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Original question:

ADS1299: Not sure if the internal reference is working properly....

ADS1299: Internal test signal at DC is offset by 600 uV.

Gian
Prodigy 20 points
Part Number: ADS1299

I recently made a board with ads1299 and bipolar supply.

Everything works fine until I started generating internal test signals.

Register 2: 0b11010011, CHnSET: 0b00000101

What I expected to see: 1 × –(VREFP – VREFN) / 2400 = 4.5/2400 = -0.0018

What I got instead: -0.0024017

When I set CAL_FREQ[1:0] to 00, the amplitude seems correct, but still skewed a bit more towards negative side.

Register 1 Setting: 0x96, Register 2 Setting 0xE8

Measurement of Vref = 4.5V

I checked ADC conversion with against this forum, https://e2e.ti.com/support/data-converters/f/73/p/772488/2856893#2856893, and it seems to work as expected.

Conversion Snippet: Channel1  |  FFEE83  |  -0.0024017

I attached schematic as well. I'm a newbie, so I really appreciate any help I can get :)

  • 0
    TI__Guru 51341 points

    Hi Gian,

    An offset of 600uV for the test signal looks to be within normal behavior. Are you using the BIAS electrode during this measurement? 

    https://e2e.ti.com/support/data-converters/f/73/t/615010

  • 0 in reply to Alexander Smith
    Prodigy 20 points

    Thank you for the reply!

    I'm not sure how to configure bias output when using the internal test signal. However, I did use the bias electrode to measure a known signal, I tried few configuration, and nothing changed significantly. Still have about 600 uV offset. Here're my configurations:

    CONFIG3: 0xEC

    CH1SET: 000, BIASP: 0, BIASN: 1

    CH2SET: 000, BIASP: 1, BIASN: 0

    CH3SET: 000, BIASP: 1, BIASN: 1

    Function generator output: 30mVpp at 10Hz

    I connected the bias pin to both P and N inputs separated by 500k resistor to roughly simulate human skin impedance. 

    I also tried to short all the input to measure noise, and I see values around 600 nV as well.

    At this point I think I can assume that's amount I have to calibrate? (Of course it also depend on my gain setting)

  • 0 in reply to Gian
    TI__Guru 51341 points

    Hi Gian,

    Happy to help! 

    Using the bias output when using internal test signal should not be necessary, I was just curious.

    Input referred noise for this device is 1uVpp, so 600nV is within spec. Whether or not this needs to be calibrated out depends on your accuracy requirements. 

    Please let me know if you have more questions!