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ADS1299: Impedance Check

FloNas
Prodigy 30 points
Part Number: ADS1299

Hello,

I am trying to measure electrode impedance using ADS1299. I use AC lead-off method to generate an ac signal that has a frequency equal to Fs/4 (Fs is 4KHz).

The thing that confuses me is that the output waveform is a square wave, not a sine wave. From what I know, impedance is measured by stimulating the load with a sine wave. Of course, from Fourier analysis, a square wave contains an infinity of sine waves, but, in my opinion, selecting the first harmonic adds a lot of complexity. 

Did I configure the ADS1299 incorrectly, or the output waveform should be actually square ? If it is indeed square, what is the best practice to determine the impedance value ?

Thank you,

Florin

  • 0
    TI__Guru 55062 points
    Hello Florin,

    Welcome to the E2E forum and thank you for your interest in the ADS1299!

    The Impedance measurement is intended to be a square wave. The square wave comes from 50% duty cycle chopping of the DC current sources. The magnitude of the DC current source is set in Lead-off Control Register bit[3:2]. In order to calculate the impedance, measure the voltage of the returned square wave, divide by the known DC current magnitude and the PGA gain, and then solve for Zin.

    Hope this helps!
  • 0 in reply to Alexander Smith
    Prodigy 30 points
    Hello Alex,

    Thank you very much for your assistance! Hope this doesn't sound silly, but why a square wave ? Many articles I have read use a sine wave for impedance checking. Are there some advantages of using square waves over sine waves ?
  • 0 in reply to FloNas
    TI__Guru 55062 points
    Hi Florin,

    You are most welcome. That's not a silly question at all!

    In terms of measuring impedance, no there is not an advantage to using a square wave over a sin wave. If you would like to use a sin wave, it's possible to disable the device lead off detection and provide your own source.

    A square wave is used is because it fulfills the functional requirement and creating a square wave by cycling the DC current source is much simpler than generating a sin wave from DC sources.