Introduction
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Is it hot in here?
Probably the most basic of measurements, that we have all wanted to make as tome point in our careers, is temperature. All equipment, including our bodies, are affected by temperature and the need to understand the local temperature is the first step to being able to control it and use it to our advantage.
You would think that temperature would be the easiest of all things to measure because it becomes quickly apparent, as you create electrical designs, that almost everything has a temperature dependence. If its output varies with temperature you have created a temperature sensor, quick file a patent. Just because a thing changes with temperature does not make it a good temperature sensor.
What’s the dirt on the sensor?
The datasheet of a sensor will tell you everything you want to know about a sensor; unless of course it is written in Chinese and then you must hope your Chinese friends will help you out. One of the things you will want to know about a sensor is its temperature range (e.g. -55degC to 150degC). You want to make sure that the temperatures you want to measure are covered by the sensor you are looking at using. Choosing an appropriate sensor range is critical. Not only must your desired temperature range fit within the temperature range of desired sensor, but you don’t want the sensor to extend too far above or below your desired range. For example, if you want to measure temperature around room temperature for say a thermostat, then a sensor that can read from 0 to 1500 degC would not be appropriate even though your temperature range fits within the sensors range. Things like precision, resolution, repeatability, response time and cost will also affect your choice.
The sensitivity of a sensor is the slope of the response curve, for example temperature vs. current. A sensor may be defined as having an output of 1uA/K. Meaning that the output current of the sensor is 1uA times the temperature in Kelvin.’
The accuracy of a sensor is the maximum difference between the actual temperature and the measured temperature. A sensor may claim an accuracy of 5.5degC, 2.5degC, 1degC or 0.5degC. The lower the temperature range specified in the accuracy, the more it will cost.
Repeatability has to do with repeated measurements under the same conditions. A sensor that has low repeatability is not considered reliable. Likewise, a sensor that drifts away from the true temperature value over time is not considered reliable.
Do I want it to be linear?
You may have heard about a response being linear or the output being linear. Most things are not linear, especially sensors. We have to work pretty hard to get things to be linear. First off, a linear response just means the output vs. the temperature is a straight line. This is one of the reasons that even though most things have a temperature sensitivity they do not make good thermometers. A sensor that has a sensitivity of 1uA/K across its whole range is really exciting.
One of the projects I worked on as an undergraduate was to turn a thermistor, a very poplar temperature sensor, into a usable thermometer. The resistance of a thermistor varies in a predictable way with temperature. The problem is that you cannot just measure the resistance and know the temperature. There is a complicated formula (a reciprocal relationship to the log of the resistance, also powers and stuff) to turn the resistance of a thermistor into a temperature. Part of the problem is that thermistors are not linear, they are logarithmic. Making a useable sensor involves linearizing the response. While you cannot make it perfectly linear, you can drive the error to zero in several places. The more places you drive it to zero the more complex the circuit.
With the advent of modern cheap microcontrollers, you could measure the resistance of a thermistor and convert the data to temperature and then output it in any form that is desired. With the dropping cost of MCU’s this might not be a bad solution. However, until just recently this was not even a possibility.
The process of turning a non-linear signal into a linear one is often called calibration. There are other things that calibration compensates for, but non-linearity is usually a large part of the calibration process.
What’s a PTAT?
PTAT is an acronym that stands for Proportional to Absolut Temperature. The numbers I was quoting above are from the AD590 temperature sensor that has been around since the 1970’s. The device is really easy to use. It is a 2-pin current device with a linear output of 1uA/K. At 25degC (298.15K) the device outputs 298uA. If you run that through a 1K resistor of high accuracy you will get a reading of 0.298 volts (1mV/K).
The devices have a range of accuracy from ±5degC to ±0.5degC. The prices range from $5.75 to $185 each. They are available in a variety of packages from flatpack, metal can to SOIC. They are even available in raw die.
They can be used for direct or differential temperature measurements. Putting them in series will give you the lowest temperature of the lot while in parallel will give you the average temperature. They can be driven directly by 5V CMOS MUX chips allowing a very simple way of taking a series of temperature measurements.
Final thoughts
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