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Application Notes

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  Accurate Temperature Sensing with an External P-N Junction &mdash AN137
  PDF, 540 Kb, File published: Sep 20, 2012
  Many Linear Technology devices use an external PNP transistor to sense temperature. Common examples are LTC3880, LTC3883 and LTC2974. Accurate temperature sensing depends on proper PNP selection, layout, and device configuration. This application note reviews the theory of temperature sensing and gives practical advice on implementation.
  Extract from the document

  Application Note 137
  May 2012
  Accurate Temperature Sensing with an External P-N
  Junction
  Michael Jones
  Introduction Temperature Sensing Theory Many Linear Technology devices use an external PNP
  transistor to sense temperature. Common examples are
  LTC3880, LTC3883 and LTC2974. Accurate temperature
  sensing depends on proper PNP selection, layout, and
  device configuration. This application note reviews the
  theory of temperature sensing and gives practical advice
  on implementation. Linear Technology devices use an external bipolar transistor p-n junction to measure temperature. The relationship
  between forward voltage, current, and temperature is: Why should you worry about implementing temperature
  sensing? Can’t you just put the sensor near your inductor
  and lay out your circuit any way you want? Unfortunately,
  poor routing can sacrifice temperature measurement
  performance and compensation. The purpose of this application note is to allow you the opportunity to get it right
  the first time, so you don’t have to change the layout after
  your board is fabricated.
  Why Use Temperature Sensing?
  Some Linear Technology devices measure internal and
  external temperature. Internal temperature is used to
  protect the device by shutting down operation or locking …

Design Notes

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  System Monitor with Instrumentation-Grade Accuracy Used to Measure Relative Humidity &mdash DN510
  PDF, 389 Kb, File published: Nov 30, 2012
  Extract from the document

  System Monitor with Instrumentation-Grade Accuracy Used to
  Measure Relative Humidity
  Design Note 510
  Leo Chen
  Because much can be deduced about a physical system by measuring temperature, it is by far the most
  electronically measured physical parameter. Selecting
  a temperature sensor involves balancing accuracy
  requirements, durability, cost and compatibility with
  the measured medium. For instance, because of its low
  cost, a small-signal transistor such as the MMBT3904
  is an attractive choice for high volume or disposable
  sensing applications. Although such sensors are
  relatively simple, accurate temperature measurement
  requires sophisticated circuitry to cancel such effects
  as series resistance.
  The LTC2991 system monitor has this sophisticated
  circuitry built in—it can turn a small-signal transistor
  into an accurate temperature sensor. It not only measures remote diode temperature to В±1В°C accuracy, but
  it also measures its own supply voltage, single-ended
  voltages (0 to VCC) and differential voltages (В±325mV). While ostensibly designed for system monitor applications, the top shelf performance of the LTC2991 makes
  it suitable for instrumentation applications as well, such
  as the accurate psychrometer described here.
  A Psychrometer: Not Nearly as Ominous as …

Articles

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  Making Accurate Temperature Measurements with Devices with Different Ideality Factors &mdash LT Journal
  PDF, 261 Kb, File published: Apr 19, 2017
  Extract from the document

  Making Accurate Temperature Measurements with Devices
  with Different Ideality Factors
  Hamza Salman Afzal and Shuilin Tian The ability to accurately measure temperature is critical
  to the reliable operation of power systems. Accurate
  temperature data can reveal the instantaneous health of a
  system, and enable real-time protection functions such as
  thermal shutdown. For instance, significant rates of change
  in temperature can indicate increased power consumption.
  If a proper temperature feedback loop is implemented, this
  information can be used to reduce power consumption
  before temperatures escalate to dangerous levels.
  Perhaps the least expensive and most
  ubiquitous temperature sensor is a diode.
  A silicon p-n junction has a forward
  drop of approximately 700mV and a
  temperature coefficient of -2.2mV/C
  at room temperature. This sensitivity
  to temperature is exploited in many
  systems as a means of measuring
  temperature by simply biasing the diode
  with a constant current and measuring
  the resulting voltage (see Figure 1)
  This method has achieved wide acceptance …

• Download
  System Monitor with Instrumentation-Grade Accuracy Used to Measure Relative Humidity &mdash LT Journal
  PDF, 1.7 Mb, File published: Jan 20, 2012
  Extract from the document

  System Monitor with Instrumentation-Grade Accuracy Used
  to Measure Relative Humidity
  Leo Chen The LTC2991 is designed to measure supply voltages, currents and temperatures
  on large circuit boards when used in system monitor applications. It is also
  capable of delivering В±1В°C accuracy when using a 1-cent MMBT3904 transistor
  as a temperature sensor, making it suitable for many instrumentation applications.
  Temperature is the most measured physical parameter, with sensor selection a
  function of accuracy requirements, durability, cost and compatibility with the medium
  being measured. An inexpensive NPN transistor is an ideal sensor for applications
  calling for disposable sensors, or those that require a large number of sensors.
  A PSYCHROMETER: NOT NEARLY AS
  OMINOUS AS IT SOUNDS A psychrometer is a type of hygrometer,
  a device that measures relative humidity.
  It uses two thermometers, one dry (dry
  bulb) and one covered in a fabric saturated with distilled water (wet bulb). Air is
  passed over both thermometers, either by
  a fan or by swinging the instrument, as in
  a “sling psychrometer.” A psychrometric
  chart is then used to calculate humidity
  from the dry and wet bulb temperatures.
  Alternatively, a number of equations exist
  for this purpose. The following equations are used in testing this circuit. Figure 1 shows the LTC2991-based psychrometer. The two transistors provide
  the wet bulb and dry bulb temperature readings when connected to the …

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Manufacturer's Classification

• Signal Conditioning > Amplifiers > Current Sense Amplifiers
• Monitor, Control and Protection > Power Monitors
• Monitor, Control and Protection > Temperature Monitors > Digital Temperature Measurement Systems