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ADS1218 and ADS1217

Abdul Abumusallam
Prodigy 30 points
Other Parts Discussed in Thread: ADS1217, ADS1218

Hi ,

I'm using the ADS1217 in one of our designs, we have it working well, however the initial design called for the ADS1218 and they are pin for pin compatible except for the ADS1218 having internal flash(which we are not using)

when we try to use the ADS1218, the device is not giving us correct readings, completely wrong? when we look at the two chips we see that they are identical , at least from the point of reading the internal registers, and the conversion is pretty the same? is there any difference at all between the two chips that cause wrong reading on the ADS1218?

  • 0
    TI__Guru* 94055 points

    Abdul,

    The big difference between the two ADCs are that the full scale range of the ADS1217 is 2*VREF/PGA, while the ADS1218 full scale range is VREF/PGA.

    Are you off by a factor of two in the data?

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 30 points

    Hi Joseph,

    The data is not exactly is off by a factor of two, because we have offsets and scale that we do to the input, my question is:

    I have set the VREF to 1.25 and the PGA =1, what would be the full scale analog input allowed , VCC=5v, and what would the full scale digital output be? I'm a little puzzled by the readings I'm getting? on the ADS1217 we have PGA=1 and the VREF=2.5v. and that is used for our op amp reference voltage, so our offset is 2.5V, we take that off from the reading we get, and that works well for us?

  • 0 in reply to Abdul Abumusallam
    TI__Guru** 113765 points

    Hi Abdul,

    Starting with the ADS1217 using a 2.5V reference and PGA of 1, your input range is +/- 2*(2.5V)/1, or +/-5V for your full-scale range.  For the ADS1218 using the 1.25V reference you are +/- 1.25V for full-scale range.

    In each the ADS1217 and the ADS1218 the digital output code will range relative to 24-bit, but the value of the code will change based on the full-scale range.  The value of one code (LSB) is the full-scale range divided by the total number of bits.  For the ADS1217 this is 10V/2^24 which is about 596nV.  For the ADS1218 this becomes 2.5V/2^24, or 149nV.  Notice that reducing the range by 1/4 also reduces the value of 1 code by 1/4.  If the ADS1218 is using 2.5V reference (same as the ADS1217) the full-scale range is 1/2 of the ADS1217, which is what Joseph told you in the previous post.  These numbers are assuming that you are using the default Data Format of Bipolar in the M/DEC1 register. 

    If this is still confusing to you, please send the exact registers settings and voltages applied along with the results for each device.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 30 points

    Thank you Bob,

    I will try to figure out my offsets and scales , I will keep you posted, thanks for the help.

    Abdul