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Reduce measurement errors in RTD circuits

16 Feb 2016  | Gordon Lee

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The most commonly measured physical parameter is temperature. With so many new ideas for connected devices in the works for consumer and industrial applications, you often need high-accuracy temperature measurements to ensure both product quality and safety. Many types of temperature sensors are available, and each one has its advantages and disadvantages. Resistance Temperature Detectors (RTDs) are one of the most common. To get the most from an RTD, you need to properly adapt it to an ADC for digitizing. These circuits can help you get quality temperature measurements from RTDs.

RTD's contain a resistive element whose resistance changes with temperature. Most elements are either platinum, nickel, or copper. A platinum RTD provides the best performance because platinum has the most linear and repeatable temperature-to-resistance relationship over a large temperature range. For a typical platinum resistor, the temperature range is -200˚C to 850˚C. The most common RTD is the Pt100, which has a resistance of 100 Ω at 0˚C.

Generally, RTDs generate more stable and repeatable outputs compared to thermocouples and thermistors. Hence, they achieve higher measurement accuracy.

RTD measurement circuits
The two most common methods to measure an RTD are constant current excitation (figure 1) and constant voltage excitation (figure 2).

Figure 1: A 2-wire constant-current excitation configuration. Simple, but wires add errors.

Figure 2: A 2-wire constant-voltage excitation configuration.

The goal is to accurately measure the RTD resistance and convert it to temperature using an equation or a lookup table. For ideal cases:

for constant current excitation, or

for constant-voltage excitation.

As with many two-wire resistance measurements, an RTD's lead wires have resistance and long lead wires will greatly affect the measurement accuracy. Therefore, the actual resistance measured by the circuits shown in Figs. 1 and 2 is (RTD + 2 × RWIRE), where RWIRE is the resistance of the lead wires, assuming both wires have the same resistance. Although theoretically acceptable, the same RWIRE implies that both wires are of the exact same length and made with the exact same material. Such an assumption can't be guaranteed in critical temperature-sensing applications. For this reason, RTDs feature 3-wire, and 4-wire configurations to help reduce the measurement error contributed by lead wires.

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