Silicon Simplifies Sensors

If you need simple electronic sensors that produce a direct temperature value, look no further than semiconductor sensors. In most cases, you need no involved math to convert a value from one of these sensors into a temperature.

You can use this type of direct-readout sensor only within the operating range — usually -55C to about 125C — specified for the semiconductor device itself. This type of sensor works well when you must measure temperatures within electronic equipment, consumer products, equipment under test and so on. Semiconductor sensors require local power and many must operate close to a measuring instrument. (Simple electronics can extend a sensor's operating distance.)

Analog Devices' AD537 IC converts a voltage to a frequency you can measure with a counter. One of the converter's reference voltages varies linearly with temperature and will cause the frequency output to vary by 10 Hz/K. So, at 20C (293K), the converter produces a frequency of 2.93 kHz. A lab counter/timer can provide an accurate readout. In an embedded system, a microcontroller (MCU) can accumulate a count over a programmed period based on the CPU's clock frequency. If timing does not allow for an exact one-second period, you usually can get close enough. The AD537 provides an accuracy of about ±2C. I have used this device and it works well.

Some sensors, such as the Dallas Semiconductor DS1726 and the National Semiconductor LM70, convert temperatures to digital values and transmit those values as serial bit streams. The DS1726 produces a user-selected 9-to-12-bit binary value that directly represents temperature in degrees Celsius. The DS1762 offers an accuracy of ±1C. A similar sensor, the DS1626, offers an accuracy of ±0.5C. A three-wire serial interface connects the sensor to an MCU. The LM70 operates with a resolution of ±0.25C. It produces signed 10-bit values and communicates over an SPI or Microwire serial interface.

If you decide to use a semiconductor direct-readout sensor, ensure its output signal will properly drive your measurement equipment or MCU. You could not apply a digital output from a 5V sensor to an MCU that operates from a 1.8V supply, for example. Also, operate these semiconductor sensors within manufacturers' specifications. Overloading a sensor's output can lead to self-heating, which will affect temperature readings. Keep in mind, these types of sensors measure the temperature of the internal silicon die. So, follow vendors' mounting recommendations to ensure you position the IC package so as to put the die as close as possible to the spot you want to monitor. Consider using thermal grease or a heat-sink compound to provide a low thermal-resistance path.

	The LM70 sensor requires only a three-wire interface - plus power and ground - to receive commands from and transfer actual temperature values to a microcontroller.