What makes industrial sensors go awry?
Jason Seitz, Texas Instruments - February 25, 2013
One of the most popular measurements in industrial processes is temperature. Temperature can be measured by a variety of sensor types including thermocouples, resistance-temperature detectors (RTDs), and thermistors, to name a few. To measure the largest temperature range, a system designer often uses a thermocouple.
For example, a Type C thermocouple has a measurement temperature range between zero and 2320°C. Thermocouples principle of operation is based on the Seebeeck effect where, if two dissimilar metals are placed together, a voltage is produced that is proportional to the temperature at the junction. Thermocouples are bipolar devices that produce a positive or negative voltage, depending on the sensing or “hot” junction temperature relative to the reference or “cold” junction temperature. First, a bias to the thermocouple is needed so it won’t rail against ground in a single-supply system. Next, measure the cold junction temperature to obtain the temperature being measured. Cold junction temperature can be measured with an IC temperature sensor, like the LM94022. One drawback to thermocouples when compared to other temperature sensors is limited accuracy, typically worse than ±1°C.
If the system requires greater precision over a reduced temperature range, say less than 660°C, a designer can implement the measurement with an RTD, which can be as accurate to below ±1°C. RTDs are resistive elements whose resistance depends on the ambient temperature they are placed in. They come in two-, three-, and four-wire configurations. Increasing the number of wires increases accuracy. RTDs require an excitation in the form of a current source. Current source values often are 100 µA to 1 mA to handle PT100 (100 Ohm at 0°C), and PT1000 RTDs (1000 Ohm at 0°C).
For accuracy up to ±0.1°C, with the tradeoff of an even smaller temperature range (less than 100°C), thermistors can be used. Similar to RTDs, thermistors’ resistance also change with temperature. Thermistors typically are connected in a resistor divider configuration where the other resistor in the divider is the same as the nominal value (value at room temperature, 25°C) of the thermistor. One end of the thermistor is connected to the supply voltage and the other end is connected to the other resistor, which in-turn is connected to ground (Figure 1). To determine temperature, measure the voltage at the divider’s center point. You would expect +V/2 at 25°C. Any deviations from this you can calculate the thermistor’s resistance and use a look-up table to determine the ambient temperature being measured.
In summary, temperature sensors need bias (voltage or current). When it comes to thermocouples, cold junction compensation is needed. TI offers a complete solution to these requirements. The LMP90100 is a 24-bit sensor AFE system with four differential or seven single-ended inputs, with two matched programmable current sources, and continuous background calibration (Figure 2). The LMP90100 is an integrated configurable chip perfect for addressing the various design challenges associated with temperature sensors.
Strain-gauges and load cells that utilize the Wheatstone bridge circuit are popular implementations for measuring pressure, force, and weight. Any strain or stress on the gauge causes a resistance change and subsequent voltage differential change on the sensor output (Figure 3). The voltages that come out of these sensors are low, typically in the mV range. To make the most accurate measurements, amplify this small voltage up to the data converter’s full dynamic range. To allow interfacing with multiple sensors as well as optimum flexibility, use a programmable gain amplifier (PGA) stage. This stage should have low noise, low offset, and low-offset drift to ensure best system performance.