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ADS1299 lead off current sources

Ed Rapp
Prodigy 160 points

Hi,

I am having problem understanding how the lead off current sources work.  I have a single-ended EEG system (common reference electrode).  When I turn on the current source (loff sensp & loff sensm, 24nA, 31.2 Hz) on one channel, I see the signal with the same amplitude on all channels.  If I turn on 2 negative current sources the signal doubles on all channels.

It appears that the positive and the negative current sources are NOT independent.  Can you explain how the current sources operate?

Thanks,

Ed

  • 0
    Intellectual 435 points

    Hi Ed,

    I've used current sources for electrode impedance measurement. I've also had setup as your, single-ended EEG system with common reference electrode (all INM inputs are connected together in the same point called reference electrode). I'm using 6nA current source on all INP inputs, and only one current source on INM input (you can choose on which input you will turn-on, for example turn-on current source on first input). 

    Value of the registers must be for this example:

    EEG_LOFF_SENSP = 0b11111111
    EEG_LOFF_SENSN = 0b00000001

    EEG_LOFF = 0b00000011

    I'm not using frequency of 31.2Hz, because that frequency is in range with EEG signals, and maybe that is the reason why you see component on all inputs, if you use EEG cap. I'm using frequency of Fdr/4. My sample rate is 500Hz, and frequency of current sources is 125Hz. This works great.

    Try this setup of the registers. You also must now that when you calculate voltage in the spectrum on frequency of 125Hz, that voltage is across sum of the impedance of the electrode on the positive input and electrode on negative input. You must turn-off current source on negative input to calculate impedance of the electrode on the positive input.

    Best regards,

    Vojin Ilic 

  • 0 in reply to Vojin Ilic
    Prodigy 160 points

    Vojin,

    Thanks for following up,  I figured out what I was doing wrong.  It turns out that I had he bits reversed in the LOFF_SENSP and the LOFF_SENSN registers.  So I was looking at the the wrong input when I turned on the current source.  Since the negative inputs are common, I was seeing  the signal on the input (actually all of the inputs) due to the REF electrode impedance.  Once I got the current source on the right channel, everything made sense. 

    Best Regards,

    Ed

  • 0 in reply to Ed Rapp
    Prodigy 30 points

    Hi All,

    I'm also using the AC current source.  I'm using the 6nA setting at ~31Hz frequency.  To do the calculation of impedance given the measured voltage and the "known" current, I need a little more information about the current waveform...

    For example, with my hardware using known resistors, I seem to get the best answer if I do my calculation as:

    Impedance = Vrms * sqrt(2) / 6 nA

    where Vrms is after filtering the signal (10 Hz pass band) around the expected ~31 Hz frequency.  If this equation really is correct, it implies that the waveform of the injected current has an amplitude (not RMS) of 6 nA.  Or, said another way, that the injected current waveform is a sine wave with a peak-to-peak amplitude of 12 nA.

    Is this correct?

    Chip