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ADS1298 Lead - off detection

Other Parts Discussed in Thread: ADS1298

Hi,

I have configured the ADS1298 for lead - off connection. The leads are connected as follows:

Following configuration for lead off detection: (Using pull-up resistors).

1) 0xFF; // LOFF_SENSP

2) 0xFF; // LOFF_SENSN

3) 0x00; // LOFF_FLIP

4) 0x02; // CONFIG4. - Turn-On DC lead-off comparators.

I am always reading 0x00, 0x00 in the status field even-though none the electrodes are connected. Let me know if there anything wrong with the configuration or any issues with the hardware?.

Regards,

Purushottam

  • Hello Purushottam,

    I've tried a couple of different browsers but for some reason I am unable to see the photo you uploaded. Perhaps try uploading it using a different file format (I'm not totally sure what is causing that but I will investigate). In the meantime, I have a question regarding your register settings. Have you made the appropriate changes to the LOFF register (address 0x04)? The default setting in this register is to perform current source lead-off. If you wish to perform pull-up/down lead-off detection, you must set bit 4 of the LOFF register.

    Regards,
    Brian Pisani
  • Hi,

    Thanks for your reply.

    The LOFF register is configured for 0x13.  IMG_20150330_155023.pdf

  • Hey Purushottam,

    The lead-off detection will never trigger since you are forcing the common-mode with your biasing resistors. If the lead is off, the voltage at the input to the ADC will be the common mode voltage you are setting and the lead-off detection comparator never reaches its trigger level. You could potentially use the AC lead-off detection feature on this device, but I would recommend looking carefully at how your passive component network would affect the output AC signal that are used for the impedance measurement. Let me know if you need more assistance.

    Regards,
    Brian Pisani
  • Thanks for the reply. The passive components are added to avoid baseline drift. Do you any suggestion to correct the baseline drift?.

    On removing these passive components, Lead-Off is detected properly.

  • Hey Purushottam,

    The benefit to using such a high resolution ADC is that it's not necessary to remove the offset from your input signal since the dynamic range is large enough to capture the signal even with such a large DC offset! As long as your common mode falls somewhere in the valid common mode range for the device, there is no need for the DC blocking capacitor and common mode setting resistors. If you wish to remove the DC offset, you can do so digitally. To maintain the common mode in the valid region, I'd recommend using the right-leg drive amplifier on this device. It is meant to stabilize the common-mode voltage on the patient.

    Regards,
    Brian Pisani