right electrode is output straight from the RLDOUT pin (as well as what looks like some  filtering) but also connected to the RLDIN pin via a 100k resistor. So is this system able to support both 2- and 3-electrode configurations without needing to depopulate components or using the auxiliary IN3P/N inputs as an RLD electrode output? And if so, can you explain how? 

    Does this also mean that in that design, if the RLD electrode is detached and the ECG readings are poor from the DC-Coupled channel as a result, I can just swap to the AC-Coupled channel instead? Do you know if this would run into any issues regarding regulatory approvals?

    Thanks again for your help in advance!