Four-Wire RTD measurement:
Error Analysis/ Experimental Results
1
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
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
VREF  1mA 2.5kOhm  2.5V
VREF  1mA 2.5kOhm  2.5V
- Temperature Range: -40C to 140C.
RTD @140°C = ~ 157.74Ohms max (Class A)
VIN ( Max )  1mA157.5Ohm ~ 157.5mV
-
Choose PGA=16 for this example:
VIN @140C  PGA  16153.7  2.459V  VREF
3
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.
4
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
VIN  1mA109.735Ohm  109.735mV
RBIAS/REF Error  109.735mV  0.02%  21.937uV
• Errors due to Offset, Gain Error and INL of the ADC (Typical Case)
Offset ~ 3.75uV
GainError  109 .735 mV * 0.005 %  5.487 uV
INL _ Error  15 ppmVREF  6 ppm 2.5V  15uV
Errordue to ADC  (Offset) 2  ( INL) 2  (Gain) 2  (Ref Resistor)2  27.37uV
Measuremen t Error in Ohms due to ADC  27.37uV / 1mA  0.0273Ohms
Errorin Celsius due to ADC  0.0273Ohms 0.3883Ohms/C 0.071C
RTD Class A Tolerance at 25°C is ±(0.15 + 0.002*t)°C= ±0.2 °C
T otalErrorin Celsius  (Class A RT D Accuracy) 2  (Circuit Error) 2  0.212C
5
ADS1248 Four-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)
6
ADS1248 Four-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 .
0.5
Class A RTD
tolerance
0.4
Temperature Error (C)
0.3
Experimental
Results
0.2
0.1
0.0
-0.1
-0.2
-0.3
Class A RTD
tolerance
-0.4
-0.5
-40
-20
0
20
40
60
80
Thermal Bath Temperature (C)
100
120
140
Calculated
Accuracy
7