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Signal Chain Basics #86: Fundamentals of Temp Sensors

How does a silicon temperature (temp) sensor IC actually measure temperature? These ICs take advantage of the basic temperature-dependent behavior of silicon PN junctions. If you have two PN junctions of different areas and force a current through them, they produce two different forward voltage levels. With a constant current, the voltages rise or fall relative to the PN temperature junction. The difference between the voltages is proportional to absolute temperature:

Linear response is an important performance benefit of a silicon temp sensor over a thermistor (Figure 1). Additional processing is not required to calculate the actual temperature.

Figure 1. Temp sensor's linear response versus thermistor's non-linear response

Figure 1. Temp sensor's linear response versus thermistor's non-linear response

The earliest temp sensors produced an output voltage that was proportional to absolute temperature. Over time, engineers preferred data in °F or °C. With the correct circuitry (gain and/or level shifting), this absolute temperature can be converted to a voltage proportional to °C or °F. Analog output devices have either a positive (LM35 = 10 mV/°C) or negative (TMP20 = –11.77 mV/°C) gain. The LM34 is an example of a Fahrenheit-based temp sensor at 10 mV/°F. More recent devices have integrated an analog-to-digital converter (ADC) that allows for directly interfacing to a controller via a standard serial interface such as I2C or SPI.

Measuring temperature external to the IC
Since a temp sensor works based on the temperature experienced by the on-chip PN junctions, how do you measure the temperature of an object external to itself? Depending on which temperature you are trying to measure, for instance the IC on a PCB versus the air surrounding the unit determines where you place your sensor on the PCB and the size of the PCB. This goes back to the principle of thermal equilibrium from high school physics.

For a quick refresher, thermal equilibrium states that two thermally connected bodies at different temperatures will transfer heat until they reach the same temperature. Also the mass of the objects in question will affect the temperature flow. As you design a temperature measurement system, keep these two concepts in mind:

  • Sensor must be thermally connected to the object
  • Sensor's thermal mass should be smaller than the object's thermal mass

Measuring the temperature of another IC
In this example we measure the temperature of an on-board controller. During operation, the controller die heats up due to current flow. This increased die temperature results in a corresponding rise in the surrounding PCB temperature. Normally the largest thermal mass in the PCB is the ground plane. The rise in the die temperature causes the ground plane temperature to increase. The board layout determines how much the ground plane rises.

To accurately measure the increase in die temperature, the temp sensor IC must be thermally-coupled to the die's thermal mass: its ground connection. The temp sensor measures the board temperature at the point where its ground pad is soldered to the PCB. As long as the temp sensor and controller share a ground plane, the two should be in close thermal connection and equilibrium.

Measuring air temperature
In our second example we want to measure the air temperature. The PCB's thermal mass and how quickly it responds to a change in air temperature is critical. For best results, minimize the PCB's thermal mass where the temp sensor is mounted. Ideally, mounting the temp sensor on its own tiny board and connecting it to the main board with only the minimum wires keeps the temp sensor thermal mass to a minimum. This PCB should be thermally isolated from the main board and system housing to prevent them from affecting the results. The opposite is true relative to its thermal contact with air. Completely expose it to maximize the thermal connection.

Summary
Temp sensor ICs are very simple to use and work on very fundamental principles. Taking these principles into account allows a designer to achieve the most accurate result possible.

Please join us next time where we will discuss the finer details of the relationship between differential nonlinearity and missing codes in precision SAR data converters.

— Dan Harmon is Sensing Business Development Manager for TI's sensing group. In his 27-plus year career at TI, he has supported a wide variety of technologies and products including interface products, imaging analog front-ends (AFEs), and charge-coupled device (CCD) sensors. He also has served as TI's USB-IF Representative and TI's USB 3.0 Promoter's Group Chair. Dan earned a BSEE from the University of Dayton and a MSEE from the University of Texas in Arlington. You can reach Dan at .

