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Original question:

ADS1298 RLD lead-off detection

  • Resolved

ADS1298: Getting RLD lead status without reading the ADS1298 registers

Prodigy 60 points
Winston Rodrigues

Replies: 3

Views: 195

Part Number: ADS1298

Hi,

  I am able to get the Lead off status of all the Leads except RLD. For other leads I am using pull up / pull down method. And it works great so far

  I am reading the data at 512 samples / sec in RDATAC (read data continuous) mode. 

   As per I understand from the datasheet and by going through other threads that to get RLD status we need to power down the RLD amplifier and get the status by reading the CONFIG4 register. But in order to do that I must send SDATAC command first to stop the conversion and do the above mentioned things. Am I right ?

  If that is the case I really would not like to do that. 

   I had another idea please tell me if this works.   

   In the below schematic RLD_LPF / RLD_OUT (marked in Red)is connected to hardware low pass filter in turn it connects to patient. I observed that if RL lead gets loose the voltage on that pin goes to -2.5v almost immediately. So Can I monitor this pin by my host processor STM32F4 using a hosts 12 bit ADC ? and decide the lead off status independently ? Or do you see any problem with this approach ? 

 Regards,

   Winston

  marker

  

   

Prodigy 60 points
Winston Rodrigues
  • Mastermind 37470 points
    Ryan Andrews
    Hi Winston,

    Thank you for your post!

    Yes, you must send the SDATAC command before any other commands, including WREG to change register settings. This is what you would need to do in order to power-down the RLD amplifier and check the lead-off status. The uncertainty in this case is that there is no other source to drive the common-mode voltage of the body. If other input lead-off sources are enabled (a combination of pull-ups and pull-downs), this may be enough to hold the body near mid-supply so that the RLD lead-off comparator does not give you a false-positive while connected.

    There are other ways to monitor the RLD connection status, similar to what you've described. Some users have found that using all pull-ups, for example, can be used to indicate RLD lead-off since all inputs will show "disconnected" at once whenever RLD is disconnected.

    When you monitor the RLD amplifier output voltage before/after disconnecting the RLD electrode, do you have any of the input electrodes selected for RLD common-mode derivation (i.e. any RLD_SENSx bits enabled)?

    Best regards,

    Ryan Andrews

    Applications Engineer | Precision ADCs 

    Are you working on a bio-potential application? Check out these helpful resources: ADS129x FAQ | ECG Online Training

  • Prodigy 60 points
    Winston Rodrigues

    In reply to Ryan Andrews:

    Hi Ryan,

    Thanks for the response and sorry for delayed response from my side.

    Yes. RLD_SENSx bits are enabled as below when I was measuring the RLD voltage ..
    RLDSENSN = 0x06
    RLDSENSP = 0x06

    I do not want to complicate the firmware by using SDATAC then read reg method. Because in future I am going to use sampling rate of 8K s/sec for pacer detection.

    When I disconnect RLD none of the leads are pulled up / pulled down. But only RLD pin pulled down -2.49v.

    So I am thinking of going ahead with my approach. But I guess I need to add an inverter to invert the signal from negative to positive because my STM32F4 doesn't have a negative ADC input.
    Instead of opamp Can I use resistor divider circuit to get the signal above ground level ?

    Regards,

    Winston
  • Mastermind 37470 points
    Ryan Andrews

    In reply to Winston Rodrigues:

    Hi Winston,

    The other leads will only trigger lead-off when RLD is disconnected IF they are enabled for lead-off detection. Verify that the LOFF_SENSx bits and the LOFF register are configured correctly.

    It makes sense that the RLD output approaches -2.5 V when the electrode is disconnected because there is a current sink on the RLD amplifier output. This will pull the RLD output down to AVSS. See Figure 42 in the data sheet. I suppose a resistor divider between AVDD and AVSS with the correct ratio will allow you to monitor the output with your MCU. Alternatively, you can feed the level-shifted signal into one of the GPIO pins on the ADS1298 and read the status with the conversion data in the STATUS word.

    Best regards,

    Ryan Andrews

    Applications Engineer | Precision ADCs 

    Are you working on a bio-potential application? Check out these helpful resources: ADS129x FAQ | ECG Online Training

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