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ADS114S08EVM: Temperature sensing

Lavany Pant
Prodigy 30 points
Part Number: ADS114S08EVM
Other Parts Discussed in Thread: ADS114S08

I am using ADS114S08 evaluation module. I am currently analyzing the temperature sensors compatibility of the board and using a Pt100 sensor. When running the script of 4 wire rtd at room temperature, the voltage is 0.28V approx. When i am measuring the voltage across R68, it is 1V. I am not sure what 0.28V means

To further clarify, I am getting 0.28V on the delta-sigma software after running the 4 wire rtd script (measurement image attached). I am not sure what is this measurement conveying? As in, what is 0.28V?

What I expect is, the rtd and r68 are in series, where rtd is connected to avdd and r68 is connected avss, adc is connected across r68 and rtd is connected to avss. What i was expecting was tht voltage across r68 will be the output of adc and power supply - voltage across r68 will be the voltage across 68. voltage across rtd divided by excitation current (1mA) will give the resistance of rtd and then from the lookup table, temperature could be checked. But its this 0.28V which was unexpected.

  • 0
    TI__Guru** 113765 points

    Hi Lavany Pant,

    The circuit is designed to be a ratiometric measurement which is excited by a current source established from AVDD.  AVDD does not directly excite the RTD or the reference voltage.  Please review the schematic and the explanation of the circuit in the EVM user's guide.  The current source is set to 1mA which will create a 1V reference voltage across R68.  The 1mA of current will flow through the RTD to generate an approximately 110mV across the RTD at room temperature.  There is also a bias voltage which will also be 1V from the EXC- of the RTD to AGND.

    Make sure that you have correctly connected the 4-wire RTD as indicated in the script.  Also, in the Data Analysis window set the reference voltage to 1V.  If you divide 0.28V by 2.5V (changing the reference value from 2.5 to 1V) you get about 112mV which would be the expected voltage.  To determine the resistance you divide 112mV by 1mA to get 112 Ohms of resistance.  From a resistance chart/table you will see this is approximately room temperature for a PT100.

    A useful resource to explain the circuit is A Basic Guide to RTD Measurements.

    Best regards,

    Bob B