10 comments on “Signal Chain Basics #86: Fundamentals of Temp Sensors

  1. Victor Lorenzo
    February 10, 2014

    The infrared temperature sensor is another one deserving special attention due to its particularities, operating principles and applications. TI's TMP006 and Melexis' MLX90614 are two interesting devices. GE (among others) also provides IR temperature sensors.

  2. RedDerek
    February 12, 2014

    I've used the handheld devices one can get from Harbor Freight. Only problem is if you want to have a better focusing spot, lensing is required. And then it cannot be a simple piece of glass for the glass will cut-off the IR spectrum the devices require. The GE site does go into this a bit on one of the data sheets.

  3. Victor Lorenzo
    February 12, 2014

    @RedDerek, I've used one handheld infrared thermometer from Fluke. It included the lense and one laser pointer. The manual described the method for estimating the field of view as a function of sensor to target object distance. It measures the mean temperature for the area contained in the field of view.

    Main inaccuracy source was the fact that different materials have different emissivity values.

     

  4. amrutah
    February 19, 2014

    ” the temp sensor IC must be thermally-coupled to the die's thermal mass…”

       The best possible method I know is of using a thermistor.  Pump-in a constant current into this thermistor and monitor the voltage.  But the problem I have seen is that the thermistors have negative tempco and also they are not linear.  What kind of external temperature sensor can be used?  Are there any MEMS related temp sensors available?

  5. amrutah
    February 19, 2014

    The principle you mentioned of generating a delta voltage by pumping a constant current in pn-junctions of different areas, is similar to what we do to generate the Bandgap reference.  The bandgap reference has the additional k*IR factor to compensate the negative tempco.  By positive gain of 10mV/deg, can we say that we over compensate by changing the k factor?

  6. geek
    February 23, 2014

    @Victor: Does the infrared temperature sensor fit well for all kinds of applications/environments? In particular, are there some industrial applications that it suits more and some that it doesn't suit so well? Also, what about the cost factor?

  7. geek
    February 23, 2014

    @amrutah: It may sound like a very naive question, but as far as I know a thermistor falls under the category of analog measurement devices. These, of course, have their own limitations. Would you not prescribe a digital mechanism for measuring temperature?

  8. Victor Lorenzo
    February 23, 2014

    @tzubair,

    >> Does the infrared temperature sensor fit well for all kinds of applications/environments?

    No, it does not. I think there is no one-fits-all temperature measurement solution. Not all materials equally emit infrared radiation, it depends on several factors including how the material surface has been finished. You will find some basic information about IR temperature measurement here.

    >> In particular, are there some industrial applications that it suits more and some that it doesn't suit so well?

    IR temperature sensors are generally not well suited in the case of transparent gases and are best suited in the case of metals. As far as I know, in most cases we can obtain better accuracy using a contact temperature sensor. Thermocouples and platinum RTDs give prety good results.

    >> Also, what about the cost factor?

    Cost varies for IR sensor based solutions. TI's TMP0006 provides an integrated solution which does not require an analog front end, provides remote and local temperature, interfaces to MCU using digital lines (I2C) and has a fairly low price (below 3$ for 250 quantities and <5$ for 10 or less).

    One thing about IR sensors, the value we read is not exactly the remote object's temperature, we need to apply some relative complex math to obtain the temperature value, but the math we need is well known and most manufacturers provide recomendations aobut it.

     

  9. amrutah
    February 23, 2014

    @tzubair: “Would you not prescribe a digital mechanism for measuring temperature”

       Since the temperature is a analog in characteristic, the measurement technique has to be analog in nature, I don't know of any digital measurement technique. Once we monitor the temperature we can sample the analog data, use a ADC and store temp-data in memory or display on LCDs.

       

     

  10. geek
    February 28, 2014

    @Victor: Thank for the elaborate answer. It really added to my knowledge about IR sensors. So from what I get, IR sensors have a very limited industrial application particularlies in manufacturing processes that invole the use of metals. However, the cost factor does not seem to limit their usage. It's only the nature of components they can measure the temperature of that matters.

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