Low-error platinum RTD circuit has shutdown capability

Reza Moghimi, Analog Devices, Santa Clara, CA -- EDN, 9/14/2000

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, R_REF. The instrumentation amplifier compares the voltage drop across the platinum RTD to the drop across R_REF and provides an amplified output signal that is proportional to temperature.

The lead resistance of wires connecting the RTD and R_REF 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 R_REF. 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.