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ADS1198: ADS1198 RLD OFF

Vladimir Gomon
Prodigy 30 points
Part Number: ADS1198

Tool/software:

Hi. Would you please clarify how to activate RLD OFF with a build-in RLD comparator?

At the beginning I initialize ADS1198 with CONFIG3 = 0xEC

When I want to check RLD electrode status, I stop continuous reading SDATAC, then write to the register CONFIG3 0xEB (or 0xEA) instead of 0xEC, to switch off RLD buffer and enable RLD comparator.  (switch off RLD buff bit2 = 0, enable RLD_SENS bit1 = 1).
Than I wait for 50ms and read CONFIG3 register. IF RLD electrode OFF I should read 0xEB, if RLD electrode ON I should read 0xEA. 

After that I write to CONFIG3  = 0xEC again, and start continuous read mode RDATAC. 
 
Am I right?

What I get:
-- when I work with ECG simulator, I read CONFIG3 = 0xEA when RLD ON and 0xEB when RLD OFF (other electrodes ON - OFF system works perfect). It should be vice versa, as far as I understand? 
--when I try RLD off at the body with real electrodes: I read 0xEA, sometimes 0xEB without any correlation with RLD status. 

Would you please suggest where I am wrong?

With regards

Vladimir

  • 0
    TI__Guru 66446 points

    Hello Vladimir,

    Thank you for your post.

    Yes, your RLD lead-off routine sounds correct. Note that bit 0 is read-only and does not need to be written to 0b to be cleared. You should only be writing 0xEA for RLD lead-off detection and 0xEC to resume normal operation. Once configured for RLD lead-off (0xEA), then you can poll this register to detect if there is any change in bit 0. Reading a value of 1b from bit 0 indicates a lead-off event has occurred, so you should expect to read 0xEB when the RLD electrode is disconnected.

    When the RLD amplifier is powered down, there is no driving source to define the common-mode voltage at the RLD amplifier output. The "lead-off status" will largely depend on how quickly the voltage at this node discharges to AVSS through the current sink when the lead is on vs. off. You can try adjusting the lead-off current magnitude or comparator thresholds to adjust the sensitivity.

    A more robust approach might be to enable lead-off detection only on input electrodes with the same polarity lead-off current (i.e. all pull-up or all pull-down). In the event the RLD electrode becomes disconnected, those input electrodes should indicate lead-off simultaneously, indicating the common root cause is the RLD electrode itself. This method would allow you to keep the RLD amplifier enabled at all times.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 30 points

    Hi Ryan. 
    Thank you for your reply. 

     more robust approach might be to enable lead-off detection only on input electrodes with the same polarity lead-off current (i.e. all pull-up or all pull-down). In the event the RLD electrode becomes disconnected, those input electrodes should indicate lead-off simultaneously, indicating the common root cause is the RLD electrode itself

    I observed this, but I can't rely on this, because the appearance is not regular. Sometimes all minus electrodes fall down, sometimes nothing happens. 

    When the RLD amplifier is powered down, there is no driving source to define the common-mode voltage at the RLD amplifier output. The "lead-off status" will largely depend on how quickly the voltage at this node discharges to AVSS through the current sink 

    I see RLD OFF confident behaviour only when I connect RLD wire to AVDD  - I always see stable 0xEA and when I connect it to AVSS   - I always see stable 0xEB. Does it mean I have to insert a longer (comparing with other electrodes) delay? For other electrodes I use 30..50ms delay between setting current source and result reading. What delay for RLD seems relevant to you? 

    With best regards
    Vladimir