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Four-Wire RTD measurement: Error Analysis/ Experimental Results. Four-wire RTD Measurement Estimate of Accuracy. Ratiometric: IDAC Accuracy/Drift does not add error. Accuracy/Resolution of A/D. Accuracy of Class A RTD. Accuracy of RBIAS Reference Resistor.

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Four-wire RTD Measurement Estimate of Accuracy

Ratiometric:

IDAC Accuracy/Drift does not add error

Accuracy/Resolution of A/D

Accuracy of

Class A RTD

Accuracy of

RBIAS Reference Resistor

four wire rtd measurement estimate of typical accuracy
Four-wire RTD Measurement Estimate of typical Accuracy

Below is a possible ADS1248 example of a PT-100 4-Wire RTD circuit in a Ratiometric configuration.

  • Select IDAC current/RBIAS resistor:
    • Beware of RTD self heating; in this example a large PT-100 probe was used to measure temperature in a thermal oil bath, where the RTD self heating was negligible. Note: In applications where a small RTD is used to measure Air/Gas stream measurements; consider using a lower IDAC current setting to avoid self-heating.
    • Selected IDAC = 1mA
    • Temperature Range: -40C to 140C.

RTD @140°C = ~ 157.74Ohms max (Class A)

    • Choose PGA=16 for this example:
four wire rtd measurement estimate of typical accuracy using typical specifications
Four-wire RTD Measurement Estimate of Typical Accuracy (Using Typical Specifications)
  • Using the typical Specs in the datasheet, an approximate estimate of the typical expected accuracy can be performed.
    • The user may calculate the approximate accuracy at each point in temperature using a spreadsheet…
  • Example using a Class A RTD at 25°C

RTD Class A resistance at 25°C = 109.735 Ohm;

Class A Tolerance at 25°C is ±(0.15 + 0.002*t)°C= ±0.2 °C

  • IDAC absolute accuracy and drift does not matter since we are using a ratiometric approach.
four wire rtd measurement estimate of typical accuracy using typical datasheet specs
Four-wire RTD Measurement Estimate of typical Accuracy (Using Typical datasheet specs)
  • The Reference/Rbias resistor will cause an error proportional to the input voltage:
    • In this example, a 0.02% high accuracy resistor with 5ppm/C low drift
  • Errors due to Offset, Gain Error and INL of the ADC (Typical Case)

RTD Class A Tolerance at 25°C is ±(0.15 + 0.002*t)°C= ±0.2 °C

ads1248 four wire rtd measurement experimental results
ADS1248Four-wire RTD Measurement Experimental Results
  • RTD: 4-wire Class A RTD Probe
  • Omega PR-10-3-100-3/16-6-E
  • Fluke/Hart Scientific High Precision Thermal Bath 7340 w/

ADS1248EVM-PDK Board

Thermal Bath Temperature Sweep from -40°C to +140°C

Precision Thermal Bath Temperature Display Resolution is 0.01°C; calibrated to absolute accuracy of 0.02°C.

Calibration Baths: Thermal Baths produce use a mixed fluid to provide great thermal contact and stability

Used for temperature calibration they provide stability to 0.005C and uniformity 0.006C

Display set point Temperature Resolution 0.01C

Thermal Bath Fluid:Propene (Galden HT200)

ads1248 four wire rtd measurement experimental results1
ADS1248Four-wire RTD Measurement Experimental Results

Important Note: The actual performance in a given system may be different and depends on many variables, including (but not limited to) the application schematics, PCB layout, temperature-forcing system accuracies, and environmental noise contributions, among other factors. TI offers no assurance of system performance other than the performance parameters detailed in the Electrical Characteristics section of the ADS1248 product data sheet.

Class A RTD tolerance

Experimental Results

Class A RTD tolerance

Calculated Accuracy