Component selection

The  LMP2011  precision amplifier is a high gain and almost zero offset voltage amplifier. The  LM4140A -2.500 is a trimmed, low drift, voltage reference with an error of less then 0.05% and a drift less then 10 PPM/°C. These components, along with several precision resistors assemble into a very accurate and stable thermometer.

The circuitry in Figure 1 is designed to take advantage of the four-wire PRTD, which is the most accurate configuration to use. The four-wire PRTD implements a Kelvin connection force and sense leads. The two wires (W1 and W4 in Figure 1 ) are the force leads and connect the RTD to the constantcurrent source. The other two wires (W2 and W3) are the sense leads and connect voltage across the RTD to the amplifier. This arrangement separates the constant-current source driving the RTD from the measurement circuit. The voltage drop in the wires W1 and W4 are removed from the measurement of the voltage across the RTD.
As an example of the error that can be introduced by the lead wires: If a probe uses 10 feet of 24-gauge copper wire to connect the resistance element to the instrument, then the lead resistance is 2 x 10 ft x 0.0257 W /ft = 0.514 W . The resistance change of the RTD at 0°C is 0.39 W /°C and the lead wire represents a 0.514 W /0.39 W /°C = 1.31°C error. At higher temperatures, the error will be larger. As an example, the resistance change of the PRTD at 400°C is 0.35 W /°C and the lead wire represents a 0.514 W /0.35 W /°C = 1.46°C error.