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ADS1258-EP: accuracy issue

Errol Leon6
Expert 5830 points
Part Number: ADS1258-EP
Other Parts Discussed in Thread: ADS1258

Hello team,

My customer is performing some testing with ADS1258-EP. They wanted to share these results and see any suggestion on how to improve accuracy from the ADC.

They are using the following  configurations for this testing:

  1. Chopping is Enabled.
  2. Data Rate is set to 11
  3. Last 11 bits are masked off.
  4. Testing is done at ambient.
  5. Apply 1V precision input on AIN 11 of ADC, take 1000 samples, average them and read results.
  6. Apply 3V precision input on AIN 11 of ADC, take 1000 samples, average them and read results.

They are seeing the following results with this testing.  There seems to be a lot of error in the results in un-calibrated section.

The customer has a few questions regarding these results below:

  1. Is there a way I can reduce the error in uncalibrated output? Because it is delta sigma converter so as per my understanding the results should be very good.
  2. The calibration (Straight Line Method) has been applied on only one of the chips to reduce offset and gain errors. Can we store these calibration coefficients in our micro controller memory and apply these coefficients to all the chips for different controllers?? Because offset and gain error will be different for different chips so I am not sure we can apply these calibration coefficients on other chips or not??

 

4353.TEST.docx

Thank you for your support!

Errol

  • 0
    TI__Mastermind 34901 points

    Hi Errol,

    Let me look into this and get back to you

    Do you have a schematic to share?

    -Bryan

  • 0 in reply to Bryan Lizon86
    TI__Mastermind 34901 points

    Hi Errol,

    To follow up on this, I wanted to clarify that this issues was resolved offline with these suggestions:

    1. The ADS1258 input impedance is pretty low, so feeding input signals directly into the ADC could result in a significant amount of leakage current depending on any resistances in the input path e.g. anti-aliasing RC filter. I would suggest buffering these inputs (this may not matter for lab tests, but in the real world system measuring real world signals, this could have an effect)
    2. Using a precision buffered reference (REF50xx could be okay, REF6xxx would be better) is more appropriate than the chosen shunt reference if best performance is necessary. Again, the ADS1258 reference inputs are not buffered internally.
    3. Make sure the REFN connection on the ADC ties directly back to the GND pin on the external VREF. In other words, don’t drop a via to GND right at REFN and a second via at VREF GND, but add a trace from REFN back to the GND connection of the VREF. This ensures a cleaner VREF signal
    4. Add small resistors in series with the digital lines (usually 30-50ohm) to help dampen reflections

    -Bryan