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Original question:

ADS1299: Electrode impedance measurement with ADS1299

ADS1299: Impedance testing error margin?

Lakshay Sahni
Prodigy 80 points
Part Number: ADS1299


In addition to the thread: https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/691122/ads1299-electrode-impedance-measurement-with-ads1299  

I want to ask a few questions:

1. As stated by Ryan Andrews in the above thread that ADS1299 definitely won't give accurate impedance. How much error percentage does it give generally?
I analysed impedance by attaching a 100kOhm resistance on the 8th channel. It gave me an impedance of 192k ohm. For 1k ohm, it gave an impedance of 5kOhm. I have setup the circuit and calculated the impedance as stated by an openBCI blog: http://eeghacker.blogspot.com/2014/04/openbci-measuring-electrode-impedance.html 

2. During AC impedance setting, at any frequency (31.2Hz or 7.2Hz) as per datasheet there should be a square wave of 6nA Vpp (if 6nA is set in LOFF register). But on the digital multimeter and Tektronix oscilloscope, it showed only 5.65nA Vpp. Is that fine? Is that what is causing the errors in the impedance results I am getting. 

3. In the impedance testing, is the Lead-off comparison performed by the same PGA which is used for EEG or those comparators are separate? I couldn't see anything about this in the datasheet. The only thing visible in the block diagram is PGA. 

  • 0
    TI__Genius 11550 points

    Hi Lakshay,

    Thanks for the post.

    The typical accuracy of the lead-off current source is ±20% (see datasheet electrical characteristics). Each INxP and INxN input has its own comparators, and the comparator output can be read either from the LOFF_STATP and LOFF_STATN registers or the status words.

    Thanks

    -TC

  • +1 in reply to TCT
    Prodigy 80 points

    Thank you for the support. I was getting error as large as 95% so I had a doubt. I also have doubt in two of the registers as there is not much detail and explanation available in the datasheet. 

    What should be the Bit2, Bit1 and Bit0 values of CONFIG3 and CHnSET when using EEG data extraction and in Impedance testing. 

    In CHnSET, I am confused in MUXn[2:0] = 110 (BIAS_DRP) and 000 (Normal electrode) for impedance testing. For EEG I am using 000, but for impedance I am attaching inbuilt current source on channel as a source so should I use 110? There is nothing written 

    In CONFIG3, I am confused in the 'BIAS sense' ( and 'BIAS buffer'. What should I keep these for EEG collection and impedance testing?. 

  • +1 in reply to Lakshay Sahni
    TI__Genius 11550 points

    Hi Lakshay,

    The CONFIG3[2:0] is primarily related to the BIAS sense amplifier. Please refer to Figure 37 of the datasheet for the bit setup. The BIAS_LOFF_SENS (BIAS_SENS) is to enable the lead-off current to this electrode. There is more information in Section 9.3.2.4.5 for the uses of Bias Drive.

    The MUXn[2:0] = 110 (BIAS_DRP) and 111 (BIAS_DRN) is to enable the routing of the BIASIN signal (Pin#62) to either INxP or INxN input pins. Please refer to Section 9.3.2.4.1 for the detail.

    Thanks

    -TC