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Signal Chain Basics #79: Digital Temperature Sensors Can Replace Thermistors

Temperature sensor versus thermistors
Many methods exist to measure temperature in electronic systems. To measure the local temperature of an object or area of a PCB, the most common methods are to use a thermistor or local temperature sensor. Table 1 summarizes the basic principles of operation for both thermistors and silicon temperature sensors, and briefly summarizes the advantages and disadvantages of each.

Table 1

Thermistor versus silicon temperature sensor. For a larger view, click here.

Thermistor versus silicon temperature sensor. For a larger view, click here.

Let's look specifically at digital temperature sensors. These integrate additional signal conditioning including an analog-to-digital converter (ADC). The output is a digital word that represents the temperature experienced by the bandgap junction. Here are some of the advantages they provide over thermistors.

Noise immunity
A digital interface, such as I2 C, SMBus, or SPI, by definition, has significantly better noise immunity versus an analog interface. As the analog output of a thermistor transits the PCB trace to the signal conditioning circuitry, it is subject to multiple noise sources, whether from electromagnetic interference (EMI) from off-board sources, or potential cross-talk with neighboring traces.

Digital signals are subject to the same noise sources. However, since only a threshold (digital 0 or 1) is important, it takes a significant noise source to create an error in a bit stream. Analog signals, by nature, are continuous. So any noise that adds to or subtracts from the signal level creates additional error in the measured temperature reading.

Power consumption
A thermistor requires a constant current excitation to create a voltage drop across it that is representative of the temperature being measured. This current is usually on the order of 100μA. In addition to this current flow, any signal conditioning circuitry (amplifiers and/or ADCs) add to this value. Digital temperature sensors can vary greatly in power consumption. However, Texas Instruments has focused on minimizing power consumption and have multiple digital temperature sensors that are 50μA and under. Included is the TMP103, which has a maximum power consumption of 3μA, or 3 percent of just the excitation current required by a thermistor. System management resources
A digital temperature sensor can free up system management resources in many ways. The most obvious is that with the ADC integrated on chip, no additional signal conditioning is required to provide temperature to the system management decision making module.

Additionally, the resistance change of a thermistor relative to temperature is non-linear (Figure 1). This requires the system management controller (SMC) to implement a look-up table to accurately understand the temperature. Also, this non-linearity causes the ADC error to be non-uniform across the full input range as each LSB represents a different change in temperature.

Figure 1

Thermistor non-linearity versus temperature

Thermistor non-linearity versus temperature

Many digital temperature sensors offer an ALERT capability for over/under temperature conditions which can be used as interrupts to the SMC. This allows it to ignore the actual temperature that the system is experiencing until it reaches a threshold at which point the SMC can start monitoring the temperature. Otherwise, you'd have to monitor the thermistor output in real time and make decisions on every reading.

Multiple temperature readings
Finally, if multiple temperature reading locations are required to ensure system management, thermistors require signal conditioning resources for each. This ties up multiple pins and ADCs within the SMC, if not separate external ADCs for each thermistor. Power consumption becomes even more evident as each thermistor requires its own current excitation source of 100μA. Since the industry standard interfaces utilized by digital temperatures all allow for multiple devices to exist on a single bus, SMC resource requirements are minimized.

For example, the TMP103 allows for up to eight devices on a single I2 C master via eight unique address options. The TMP104 utilizes a novel one-wire interface called SMAART wire that allows for up to 16 devices to be daisy chained and managed via a single serial (UART) interface on the SMC (Figure 2).

Figure 2

TMP104 digital temperature sensor with one-wire interface. For a larger view, click here.

TMP104 digital temperature sensor with one-wire interface. For a larger view, click here.

Please join us next month where we will see how to optimize a SAR ADC driver based on a given precision versus power requirement.

References:

For more information, visit: www.ti.com/tempsensor-ca.

About the author:
Dan Harmon is Sensing Business Development Manager for TI's sensing group. In his 25+ 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 . Related posts:

19 comments on “Signal Chain Basics #79: Digital Temperature Sensors Can Replace Thermistors

  1. Dirceu
    July 12, 2013

      Completely analog sensors are unidirectional devices, providing just the associated quantity. Otherwise, digital sensors are very flexible as they can accept commands and settings; in addition to providing the measurement directly in degrees Celsius / Fahrenheit. The downside (here I'm a PA advocate) is the need for a digital front end, such as microcontroller / state machine / pld in order to handle the I2C / SPI protocols.

  2. DHarmon
    July 12, 2013

    Dirceu,

    Thanks for reading and commenting.  

    Since temp sesnors are often used in systems in whcih a MCU of some sort is present, the digital interface is usally already available.  There are areas where the analog output temp sensors are great as well.

    The great thing is that at TI we have a whole range of both analog and digital temp sensors for whatever the system needs.

  3. eafpres
    July 13, 2013

    Hi Dan–I looked up the TMP-103.  Pretty impressive littie device.  1µA typical consumption, works from -40 to +125, 1ºC resolution.

    I'm guessing it might be used in cell phones.  Is it also used in automotive?

