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ADS1299 - DC lead-off detection

Ashutosh Kulkarni
Expert 1005 points
Other Parts Discussed in Thread: ADS1299

Hi,

I am working on DC lead-off detection in ADS1299.

I have configured ADS as per the datasheet sequence given on page 59 -

WREG LOFF 08h // Comparator threshold at 95% and 5%, current source = 6uA
WREG CONFIG4 02h // Turn-on dc lead-off comparators
WREG LOFF_SENSP FFh // Turn on the P-side of all channels for lead-off sensing
WREG LOFF_SENSN FFh // Turn on the N-side of all channels for lead-off sensing

This is sort of working but i am getting only Lead off detection in LOFF_STATP bits but not in LOFF_STATN register bits. I get all 0xFF in LOFF_STATN registers though sensors are not connected at all. But get proper bits in LOFF_STATP register for particular sensor not connected.

I think I should get respective bits in LOFF_STATP and LOFF_STATN registers as 0, if the sensor is not connected.


I tried to reverse the direction of current by setting LOFF_FLIP register to 0xFF but no difference.

Am I doing something wrong here / missing in terms of understanding?

Thanks,

Ashutosh.

  • 0
    TI__Mastermind 38745 points

    Hey Ashutosh,

    You should expect the bit in the LOFF_STATP/N register to become set when the electode is no longer connected. Referring to your situation, when the negative side sensors are disconnected and the LOFF_STATN register becomes 0xFF, that is expected behavior. If the P side remains as 0x00 even when electrodes are unconnected, I'd like to look at your schematic to see if I can tell what is happening there.

    Regards,

    Brian Pisani

  • 0 in reply to Brian Pisani
    Expert 1005 points

    Hello Brian,

    Thanks for the reply.

    Just to elaborate further, I have connected below circuit which simulates Skin, Electrical contact model to VINP and VINN pins of ADS1299. So the junction of 51K and 47nF is going to the VINP / VINN pins of ADS1299.

    When this circuit is not present at the ADS inputs I get particular channel bit in LOFF_STATP and LOFF_STATN as '1'.

    But when this circuit is present at both VINP and VINN pins of the ADS inputs, I get ONLY LOFF_STATP bit for particular channel as 0, which is as expected but LOFF_STATN bit for that particular channel remains '1', so this is not expected.

    As explained earlier, i tried by reversing the current direction by setting LOFF_FLIP register, but no change in the LOFF_STATN bits.

    Thanks,


    Ashutosh.

  • 0 in reply to Ashutosh Kulkarni
    Expert 1005 points
    Hi Brian,

    Please note that I havent connected anything to BIAS OUT pin and my ADS1299 EVM has default jumper settings as far as BIAS connections are concerned.

    I dont know what to do there and what to expect, so i havent touch that part of hardware.

    Thanks,

    Ashutosh.
  • 0 in reply to Ashutosh Kulkarni
    Expert 1005 points
    Hi Brian,

    I have solved this by connecting BIAS OUT signal from ADS1299 to the reference electrode / sensor as shown in fig. 48 in the datasheet.

    Now I get proper LOFF_STATP and LOFF_STATN bits when sensor are connected / disconnected.

    Thanks for your help.

    Ashutosh.
  • 0 in reply to Ashutosh Kulkarni
    Prodigy 130 points
    Hello,

    Can you please elaborate on how you were able to solve the problem? How were you able to connect the BIAS_OUT signal from the ADS1299 to the reference electrode?

    I have been looking at figure 48 to try to understand what you mean but I have not been able to do so.

    Thank you!