AN3229| Application Note
Maxim > App Notes > AUTOMOTIVE CIRCUIT PROTECTION TEMPERATURE SENSORS and THERMAL MANAGEMENT Keywords: MAX6674, MAX6691, MAX6675, MAX6642, temperature sensors, system measurement and protection
May 11, 2004
APPLICATION NOTE 3229
Selecting Temperature Sensors for System Measurement and Protection
Abstract: A number of technologies are available to designers who need to measure temperature within a system. Thermistors, thermocouples, RTDs, and temperature-sensor ICs each have advantages and disadvantages in any given situation. This article compares the most popular temperature sensing technologies. It discusses the suitability of each technology for monitoring common targets such as PC boards, ambient air, and high-power circuits like as CPUs and FPGAs.
Temperature-Sensing Technologies
Sensors are often used within electronic systems to monitor temperature and provide protection from excessive temperature excursions. The most common technologies for use within systems are listed below. Thermocouples are made by joining two wires of dissimilar metals. The point of contact between the wires generates a voltage that is approximately proportional to temperature. Characteristics include wide temperature range (up to 1250 C), low-cost, very low output voltage (on the order of 40uV per C for type K), reasonable linearity, and moderately complex signal conditioning (cold-junction compensation and amplification). There are several thermocouple types, which are designated by letters. The most popular is type K. Maxim manufactures ICs (MAX6674 and MAX6675) that perform the signal conditioning functions for type-K thermocouples, simplifying the design task and significantly reducing the number of components required to amplify, coldjunction compensate, and digitize the thermocouple's output. Thermoco
uples are available in probes and with bare leads. RTDs are essentially resistors (often made from platinum wire) whose resistance varies with temperature. Characteristics include wide temperature range (up to 750 C), excellent accuracy and repeatability, reasonable linearity, and the need for signal conditioning. Signal conditioning for an RTD usually consists of a precision current source and a high-resolution ADC. Cost can be high. RTDs are available in probes, in surface-mount packages, and with bare leads. Thermistors are temperature-dependent resistors, usually molded from conductive materials. The most common thermistors have a negative temperature coefficient (NTC) of resistance. Characteristics include moderate temperature range (up to 150 C), low-to-moderate cost (depending on accuracy), poor but predictable linearity, and the need for some signal conditioning. Thermistors are available in probes, in surfacemount packages, with bare leads, and in a variety of specialized packages. Maxim manufacture
s ICs that convert thermistor resistance to digital form. IC temperature sensors are complete, silicon-based sensing circuits with either analog or digital outputs. Characteristics include moderate temperature range (up to about 150 C), low cost, excellent linearity, and additional features like signal conditioning, comparators, and digital interfaces. Digital formats are numerous and include 3- wire and 4-wire (such as SPITM), 2-wire (I2CTM and SMBusTM), and single-wire (1-Wire , PWM, frequency, and period). Note that signal conditioning, analog-todigital conversion, and thermostatic functions all add costs to the other sensing technologies, but are normally included within sensor ICs. IC temperature sensors are available primarily in surface-mount packages.
AN3229 Application Note maxim Download PDF
Add this permalink to your bookmarks for future download of AN3229 ApplicationNote
Permalink: http://application.emcelettronica.com/maxim/AN3229
