SIGNAL CONDITIONING FEATURE
Sensing the challenge T
Michael Mayes, design section head, Mixed Signal
Products, Linear Technology, examines how the challenges of temperature measurement can be solved by using a precise temperature-to-bits converter
emperature is one of the most commonly measured
physical parameters. Digitizing common temperature sensors – such as thermocouples, RTDs, thermistors and diodes – requires expertise in analogue circuit design, digital circuit design, and firmware development. The LTC2983 from Linear
Technology includes this expertise in a single IC, solving the challenges associated with the temperature sensors mentioned above. It combines all the analogue circuitry necessary for each sensor type with temperature measurement algorithms and linearization data to directly measure each sensor and output the result in ˚C. While methods for extracting temperature from these elements are well known, measuring temperatures to better than 0.5˚C or 0.1˚C accuracy is challenging.
THERMOCOUPLES Thermocouples generate a voltage as a function of the temperature difference between the tip (thermocouple temperature) and the electrical connection on the circuit board (cold junction temperature). To determine the thermocouple temperature, an accurate measurement of the cold junction temperature is required. This is known as cold junction compensation. The cold junction temperature is usually
determined by placing a separate (non- thermocouple) temperature sensor at the cold junction. The LTC2983 allows diodes, RTDs and thermistors to be used as cold junction sensors. To convert the voltage output from the thermocouple into a temperature result, a high order polynomial equation (up to 14th order) must be solved for both the measured voltage and the cold junction temperature. The LTC2983 has these polynomials built in for all
Measured accuracy of LTC2983 with a precision temperature calibrator
eight standard thermocouples (J, K, N, E, R, S, T, and B) as well as user programmed table data for custom thermocouples. This simultaneously measures the thermocouple output, the cold junction temperature and performs all required calculations to report the thermocouple temperature in ˚C. In addition to this it includes an open circuit detection circuit that checks for a broken thermocouple just prior to the
measurement cycle; and reports faults related to the cold junction sensor.
DIODES Diodes are inexpensive semiconductor based devices that can be used as temperature sensors and are typically used as the cold junction sensor for a thermocouple. When an excitation current is applied to a diode, they generate a voltage as a function of temperature and the current applied. If two perfectly matched excitation current sources of known ratio are applied to the diode, a voltage of know proportionality to absolute temperature (PTAT) is produced. The LTC2983 generates these current sources, performs the measurements and calculations and reports the results in ˚C.
RTDS
RTD temperature measurement using the LTC2983
RTDs are resistors that change value as a function of temperature. In order to measure one of these devices a low drift precisely known sense resistor is tied in series with the RTD. An excitation current is applied to the network and a ratiometric measurement is made. The value, in ohms, of the RTD can be determined from this ratio. This resistance is used to determine the temperature of the sensor element using a table lookup. The LTC2983
automatically generates the excitation current, simultaneously measures the sense resistor and RTD voltage, calculates the sensor resistance and reports the result in ˚C. RTDs can measure temperatures over a wide temperature range, from as low as -200˚C to 850˚C. The LTC2983 can digitize most RTD types (PT-10, PT-50, PT-100, PT-200, PT-500, PT-1000, and NI-120), has built in coefficients for many standards (American, European, Japanese, and ITS-90) and can also measure custom table driven RTD sensors. RTDs come in many configurations.
2-wire, 3-wire, and 4-wire. The LTC2983 can accommodate all three configurations with a configurable single hardware implementation. It can share a single sense resistor among multiple RTDs; and its high impedance input allows external protection circuits between the RTD and ADC inputs without introducing errors. It can also auto rotate the current excitation to eliminate external thermal EMF errors (parasitic thermocouples). In cases where parasitic lead resistance of the sense resistor degrades performance, the LTC2983 allows Kelvin sensing of Rsense. It also includes fault detection circuitry capable of detecting and reporting common faults associated with RTDs.
THERMISTORS Thermistors are resistors that change value as a function of temperature. Unlike RTDs, a thermistors resistance varies many orders of magnitude over its temperature range. In order to measure one of these devices a sense resistor is tied in series with the sensor. An excitation current is applied to the network and a ratiometric measurement is made. The value, in ohms, of the thermistor can be determined from this ratio. This resistance is used to determine the temperature of the sensor solving Steinhart-Hart equations or table data. The LTC2983 automatically generates the excitation current, simultaneously measures the sense resistor and thermistor voltage, calculates the thermistor’s resistance and reports the result in ˚C – thermistors typically operate from -40˚C to 150˚C. The LTC2983 includes coefficients for calculating the temperature of standard 2.252kΩ, 3kΩ, 5kΩ, 10k Ω, and 30k Ω thermistors. Since there is a large variety of thermistor types and values, the LTC2983 can be programmed with custom thermistor table data (R vs. T) or Steinhart-Hart coefficients. Of additional benefit, its fault detection circuitry is capable of detecting and reporting common faults associated with thermistors.
Linear Technology
www.linear.com INSTRUMENTATION | JULY/AUGUST 2016 29
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