RTD measurements can be done using discrete op-amps and data converters or integrated front-end solutions. TI has solutions that cover each of these topologies. Depending on various parameters such as cost, board space, resolution and performance an engineer may prefer to choose one topology over another. RTD: Op-amp approach
RTD: with 4-20mA output: XTR approach: Using XTR105 RTD: AFE approach: Using LMP90100 OP-AMPS
Excitation with Current Source
With an excitation current, the RTD voltage is directly proportional to the RTD resistance. The output voltage is
proportional to the excitation current so it is tempting to make this current large enough to produce a substantial voltage change with temperature. But with increased excitation current comes increased power dissipation and self-heating that can affect measurement accuracy. This may require careful consideration of RTD characteristics and accuracy requirements.
Excitation currents in the range of 100uA to 1mA are common for a Pt100 RTD. Output voltage changes by approximately 38uV/ºC with 100uA excitation.
Excitation with Voltage Source and Resistor
Excitation current can come from a voltage source and a resistor. The current, however, is not truly constant in this configuration. As the RTD resistance increases with temperature, the excitation current decreases. This can be accounted for in calibration at temperature extremes but nonlinearity is increased, especially over wide measurement temperature range.
Still, if the R1 is made large compared to
the change in RTD resistance throughout the measurement range, results may be satisfactory.
Basic Op Amp Amplifier Circuit
With two matched excitation current sources, the output voltage can be adjusted to zero at the desired low-scale temperature. For example, the RTD voltage will be (100uA)(100Ω)=10mV at 0ºC. To provide 0V output at this temperature requires a negative offset provided by IS2. The REF200 dual 100uA current source is well suited to this application.
Instrumentation Amplifier Circuit
An instrumentation amplifier offers some advantages over single op amp circuits. Here, the offsetting circuitry, IS2 and R10, does not affect the gain of the circuit. Gain is solely controlled by R7. Furthermore, the circuit allows a so-called three-wire connection to the RTD. Equal wire resistance in the connections to the RTD create a common-mode voltage that is rejected by the instrumentation amplifier. This allows longer connections to be made to the RTD without creating excessive error due to wire resistance.
RTD measurement with current-loop output using XTR105
Objective:
Design an easily configurable circuit that produces a 4-20mA output based on an RTD temperature range of 0C – 200C using the XTR105. Solution:
An example of this solution using a two-wire RTD can be seen in Figure 1 of the XTR105 datasheet.
To complete this design with the XTR105 the only requirements are to calculate the three external components, Rg, Rz, and Rlin1. Equations for these calculations can be seen in Figure 1 and the calculations are shown below:
RTD Temperature Range
The resulting circuit can be seen in the SPICE simulation below. With the RTD resistance set to the 0C value of 100Ohms, the circuit output is 4mA.
When the RTD resistance is at the 200C value of 178.845 the circuit output is 20mA.
The simulations use the exact calculated resistance values to achieve the ideal circuit output. Standard 1% or 0.1% values will have to be used in the final circuit and the deviations between the calculated values and the real values will appear as an error. Also, the effect of the analog linearization can be seen by looking at the change in the currents, i_lin and i_rtd, as the RTD resistance changes from zero-scale to full-scale. An example accuracy calculation for this system can be seen in Table II in the XTR105 datasheet and is shown below:
If a three wire RTD is used, the circuit changes and looks Figure 3 in the XTR105 data sheet.
To complete the design with the XTR105 an additional resistor, Rlin2, needs to be calculated along with Rg, Rz, and Rlin1. Although calculations can be used to determine the exact values for the resistors, Table 1 in the XTR105 datasheet provides the user with the nearest 1% values for the four external resistors based on the initial temperature and span.
For a design from 0C – 200C the 1% values can be found in the highlighted cell in the table above and are shown below.
If the equations are used to calculate the exact values the results are shown below. The deviation between the actual calculated values and the final resistor will appear as an error in the system.
AFE approach: Using LMP90100
Download Using LMP90100 PDF
