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ADS122U04: Don`t understand data output

user6244799
Intellectual 950 points
Part Number: ADS122U04

Hello
I don`t understand ADC `s data output. I finded in the datasheet an explaination about it, but it didn`t clear for me.



  • 0
    TI__Guru** 112715 points

    Hi user6244799,

    You ignore the LSB and only consider the MSB and mid-byte data.  In your data the MSB = 0x00 and the mid-byte data is 0x32.  The temperature sensor result is 14 bits left justified in the returned data, so 0x00 32 would be shifted right by two (>>2) which would leave a 14-bit result as 0x000C or decimal 12.  12 times the coefficient of 0.03125 deg C per code is 0.375 deg C.  

    I'm not sure if you have the part in an environment of that temperature, but that is the result from the data given.  If you are expecting a room temperature value, then make sure that you have the TS bit set and also that you are reading the conversion data after setting the TS bit and the conversion has completed.  The RDATA will read the last conversion completed and not necessarily the conversion after setting the TS bit.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 950 points

    Thanks for your explain.
    If I use no internal temperature sensor should I set TS bit on? As I understand it`s unneeded.

  • 0 in reply to Bob Benjamin
    Intellectual 950 points

    And please check do I right grasp it.
    When I have TX (like on the scan lower) my geted temperature has the value approximately equal 25?

    counter                             LSB                          Data                           MSB

  • 0 in reply to user6244799
    TI__Guru** 112715 points

    Hi user6244799,

    To be more clear, when you want to use the internal temperature sensor you need to set the TS bit to 1.  If you want to convert the analog inputs you need to set the TS bit to 0.  You cannot measure both the internal temperature sensor and the analog inputs at the same time.  It is just one or the other.

    If the TS bit is 1 and the data retrieved is as you have shown, you totally ignore the LSB.  You combine the MSB and mid-byte together and right shift by 2 (as the data is 14-bit left justified) to get a 14-bit result from the 16 bits of data.

    (0000 1100 1110 0100b) >> 2 = (0000 0011 0011 1001b) = 339h = 825d

    825 counts * 0.03125 deg C/count = 25.78 deg C

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 950 points

    If I use an external temperature sensor how receiving data should be converted? Like an internal one data?

  • 0 in reply to user6244799
    TI__Guru** 112715 points

    Hi user6244799,

    If you use an external temperature sensor such as a thermistor, thermocouple or RTD the measurement is much different between the sensors and is not at all calculated in the same way as the internal temperature sensor of the ADS122U04.

    I would suggest reading the following:

    A Basic Guide to Thermocouple Measurements

    A Basic Guide to RTD Measurements

    Best regards,

    Bob B