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ADS1298: RLD derivation and lead-off detection

Maciej Lasota
Prodigy 170 points
Part Number: ADS1298

Hello,

I am playing with EVM ADS1298RECGFE-PDK with channels assignment as follows: LA = IN2P; RA = IN2N, IN3N; LL = IN3P; C1 = IN8P; N = ECG_RL. I use unipolar 2.5V supply voltage. Which bits from RLD_SENSP and RLD_SENSN registers should I use as "RLD derivation"? Is there any general rule which inputs should be taken for "RLD derivation"? I was not able to find such guidelines, therefore I have set "RLD derivation" from all inputs I use, but I am not sure if this is correct (see question below regarding lead-off).

Another question (somehow related to RLD) is about lead-off detection. I have configured lead-off detection to DC current mode 12nA and selected LOFFP and LOFF bits for all inputs I use:

I noticed that in such configuration I am not able to detect when RA electrode (IN2N, IN3N) is connected. It is permanently detected as "LEAD OFF" (even if shorted directly with RLD). Lead-off detection for LA , LL and C1 works correctly. Lead-off detection for RA works correctly if I set all bits from RLD_SENSP and RLD_SENSN to 0 (no RLD derivation). What am I doing wrong? What should I do to have RLD and lead-off detection working correctly at the same time? Should I make any changes in LOFF_FLIP register?

Thanks & regards, Maciej

  • 0
    TI__Guru 57326 points

    Hello Maciej,

    Thank you for your post.

    The ADS1298 does not support a 2.5-V, unipolar supply. Do you mean that you are using AVDD = 2.5 V and AVSS = -2.5 V? This is known as a bipolar 5-V supply (i.e. +/- 2.5 V). It appears that you are using our EVM hardware. You may configure the analog supplies as unipolar 3 V and bipolar 5-V using JP2 and JP20.

    In general, each of the primary input electrodes (RA, LA, LL) are used to sense the common-mode signal for RLD. Adding these electrodes to the RLD circuit changes the RLD closed-loop gain, so one must be careful not to saturate the RLD amplifier output.

    Pay careful attention to the voltage seen at the input pins when debugging lead-off detection problems. You may need to adjust the comparator voltage thresholds to avoid unwanted lead-off indications.

    Lead-off detection and RLD common-mode derivation are not "one-size-fits-all" configurations. I recommend reviewing the collateral on the ADS129x BIOFAQ. You'll find some application notes and links to online videos about ECG applications.

    Best regards,

  • 0 in reply to Ryan Andrews
    Prodigy 170 points

    Hello Ryan,

    Thank you very much for your reply.

    I have done many measurements of RL voltage for all electrodes I use (RA, LA, LL, C1 and RL) connected to the common point via skin impedance simulator (51k and 47nF in parallel on each) and different settings of RLD_SENSP and RLD_SENSN registers bits related to these channels.

    When all electrodes are connected to the skin impedance simulation circuit, then the voltage at RL is equal to AVDD/2 and is independent on settings in RLD_SENSP and RLD_SENSN.

    But for measurements where I left one of electrodes (RA, LA, LL, C1) disconnected I noticed some strange rule: if all bits in RLD_SENS registers were 0, then the RL voltage was close to VDD/2. Setting ANY of bits related to my channels in RLD_SENSP or RLD_SENSN increases the RL voltage. I was a bit surprised because I expected that setting bits from RLD_SENSP would increase RL voltage, but setting bits from RLD_SENSN would decrease it. Why it is like that?

    Summary of my measurements:

    0 bits selected -> RL =~1.25V

    Any 1 bit selected -> RL =~1.47V

    Any 2 bits selected -> RL =~1.7V

    Any 3 bits selected -> RL =~1.93V

    Any 4 bits selected -> RL =~2.16V

    Any 5 bits selected -> RL =~2.38V

    All 6 bits related to my channels selected (RLDN2, RLDP2, RLDN3, RLDP3, RLDN8, RLDP8) -> RL =~2.48V (saturation?) I am not sure if I should use RLDN8, beacuse IN8N should be driven from WCT.

    I guess I could decrease the feedback resistor between RLDOUT and RLDINV to not be so close to saturation when 5 bits from RLD_SENS are used? Is there any recommended RL "target" voltage when desired number of bits in RLD_SENS registers are used?

    Thanks & regards, Maciej