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ADS1293EVM: The device cannot report lead off information when the DC LOD detection current lower than 100nA

John11513
Expert 5360 points
Part Number: ADS1293EVM
Other Parts Discussed in Thread: ADS1293

Hi,

My customer reported an issue with ADS1293: the DC LOD function cannot work properly when the detection current is configured lower than 100nA. While the function seems correct once the detection current is high, such as 0.32uA. However, Chinese regulation require the current should not exceed 0.1uA. They had tested the LOD function both on their own PCB and on our EVM with same configuration. The issue is same. (The test was done at room temperature) I am not sure how should they close this problem?

Thank you!

John

  • 0
    TI__Genius 11550 points

    Hi John,

    Thanks for the support.

    The ADS1293 DC lead-off will trigger a lead-off condition on the associated input pin whenever the pin voltage exceeds VDD-0.5V. Depending on the design of the input, the smaller current may lead to a longer settling time due to external capacitance. The lead-off detection circuits also require a low impedance return path for the LOD currents for proper operation.

    Thanks

    -TC

  • 0 in reply to TCT
    TI__Expert 5360 points

    However no mater how long time it always cannot report lead off condition. Except lead off electrode, all other electrodes are shorted to right leg electrode, so there is no issue with low impedance return path.

    Thank you!

  • 0 in reply to John11513
    TI__Genius 11550 points

    Hi John,

    The leadoff condition will be triggered when the input pin voltage rises towards VDD. There may be other leakage paths for the leadoff current in the system, preventing the input voltage from rising and triggering the leadoff condition. It may be helpful to check the input pin voltage with a high impedance voltmeter (>GΩ) and verify that this input voltage should be close to VDD.

    Thanks

    -TC

  • 0 in reply to TCT
    TI__Expert 5360 points

    Hi TC,

    A further test shows it is sure the issue is not due to leakage. 

    The LOD always working properly if one input is only connected to one INA input. However, the device cannot report lead off condition once the input is connected to two INA inputs until the detect current increased up to >2uA.

    For example, if RA electrode is connected to IN1 pin, and IN1 is configured to connect to CH1- and CH2- in parallel, the device cannot report leadoff condition of RA. While if we only connected IN1 to CH1-, the device can correctly report RA lead off condition with only 96nA LOD current.

    Thank you!

    John

  • 0 in reply to John11513
    TI__Genius 11550 points

    Hi John,

    Have they tried turning on only one DC leadoff detect circuit (either CH1- or CH2-) when both channel inputs are connected to the IN1 pin or RA electrode? There should not be any difference if the RA electrode is connected to CH1- only or to CH1- and CH2- in parallel. The voltage at this pin should rise towards VDD when the lead is off.

    Thanks

    -TC