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ADS1232: INL for weight Scale

user5163865
Genius 3600 points
Part Number: ADS1232

Hi,

A customer is using this ADC in a weigh scale application. There is a phenomenon where INL deteriorates around a certain weight.

Questions
・Is the point at which nonlinearity worsens approximately constant in the ADC input range?
・Why is the maximum value of nonlinearity not specified when used with gain = 64,128?
・Do the characteristics of the PGA Bypass Capacitor affect nonlinearity?

//conditions
GAIN 128, VREF 3.3V
The input to the ADS1232 is the output of the strain gauge bridge 4000Ω, AINP1-AINN1) 0 mV 4 12 mV.

Customer declined to publish INL measurement data.
If you wish, I can send the data to a private email addressed to a TI engineer.

Since the INL of Gain128 is only TYP, I think it is difficult to judge, but I would like to know TI's opinion.



best regards,
Hiroshi

  • 0
    TI__Mastermind 41010 points

    Hello Hiroshi-san,

    We do provide a typical plot for INL for the 128 gain range.  Take a look at Figure 6-22 in the datasheet.  You can see the maximum magnitude of INL occurs near +/-0.5 full scale.

    Maximum non-linearity was not specified due to the difficulty of measuring nV level voltages in a short amount of time.  However, I would expect it to be about 2x the maximum specification for the lower PGA gains, or roughly 0.002% of FSR.

    The PGA bypass capacitor can effect performance, and should be a very high quality, low dielectric absorption, material.  We typically recommend using NP0/C0G ceramic capacitors for the PGA output.  Also, if there are contaminants on the board, solder flux or oils, this can cause leakage currents resulting in nonlinear behavior.

    In addition to the ADC, the strain gauge can also have non-linearity that may contribute to the overall INL. 

    If possible, can the customer share the schematic, including the PGA bypass capacitor type used?

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Genius 3600 points

    Hi Keith-san,

    Thank you for your detailed explanation and support.
    I have emailed the circuit diagram and other information to your PM address.

    Best regards,

    Hiroshi

  • 0 in reply to user5163865
    TI__Mastermind 41010 points

    Hello Hiroshi-san,

    Thank you for the additional information.  I will not be able to provide a follow-up response until Monday or Tuesday of next week.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Genius 3600 points

    Hi Keith-san,

    Thank you very much for your kind support.
    I will temporarily close this thread as resolved.

    Best regards,

    Hiroshi