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ADS125H01: output conversion

Frank Giordano
Prodigy 20 points
Part Number: ADS125H01


Tool/software:

Hello, I am having an issue with the output conversion from ADS125H01.  I am applying a 49 mV differential signal at input (+5.030 V to AINP, +4.981 V to AINN), and at the output after conversion I am receiving approximately 88 mV.  I not sure why the difference.  Below is my setup and calculations.

AVDD  +5 V

DVDD  +3.3 V

HV_AVDD  +12 V

HV_AVSS  GND

REFP  +5 V

REFN  GND

MODE0 Register set for 40 kSPS

MODE1 Register set for Pulse-conversion

MODE4 Register set for Input External (AINP – AINN) and PGA Gain 1

REF Configuration Register set for External Reference (REFP – REFN) for a VREF = 5 V

 SPI Reading approximately 0x2428a which equals 148,106. 

(SPI reading) x (VREF / 2(23)) = voltage reading

148,106 x (5V / 8,388,608) = 88 mV.

I have been monitoring Registers Status0 and Status1 and I do not see any errors.

 I am not sure what I am doing wrong, any help would be appreciated.     

  • 0
    TI__Mastermind 45955 points

    Hi Frank,

    Is the output code/voltage always approximately double if you try other input voltage? Can you share a SPI timing plot so that I can check if the code is properly read?

    There is a ADS125H0x Design Calculator tool on TI.com, you can use it to check if the common-mode range is violated or code conversion is correct. I did not see an issue with your setup.

    BR,

    Dale

  • 0 in reply to Dale Li
    Prodigy 20 points

    Hello Dale, thanks for the reply.  Below is sample plot for a SPI reading from today, showing 0x02454E which is comparing to the incorrect results from last week.  I am checking the CRC, and the 0x0A in the reading is the correct CRC for 0x02454E.  I will try to get more information for changing input voltage and see affect on output voltage.  

  • 0 in reply to Frank Giordano
    TI__Mastermind 45955 points

    Hi Frank,

    Thanks for providing the timing. Your timing looks good to me. You got 02h status0 data and also 454E0Ah conversion data. Let me know if you have any trouble to verify.

    BR,

    Dale

  • 0 in reply to Dale Li
    Prodigy 20 points

    Hello Dale:

    I do not have the optional STATUS0 byte enabled, therefore I believe the reading is 0x02454E with correct CRC for 0x0A.  I tried inputting different inputted voltages and read approximately the below outputs.  The output readings appear to be incorrectly ranged by approximately 1.8

    AINP – AINN = 43 mv then outputted SPI reading for 76 mv

    AINP – AINN = 49 mv then outputted SPI reading for 89 mv

    AINP – AINN = 69 mv then outputted SPI reading for 126 mv

    Best regards,

    Frank

  • 0 in reply to Frank Giordano
    TI__Guru 53156 points

    Hi Frank Giordano,

    Thanks for providing additional information

    Can you read back all of the ADC register settings once you have programmed the device and provide that information here?

    What are you using as a voltage source to supply the 49mV? How is this source connected to the ADC?

    Can you connect a small battery e.g. AA or AAA, to the ADC inputs and see if you can measure this voltage correctly? Make sure to connect the negative end of the battery to some bias voltage within the range of the ADC. You can for example connect REFOUT on the ADC to the battery negative terminal. Let us know the outcome of this test.

    -Bryan