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ADS1258 gain deviation

Alon Gutman
Prodigy 30 points

Other Parts Discussed in Thread: ADS1258, TLV2711

Hello.

We're experiencing some inaccuracies in signal data conversion in ADS1258.
We have investigated the phenomenon in our several product boards. By applying different values of a stable DC voltage directly to the analog signals input and reading the converted data, we have created a conversion chart in several boards.
It seems to be a gain deviation factor between different boards. (Deviation range of -7% ... +7% from the expected value)
In each board, all the 15 input signals behave the same, so the deviation is general per each ADS1258 unit.
We have tried removing and installing new ADS1258 units in each board, but the gain factor remained the same in each board, which means that cause is in our peripheral hardware supplying the ADS1258 unit.
The 5v Vref. in all our boards is pretty stable, and the difference between the boards is less than 0.2%.
In general we could calibrate the deviation per each board in the software, but it would help a lot if we could find the factor causing it.


Attached below are some relevant schematics:

The ADS1258 unit:


CLKSEL is drivven Low.
CLKIO is left floating

The Vreff circuit:

 

Appreciate your help,

Alon Gutman 

  • 0
    TI__Mastermind 44770 points

    Hi Alon,

    Welcome to the TI E2E forums! Also, thank you for sharing your schematic!

    The reference circuit you've shown may be accurate while the ADC is not converting, but could easily be loaded and create gain errors. The ADC's reference input looks like a switched capacitor circuit. The TLV2711 is a very low bandwidth op amp. You may need to use a faster op amp and adjust the RC filter between the op amp and ADC. Try monitoring the reference voltage at the ADC's input while it is continuously converting to verify if this circuit is providing a stable reference.

    Another possible source of gain error may be due to the output impedance of your analog inputs. You have not shown any buffers or signal conditioning circuitry on your inputs. You may need to consider adding a buffer between the MUXOUT and ADCIN pins.

    Finally, I would also consider the grounding scheme. You have separate analog and digital grounds, however, the ADC's digital ground needs to be at the same potential as the analog ground. Is there a low impedance connection between these grounds, or are they isolated?

    Best regards,
    Chris