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ADS8866: Nonlinear data acquisition

Philipp Kloth
Prodigy 20 points
Part Number: ADS8866

Hello,

I have a problem with the nonlinearity of the ADS8866. In the picture bewlow you can see a Bending of the data (at around 17000 bits). I drive the ADC with a precise function generator, generating a triangle signal at around 1 kHz. The ADC itself is running at around 80kHz via SPI connection.

The schematics of the ADC are shown here:

VCC is 3.3V, supplied by a laboratory power supply. I tested the stability of AVDD and DVDD, but it seems to be stable.

Can you help me with this issue somehow?

  • 0
    TI__Mastermind 41390 points

    Hi Philipp,

    Can you double check either the input frequency or the sampling frequency?  Based upon your results, you have about 280 samples per input period.  If the input triangle wave is 1khz, this would suggest your sample rate is closer to 280ksps, which the ADS8866 does not support.

    Also, it looks like you are driving the input all the way to 0V and a little above 2.5V.  If you overdrive the input, it can cause the reference voltage to ring, which would effect measurement accuracy.

    I suggest changing your input waveform to a lower amplitude; 0.25V min to 2.25V max, and to reduce the frequency to 100hz.  This should help with debugging the circuit.

    Also, how do you connect the signal generator to the ADC input?  The AinN input path should be connected to GND (Connect pin 2 of connector J3 directly to ground)  Connect the ground of the signal generator to GND on your board, and the signal gen output to pin1 of J3.

    If you have an oscilloscope, it would be helpful to measure the voltage at AINP pin 3, to confirm that you have a clean input triangle wave.

    Regards,
    Keith N.

    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 20 points

    Hey Keith,

    shame on me. The OPA in front of the ADC Input was causing the troubles. Even when I was applying my signal directly into the ADC Input. But anyhow, thank you very much for your help!

    Regards

    Philipp