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ADS1291 lead-off resistor or current source mode?

Other Parts Discussed in Thread: ADS1291, ADS1292, ADS1298

I am trying to use the lead-off functionality on the ADS1291. I am using 3.3V digital supply and a +/-2.5V bipolar analog supply. I am able to acquire the ECG signal perfectly fine, but I cannot get lead-off detection to work: the LOFF_STAT bits always return zero (leads-on status). My relevant register settings are as follows:

LOFF: 0x10

CONFIG2: 0xE0

LOFF_SENS: 0x03 

I am not using pull-up/pull-down resistors on the input lines and the inputs are not ac-coupled. I have a lowpass filter on each input and nothing more. I am using dry stainless steel electrodes. I noticed on the ADS1298, there is an additional setting on the LOFF register to select VLEAD_OFF_EN for resistor or current source mode. This option does not seem to be available on the ADS1291/ADS1292. Is there a reason for this? Is only one of these options possible? It's not clear from the datasheet.

Thank you!

  • Hey ccent,

    Your register settings look good. Have you tried decreasing the comparator threshold or increasing the current magnitude in the LOFF register? Are you sure there is nothing on the electrode path that could be leaking any current (i.e. protection diodes)?

    On the ADS1298, there are internal pull-up/down resistors that can be used to do pull-up/down lead-off detection internally. On the ADS1291 there are the internal current sources but no pull-up/down resistors. Therefore there is no option to switch between them. Pull-up/down lead off detection can still be performed on the ADS1291 with external resistors.

    Regards,
    Brian Pisani
  • Hi Brian,

    Thank you for the reply. I tried changing the comparator threshold with no success. When I changed the current to 6uA, one of the leads indicated that it was off, while the other still showed that it is on. However, when I connected the electrodes to the body, the lead status did not change. 

    I only have two RC lowpass filters consisting of 1Mohm and 2200pF components on each ECG input line. Do you have any suggestions regarding possible leakage pathways?

    Thank you!

  • Hey ccent,

    With only a series resistor and shunt capacitor in the path of the inputs, I doubt those components would ideally be leaking 6 uA of current. To ensure that it is not a device issue, you could probe the inputs to the device when the leads are off. You'd expect the lead-off current sources to saturate the channel so the input voltages should be voltages far from mid-supply when the electrodes are disconnected. Have you tried a different board? Perhaps there is a solder bridge somewhere on the one you were using.

    Regards,
    Brian Pisani