Advances in Measuring with Nonlinear SensorsApril 1, 2005 By: Bonnie C. Baker, Microchip Technology Inc. Sensors
Obtaining data from a sensor that is less than linear is easy enough if you need only a small portion of the sensor's output range. With this scenario, you can implement a simple, piecewise, linearization algorithm in your controller or processor. If you want a wider output range from the sensor, you can use analog linearization circuits to help tame that output. For instance, a resistor in series or parallel with a (nonlinear) thermistor will linearize a portion of the output, usually ±25°C (10-bit accurate) around a center point you have designed into the circuit. You can tune the center point of a thermistor response with the value of the added resistor. These circuit techniques can usually help you capture a wider but not total range of the sensor output. Hardware linearization techniques can be effective enough to serve most applications, but if the range limitations still affect the usefulness of your system, you can infuse flexibility into your circuit by using the programmability features of a microcontroller and a programmable gain amplifier (PGA).
The Nonlinear Thermistor
The word thermistor originated from the descriptor thermally sensitive resistor. There are two basic types: negative temperature coefficient (NTC) and positive temperature coefficient (PTC). NTC thermistors are best suited for precision temperature measurements; PTCs, for switching applications. In this article we will focus on NTCs. Their applications range from devices that monitor and control automotive exhaust emissions, ice detectors, skin sensors, blood and urine analyzers, refrigerators, freezers, mobile phones, and battery pack chargers. They are also components of precision instrumentation such as handheld meters and temperature gauges.
NTC thermistors have three modes of operation. One is based on the thermistor's resistance vs. temperature characteristics. The other two take advantage of voltage vs. current and current over time. Applications using resistance vs. temperature are by far the most prevalent in industry. In contrast to applications using voltage vs. current and current over time, resistance vs. temperature circuits depend on a "zero-power" operating condition. Zero power implies that there is minimal self-heating of the thermistor. Figure 1 shows the resistance vs. temperature response of a 10 kΩ NTC thermistor. The 25°C rating for individual thermistors is typically 1 kΩ to 10 MΩ.
Figure 1. For precision temperature measurement applications, the thermistor is used in a "zero power" state that keeps the current through the thermistor low and thus greatly reduces self-heating effects on the element's resistance. The negative temperature coefficient 10 kΩ thermistor whose performance is graphed here is from Vishay/BCcomponents.
Since the thermistor is a resistive element, current excitation is required. You can apply the current with a voltage or current reference. The performance of the thermistor in Figure 1 is reasonably repeatable as long as you keep the power across the device below the power dissipation capability of the package. Once you violate this thermal condition the thermistor will self-heat and artificially decrease in resistance, giving a higher-than-actual temperature reading.
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