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Original question:

ADS1299: Performing impedance measurement using the lead-off current sources.

ADS1299: Impedence calculation using internal current sources.

Raghav Taneja
Prodigy 30 points

Part Number: ADS1299
Other Parts Discussed in Thread: TINA-TI

Hi

I am trying to calculate skin to electrode impedance using 6nA current at 31.2hz generated internally of a single electrode connected to negative end of the 8th channel and positive end is connected to bias electrode. 

I have connected an anti aliasing filter(2.2K ohm and 1nF) at the input. My configurations are - 

LOFF_SESN -> 0x80.

LOFF  -> 0x02

gain is 24 and data rate is 500sps.

when i add an additional 660K  ohm resistor at the input the digital output does not change and comes to be 1-2 mV. Shouldn't it be a minimum of 3.6 mV ((660K+2.2k)*6nA)?

 

  • 0
    TI__Guru 69076 points

    Hello Raghav,

    Thank you for your post.

    If you can download TINA-TI spice, I recommend trying to build you circuit and simulating the lead-off current to see what the expected input voltage is. You are correct that you should see about (660k + 2.2k) x 6 nA = 3.97 mV.

    You'll need to check the common-mode voltage of IN8N and IN8P to ensure they are still close to mid-supply and not causing the PGA to rail.

    If you want to post your register settings below and a circuit schematic, I can give them a review.

    Best regards,