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ADS1298 RLD lead-off detection

Carlo Nenna
Prodigy 40 points
Other Parts Discussed in Thread: ADS1298

Dear all,

 

I'm trying to understand how to obtain the status of the RLD with ADS1298. Reading through the datasheet my undertanding is that bit0 of Config3 register should provide the status of RLD, but it's not clear to me what shall be done to obtain a reliable information from that bit. I tried to play with LOFF and Config3 registers, but I could not find the correct sequence of values to be programmed to obtain a reliable RLD status (sometimes I get bit 0 of config3 register reporting RLD connected while it is not, and viceversa. Some other times bit0 reports a correct status...but it's quite random..)

 

please, can anyone advice on the correct sequence of values to be programmed into the relevant registers?

 

Thanks in advance

 

regards,

 

carlo

  • 0
    TI__Mastermind 43915 points

    Hi Carlo,

    For right leg drive lead off detection, three things need to be done/set; The RLD drive internal amp needs to be powered down, the RLD_LOFF_SENS bit needs to be enabled, and the VLEAD_OFF_EN bit needs to be set either high or low depending on which method of lead off detection you would like to do. If you are randomly reading back the bit0 set as high or low regardless of if the lead is connected, it sounds like you do not have the RLD_SENS bit set high. The RLD_LOFF_SENS bit is used to connect the line to the comparator for the lead off detection to take place. If not set properly, the comparator will see a 'floating' input from the RLD signal as it is not internally connected without that specific bit set which would lead to the behavior it sounds like you are experiencing. 

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 40 points

    Hi Tony,

    I tried to do as you said, which was somehow what I did already in my first test, but still no luck. Actually, here's exactly what I'm doing:

    1) power on the ADS1298 and read all registers:

    ID                0x92   
    CONFIG1            0x06   
    CONFIG2            0x00   
    CONFIG3            0x40   
    LOFF            0x00   
    CH1                0x00   
    CH2                0x00   
    CH3                0x00   
    CH4                0x00   
    CH5                0x00   
    CH6                0x00   
    CH7                0x00   
    CH8                0x00   
    RLD_SENSP        0x00   
    RLD_SENSN        0x00   
    LOFF_SENSP        0x00   
    LOFF_SENSN        0x00   
    LOFF_FLIP        0x00   
    LOFF_STATP        0x00   
    LOFF_STATN        0x00   
    GPIO            0x0F   
    PACE            0x00   
    RESP            0x00   
    CONFIG4            0x00   
    WCT1            0x00   
    WCT2            0x00

    2) set CONFIG3 to 0xCA

    3) read back CONFIG3: 0xCA, that is RLD_STAT bit is reporting RLD connectd, but no cable is connected to ADS1298 board!

    4) set CONFIG4 to 0x2 (LOFF comparators enabled), and reading  CONFIG3 i have 0xCB, which is RLD disconnected. And this is correct.

    5) connect  the ECG cable, with RL lead only connected to my body (other leads floating) and  read CONFIG3: still 0xCB. Not correct

    6) set LOFF register to 0x13 but still 0xCB from CONFIG3

     

    maybe something else shall be configured in other registers?

     

  • 0 in reply to Carlo Nenna
    TI__Mastermind 43915 points

    Hi Carlo,

    Edited post -

    Talking this over with the design group, here is the details on lead off detection for right leg drive. As a stand along chip, using the RLD lead off detection is going to be a little difficult. The lead off detection uses a comparator that takes a leadoff threshold voltage to compare to the RLD electrode. While the RLD internal buffer is enabled to drive the body to some common point, the buffer also directly drives the comparator once the RLD_LOFF_SENS bit is enabled. Therefore, the buffer needs to be disabled in order for that comparator to work. However, as soon as that buffer is disabled, the source (or body) is floating relative to the chip. Therefore, the comparator is comparing some threshold voltage to essentially a floating node. You will just read back connected and disconnected randomly as soon as the buffer is disabled regardless of if the electrode is connected. You will need to somehow use a different pin to set the common mode voltage to the body in order to use the RLD lead off.

     

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    TI__Mastermind 43915 points

    Hi Carlo,

    I edited my previous post after talking with the design group explaining how to achieve RLD lead off detect.

    Regards,

    Tony

  • 0 in reply to Tony Calabria
    Prodigy 40 points

    Thanks Tony.

     

    I'll make some more tests following your advices and I'll try to figure out the best way for me for detecting RLD connection status

     

    regards,

     

    carlo

  • 0 in reply to Carlo Nenna
    Prodigy 10 points

    Hi Carlo 

    I want to know did your circuit worked?

    Could you measure the electrode impedance individually? Or your measurement was relatively?

    e.g. relative to DRL or reference?

    How did you measure DRL (or reference impedance)?

    You know, for EEG impedance checking we must measure EEG electrode impedance relative to GND, While DRL in not connected to GND!

    How did you do that?

    Thanks

    Farzane

  • 0 in reply to Tony Calabria
    Intellectual 570 points

    Dear Tony calabria,

          I am using read data continuous command for data retrieval upon  Data ready interrupt. Please provide more details to read the CONFIG3 register value after each data retrieval for every sample. Please share a sample code to detect RLD lead off.

    Please, clarify the following.

      • Necessity of The RLD drive internal amp to be powered down for RLD detection & if required how to powered down RLD drive internal amp.

    Regards,

    Bhairu