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ads1299 using ac lead off. how to check the lead off status?

Hyeongseob Choi
Prodigy 20 points
Other Parts Discussed in Thread: ADS1299

Hi. I am working on 24ch eeg device with ads1299 chip.

And I need to check the impedance of leads.

my ads1299 setting is..

24 channels of lead connected to [-] input and all [+] input is connected to one reference

so I set the ads1299 LOFF register to 0x03(ac Fdr/4). In my case, it is 125Hz

and LOFF_SENSP to 0(zero) and LOFF_SENSN to 0xFF

I got datas in my pc and analyzed it with FFT

I expected that there shold be a power(?) in range of 125Hz only when lead is connected to reference

but I also got a amplitudes in  other channels which are not connected. other channels have 125Hz signal.

I thought that chennels affected by all [+] input tied one reference.

Is it working right? so I thought that I dont need FFT to check the lead off status. is it right?

I just calculate the impedance with only Vrms and current.

have two questions.

one. Do I need to FFT to know lead off status?

two. when measure the real eeg data, do I turn off the ac lead off function?

thanks.

have a good day.

  • 0
    TI__Mastermind 38745 points
    Hello Hyeongseob,

    You've got the idea mostly correct. Are you using the bias amplifier on the ADS1299? The current that is sourced by the lead-off current sources needs to be sinked somewhere. If you are not using the bias amplifier to sink it, then you will need to turn on the lead-off current sources for the positive inputs, too (LOFF_SENSP = 0xFF).

    When you look at the signal power stationed at 125 Hz, you are essentially looking at the voltage corresponding to the AC lead-off current source magnitude multiplied by the round trip impedance from the INxP input to INxN input (in the case you have turned on the positive side current sources). Essentially, V_125Hz = 6 nA*Z_contact, since you are using the 6 nA current source. If an electrode gets disconnected on a particular channel, then Z_conact -> infinity and V_125Hz should vary roughly between +/-Vref. So when they are disconnected, the measured signal power at 125 Hz should be larger than if they are connected.

    The answer to both of your questions at the end is no; there is a method to doing both things in the time domain in real-time. Since 125 Hz is outside of the bandwidth you expect for EEG signals, you could develop and algorithm where the all the data you collect from the ADS1299 goes through two separate filters in parallel. One of the filters would be a low-pass filter that allows the ECG data to pass as your method for collecting that data but rejects the 125 Hz and higher frequencies which contain lead-off information. The other filter would be a high-pass filter that eliminates all the information under 125 Hz. The data that was passed through the high-pass filter will only include information about the impedance of the contact. The magnitude of that high pass filtered waveform in volts divided by 6 nA will be the impedance of the contact. Does that sound like a workable solution?

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 20 points
    Hello Brian

    Thank you so much!!!

    I understand and clear now.