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ADS1299: Lead-off using external resistors

Bryan Courtney
Prodigy 120 points
Part Number: ADS1299

Hi,

I have attached external resistors to detect the lead-off. I am measuring the DC offset when leads are off and only getting -200mV offset.

I started with 100Megs to reduce patient leakage. Then i increased it to 10M as per the datasheet but still not getting pulled low enough.

Am i missing something?

Thanks

bryan

  • 0
    TI__Guru 57306 points
    Hi Bryan,

    Thanks for evaluating our ADS1299.

    Are these pull-up or pull-down resistors? Do you still have the internal lead-off current sources turned on? Are you measuring the DC voltage of each input with respect to GND or AVSS?

    Please share the ADS1299 portion of the schematic and the register settings so that I can look into this further for you.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 120 points

    These are both pull-downs (-2.5V and a -3.3V) at 100Mohm. The current sources are turned OFF. I am measuring the DC voltage with respect to GND.

    I am measuring the input impedance to the channels to GND and only measuring 10-20Mohm (datasheet says 500-1000Mohm). This is causing a voltage divider situation and my pull-downs are not getting pulled down enough for our needs. Why am i seeing 10Mohm? 

    Bryan

  • 0 in reply to Bryan Courtney
    TI__Guru 57306 points
    Hi Bryan,

    The input impedance spec is the differential input impedance between the INxP and INxN pins of each channel, not from each input pin to ground.

    I still don't understand how you have these pull-down resistors configured. You have one pull-down resistor on each input pin, correct? Why are some inputs pulled down to -3.3V? This would exceed the negative analog supply (AVSS) by more than the Absolute Maximum Rating (AVSS - 300mV). -2.5V makes sense if you are using a bipolar 5-V supply (+/-2.5V).

    If you prefer to share your schematic via email, please let me know and I will contact you offline.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 120 points
    email would be best
    thanks