SNOAA47A September   2024  – October 2024 TMP116 , TMP117 , TMP119

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Sensor - Object Surface Thermal Resistance and the Importance for Measurement Precision
  6. 3Testing
    1. 3.1 Hardware Setup
    2. 3.2 Test Setup
    3. 3.3 Test Method
      1. 3.3.1 Measurement Results
  7. 4Thermal Parameters Calculations
  8. 5Summarizing and Interpreting Test Results
  9. 6Summary
  10. 7References
  11. 8Revision History

2 Sensor - Object Surface Thermal Resistance and the Importance for Measurement Precision

A significant part of all temperature measurements is measuring the surface temperature of some object. The classical way to do this is by measuring object temperature through the PCB board as Simplified Schematic of Temperature Flow During Solid Surface Temperature Measurement shows.

Simplified Schematic of Temperature Flow During Solid Surface Temperature MeasurementFigure 2-1 Simplified Schematic of Temperature Flow During Solid Surface Temperature Measurement

Where,

  • TOBJ is the measured object temperature
  • TAIR is the ambient temperature (typically air)
  • TS is the internal sensor temperature
  • RSO is the thermal resistance between the sensor and the object
  • RSA is the thermal resistance between the sensor and the air (environment)
  • PS is the average power dissipated by the sensor during the measurement
  • MT is the combined thermal mass of device and PCB

From the schematic, these parameters can be related by

Equation 1. T O F S = R S O × T O B J - T A I R R S O + R S A

where

  • TOFS is a temperature offset between the measured object and the sensor

Equation 1 shows that the sensor temperature offset is zero only in two cases: if RSO is zero or if RSA is infinite. If there is a difference between TOBJ and TAIR (and RSA is not significantly higher then RSO), some offset between the sensor and object temperature can be observed. This shift increases as the difference between TOBJ and TAIR increases, or when RSA becomes smaller and approaches the RSO value.

It is very important for precise measurements to minimize the value of RSO and properly estimate it during design. The value of RSO directly affects the sensor offset, the sensor temperature settling time, and, potentially, the need to perform system calibration. See the Precise Temperature Measurements With the TMP116 and TMP117, application note for more the details.