  4. Netcrawl
    July 13, 2013

    Texas Instruments has made great stride in this area- its new sensor the TMP-103 is the industry's most power efficient digital temperature sensor, which consumes 97 precent less power. and this one is much smaller than its closest rivals- probably the smallest in the industry.TI has just set the gold statndard when it comes to power efficiency and form factor, TMP 103 simplifies thermal profiling enabling lonegr battery life.  

  5. Davidled
    July 13, 2013

    Digital Temperature sensor is very impressive, when overviewing disadvantage tab. But in the Auto Lab, still thermistor is so popular used for engine powertrain and other purpose. I think that digital temperature sensor may be good fit to PC to control FAN on and off or consumer electronics. 

  6. DHarmon
    July 15, 2013

    Thanks for reading the article and taking the time to look at our temperature sensors.

    There is not a real fundamental reason the TMP103 could not be used in automotive other than many automotive applications will not use wafer chipscale packaging.

    For similar functionality in a non-WCSP package, the TMP102 is in a SOT-563 package. It consumes more power than the TMP103 at 10uA, but this is still significantly lower than any competitors digital solutions as well as less than thermistor applications typically use.

  7. DHarmon
    July 15, 2013

    Thanks for reading and the great words about our TMP103.

  8. DHarmon
    July 15, 2013

    Thanks for reading the article.

    You are correct that thermistors still get used in many applications.  However, there are no fundamental reasons that temperature sensor ICs (either digital or analog) cannot be used in any of these applications.  The TI portfolio of temperature sensors are used across a wide spectrum of applications including many in the automotive space.

  9. antedeluvian2
    July 15, 2013

    Dan

    You are correct that thermistors still get used in many applications.  However, there are no fundamental reasons that temperature sensor ICs (either digital or analog) cannot be used in any of these applications

    I recently did a series of 4 blogs on the measuring of temperature on MCC. In the comments of the 3rd, I pointed to a TI blog Ditch the NTC thermistor: use an analog temp sensor. Didier Juges provided this thought provoking response:

    “The thermistor still has some advantages over a semiconductor sensor like the LMT87 but the list of advantages is dwindling… Certainly the price is no longer an issue (the LMT87 is $0.87 at Newark, my favorite thermistor, the EPCOS B57871S is over $1). There are still reasons why I prefer thermistors in some cases.

      – I like glueing a thermistor into a solder lug to make a handy sensor that can be attached anywhere with a screw. The SOT23 package limits you to a PWB mount point.

      – the thermistor's non-linearity is actually an advantage if you need excellent resolution over a narrow temperature range.

      – the thermistor with its bias resistor is a ratiometric sensor. The temperature reading does not depend on the accuracy (or stability) of the voltage reference driving the ADC.

      – if you remote the temperature sensor at the end of long wires, a semiconductor sensor will be more easily damaged by transients picked up along the wires than a thermistor, an issue in some environments.”

     

  10. TheMeasurementBlues
    July 15, 2013

    For low cost and wide temperature range, you still can't beat a thermocouple.

  11. DCH
    July 17, 2013

    You can get a Murata NCP18XH103J03RB 0603 thermistor from Newark for less than $0.06 in quantities of 1.  The TMP103 is $1.01.

    No comparison in volume.

  12. Brad_Albing
    July 18, 2013

    @DHarmon – For automotive apps, you should be able to tolerate a 10uA current draw.

  13. Brad_Albing
    July 18, 2013

    @TMB – I'm with you when it comes to thermocouples and high-temp environments – industrial ovens, exhaust manifolds, etc.

  14. TheMeasurementBlues
    July 18, 2013

    >You can get a Murata NCP18XH103J03RB 0603 thermistor from Newark for less than $0.06 in quantities of 1.

    @DCH, thanks for the info, but can you order just one?

  15. TheMeasurementBlues
    July 18, 2013

    Thermocouples are really the only way to go for high temperature such as industrial ovens, kilns, etc.

    From Omega's Type K Thermocouple conversion sheet:

    MAXIMUM TEMPERATURE RANGE
    Thermocouple Grade
    – 328 to 2282°F
    – 200 to 1250°C

  16. Brad_Albing
    July 18, 2013

    That'll do.

  17. DCH
    July 18, 2013

    @TheMeasurementBlues

    The price I gave was for quantity of 1.  I am used to >100K EAU where the prices are much better.

  18. SunitaT
    July 31, 2013

    The main concern when measuring anything is to confirm the measuring device itself does not impact the media it is measuring. With contact temperature measurement, this is particularly important. Selecting proper sensor size, encapsulation and lead configuration are key design concerns to decrease “stem-effect” and other measurement errors. Once minimal effect of the measurement media is accomplished, how precisely you can measure the media becomes vital. Accuracy includes basis sensor characteristics, measurement accuracy, etc. The most accurate sensor is useless if design concerns around stem-effect are not addressed.

  19. DHarmon
    July 31, 2013

    Thanks for reading the article and commenting. 

    You are correct, the measurement system design is very critical to maximizing accuracy.

    Placement of the sensing element is fundamentally the same for a temperature sensor IC or thermistor.  One potential benefit of a digital temp sensor is that there is no long lead from the sensing element to the processing/digitizing circuitry that could be susceptible to noise as with an analog signal.

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