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ADS122C04EVM: ADS122C04EVM

Nikolay Philip
Prodigy 20 points
Part Number: ADS122C04EVM
Other Parts Discussed in Thread: ADS122C04

please help me. how i convert 3-wire rtd data analysis reading to temperature value?

i measured 4 cases and connected the evm according yours recommendation:

case 1: i connected 100 ohm external resistor to the evm to pins 2,3 and 5 and got 715753 decimal reading result. according pt-100 datasheet 100ohm is 0 deg.

case 2: i connected 165.5 ohm external resistor to the evm to pins 2,3 and 5 and got 1180610 decimal reading result. according pt-100 datasheet 165.5ohm is 172 deg.

case 3: i connected 151.1 ohm external resistor to the evm to pins 2,3 and 5 and got 1078692 decimal reading result. according pt-100 datasheet 151.1ohm is 134 deg.

case 4: i connected 90 ohm external resistor to the evm to pins 2,3 and 5 and got 644701 decimal reading result. according pt-100 datasheet 90ohm is -25 deg.

how i can ensure that its a right result ? or not? and how to convert it to temperature value?

thank you,

Nikolay Philip

 

  • 0
    TI__Guru** 112775 points

    Hi Nikolay,

    the ADS122C04 uses a voltage reference and the conversion result is a voltage relative to the reference.  However, when making a ratiometric measurement where the excitation is common to the reference and RTD, the conversion result can be calculated as a ratio of the reference resistor with respect to the resistance value of the RTD.  This information is explained in the EVM user's guide in section 5.1.1.  Note the use of equation 4 on page 24.  The resistance of the RTD can be calculated using the equation 4 formula.  R77 on the EVM is 4.7k.  If you use the script provided for the EVM for 3-wire measurements, the PGA setting is a gain of 4.  The EVM GUI does not convert the resistance to temperature, so this will be a manual process.  Once you have determined the resistance value, you will need to use a lookup table or the polynomial equations for the RTD you intend to use to determine the temperature.  

    R77 will not be an exact value due to device tolerance.  But if we assume that it is correct I have calculated the approximate values of resistance.  I say approximate as I have rounded some of the values.

    RTD          Codes       Calculated Resistance

    100            715753            100.2

    165.5         1180610           165.3

    151.1         1078692           151.0

    90               644701             90.2

    There are a number of things that can disturb the accuracy.  The first is the true value of the reference resistor.  However the ADC may also have some offset that should be removed prior to the calculation.  Noise can also hinder the measurement for a less accurate result.  Calibration can reduce the effect of offset and gain error as well as adjusting for the correct value of the reference resistor.  One error that cannot be calibrated is the drift of the reference resistor and so the reference resistor should not only be a precision resistor, but also be low drift.

    You may also find the information in A Basic Guide to RTD Measurements useful.

    Best regards,

    Bob B