RTDs, or resistance temperature detectors, are sensors used to measure temperature. These sensors are the among the most accurate temperature sensors available, covering large temperature ranges. However, getting accurate measurements with precision analog-to-digital converters (ADCs) requires attention to detail in design of measurement circuits and calculation of the measurement. This application note starts with an overview of the RTD, discussing their specifications, construction, and details in their use in temperature measurement. Different circuit topologies with precision ADCs are presented for different RTD configurations. Each circuit is shown with a basic design guide, showing calculations necessary to determine the ADC settings, limit measurement errors, and verify that the design fits in the operating range of the ADC.
RTDs are resistive elements that change resistance over temperature. Because the change in resistance is well characterized, they are used to make precision temperature measurements, with capability of making measurements with accuracies of well under 0.1°C. RTDs are typically constructed from a length of wire wrapped around a ceramic or glass core. RTDs may also be constructed from thick film resistors plated onto a substrate. The wire or resistance is typically platinum but may also be made from nickel or copper. The PT100 is a common RTD constructed from platinum with a resistance of 100 Ω at 0°C. RTD elements are also available with 0°C resistances of 200, 500, 1000, and 2000 Ω.