Low-error platinum RTD circuit has shutdown capability
Reza Moghimi, Analog Devices, Santa Clara, CA -- EDN, September 14, 2000
Resistor-temperature detectors (RTDs), are the most stable and popular temperature sensors. Platinum RTDs allow for a much wider range of temperatures than silicon-based sensors. In many cases, platinum RTDs sit far from the measurement circuitry, which adds a great deal of error into the measurement system. The circuit in Figure 1 eliminates error by using a general-purpose amplifier and Kelvin-connected voltage references. The circuit also allows for single-supply operation and can detect temperatures of –200 to +400°C with an output-voltage-scaling factor (sensitivity) of 5 mV/°C. To reduce errors due to self-heating, the circuit uses the largest RTD—value, in this case 1 kW—that results in an acceptable response time. The larger the RTD, the longer the response time.
IC1 provides excitation and signal conditioning for the RTD, and internal current sources provide a matching excitation of 1 mA to the platinum RTD and reference resistor, RREF. The instrumentation amplifier compares the voltage drop across the platinum RTD to the drop across RREF and provides an amplified output signal that is proportional to temperature.
The lead resistance of wires connecting the RTD and RREF can add inaccuracy to the temperature measurement. Voltage reference IC2 creates a pseudo ground for IC1 to overcome this inaccuracy. IC2 has good temperature stability and low noise and can provide 5 mA of drive current. IC2 also has a sense pin for sensing the drop on the line and compensating for the drop. Thus, the circuit provides stable and identical voltages at the bottom of the platinum RTD and RREF. The circuit also buffers this voltage using the internal amplifier of IC1. A 1-kW resistor in parallel with a 1-µF capacitor at the output of IC2 provides a path for the current to flow to ground.
IC3 makes it possible for the µC to address the platinum RTD. The circuit in Figure 1 can accommodate four platinum RTDs, but you can increase this number by using other differential multiplexers. IC3's low on-resistance match between channels of 0.4W does not introduce large errors into the system.
Another feature of this circuit is that it allows a programmer to put the circuit into shutdown mode. Initiating shutdown disables the enable pin of IC3 so that none of the switch pairs are on. Also, IC1 and IC2 shut-down to conserve power.
The tracking of the current sources of IC1 is 2 µA, and, as stated, the matching on-resistance of IC3 is 0.4W. Thus, the worst-case mismatch resulting from the current sources and switches is 0.4W×2 µA=0.8 µV. If higher precision matching among current sources is necessary, you can connect a 50-kW potentiometer between the NULL-A and NULL-B pins and connect the center tap of the potentiometer to 5V.
