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Original question:

ADS1298R: Daisy chain continuous mode

ADS1298R: incorrect impedance values

jameel magzal
Intellectual 810 points
Part Number: ADS1298R
Other Parts Discussed in Thread: ADS1298

Hi all,

we have been working for some time with the ADS1298, we were having some trouble with our custom board and so we tested the evaluation model and something in the results are not clear:

1. we are not using the RLD, not sure if this would affect anything.

2. we tested the Voltage values on some resistors using 24nA, and found out that the zero ohm resistor would give about 360uv, which means all the other results we would have to substract this value to get actual voltage response, the issue is that this internal resistance would change from channel to channels and from board to board which means we would need a calibration for each board.

is the RLD required to be able to receive correct results without the 360uv offset ?

thanks in advance,

Jameel

  • 0
    TI__Genius 11550 points

    Hi Jameel,

    Thanks for the your post.

    1. For the impedance measurement using the AC/DC lead-off current, the lead-off current needs a low impedance path to flow through the biopotential electrode and the patient bias electrode to the RLD bias buffer. 
    2. Shorting the input channel with a zero ohm resistor will give you the offset error. This offset voltage will differ from channel to channel and device to device. The offset error of 360uV you get is within the device's typical value, which is +/-500uV. You will need to have a calibration operation if you want to eliminate or minimize each device's channel offset error.

    Thanks

    -TC

  • 0 in reply to TCT
    Intellectual 810 points

    TC hi, 

    Just to be clear, only first point of the response is about the impedance measurement correct ?

  • 0 in reply to jameel magzal
    TI__Genius 11550 points

    Hi Jameel,

    Yes. The 2nd comment is for the offset voltage measurement and calibration. 

    Thanks

    -TC

  • 0 in reply to TCT
    Intellectual 810 points

    Do you think you have an explanation for why we get this square wave of this amplitude while doing AC impedance test with 24nA while the channel is connected to the ground? we are getting here +- 165uV which is not supposed to be.

    We saw same behavior with EVM, is this normal behavior of the ADS1298 or are we doing something wrong ?

  • 0 in reply to jameel magzal
    TI__Genius 11550 points

    Hi Jameel,

    We will reach out to you and your colleague. 

    Thanks

    -TC