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ADS1299: ensuring lead off and bias currents are below 10 uA

Prodigy 40 points

Replies: 2

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Part Number: ADS1299

Dear TI experts,

We have configured a 64 channel system with 8 TI chips in a referential montage.  We are using 64 P inputs, single reference electrode using SRB1 connected to all chips, and a bias electrode with PD_BIAS enabled on only one chip and BIAS_SENSP/BIAS_SENSN disabled on all channels.

a) We have configured continuous AC lead off detection at 6 nA and 31.25 Hz, and have shorted the SRB1 input with a single INxN pin to access the the negative LOFF current source for that input.  We are measuring the amplitude of the 31.25 Hz component in our single to estimate the contact impedance of each channel.  From what I understand, the way we have it set up we are measuring the sum of SRB electrode impedance and each individual electrode impedance.  Is this correct?  It appears to be working properly.

b) I would like to confirm my understanding of the current path for our setup.  Is it correct that we're generating 6 nA from each of the 64 electrodes (6 nA x 64 = 384 nA) and sinking into the SRB electrode.  Is the BIAS electrode sinking current as well?

c) According to the 60601-1 standard, there is a 10 uA limit on auxiliary current put into the patient.  Is there a safety mechanism in the chip to ensure that more than a certain amount of current is not being injected into the patient?  If not, do you have any suggestions on how we can ensure that this limit is not reached?  Should we add in series impedance or measure the current somewhere and turn off the chips if a fault occurs?

Thanks in advance!

  • Hi Alex,

    Thank you for your post and welcome to our forum!

    Do you have all of the LOFF_SENSx bits enabled on all eight devices? I assume you have at least all LOFF_SENSP bits enabled since you mentioned 64 x 6 nA. What about LOFF_SENSN? At least one LOFF_SENSN bit on one device needs to be enabled per the description of your setup below. The rest can be disabled since those pins are not connected.

    When SRB1 is enabled, the input MUX will open a switch at the input of each channel, disconnecting the INxN input pin and the corresponding current source from the negative PGA input. The point here is that SRB1 does not normally see any lead-off current...unless you do you what you described. Sharing the SRB1 pin with a single INxN pin externally allows 1 x i_LOFF to return through a negative current source. The rest of the current is returned through the BIAS amplifier. Based on that configuration, I believe you are correct in saying that the measured differential impedance is from INxP, through the body, to SRB1.

    The ADS1299 does not include any form of patient protection. Some input ESD diodes are included on each channel, but they are just that - protection for the device against ESD transients. Most applications use 10s of kOhms of impedance in series with each electrode to account for a possible short to the supply rails.

    Best regards,

    Ryan Andrews

    Applications Engineer | Precision ADCs 

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

  • In reply to Ryan Andrews:

    Hi Ryan,

    Yes, we have enabled a single LOFF_SENSN bit corresponding to the channel that is connected to SRB1.

    Would there be any effects on signal quality if we add a 100 kOhm resistor on each input?  Would we only add the resistors to the measurement electrodes and SRB1 or to the BIAS electrode as well?