MLX90616 Datasheet, PDF (24/36 Pages) Melexis Microelectronic Systems – Infra Red Thermometer in TO-39 for high temperature thermometer guns

MLX90616ESF-HCA
Infra Red Thermometer in TO-39
for high temperature thermometer guns
8.7 Computation of ambient and object temperatures
The IR sensor consists of serial connected thermo-couples with cold junctions placed at thick chip substrate
and hot junctions, placed over thin membrane. The IR radiation absorbed from the membrane heats (or
cools) it. The thermopile output signal is:
  V_ir Ta,To  A. To⁴ Ta⁴ ,
Where To is the object temperature absolute (Kelvin) temperature, Ta is the sensor die absolute (Kelvin)
temperature, and A is the overall sensitivity.
An on board temperature sensor is needed to measure the chip temperature. After measurement of the
output of both sensors, the corresponding ambient and object temperatures can be calculated. These
calculations are done by the internal DSP, which produces digital outputs, linearly proportional to measured
temperatures.
8.7.1 Ambient temperature Ta
The Sensor die temperature is measured with a PTC or a PTAT element. All the sensors conditioning and
data processing is handled on-chip and the linearized sensor die temperature Ta is made available in
memory.
The resolution of the calculated temperature is 0.02 ˚C. The sensor is factory calibrated for the full automotive
range (-40…+125 ˚C). In RAM cell 006h, 2DE4h corresponds to -38.2 ˚C (linearization output lower limit) and
4DC4h (19908d) corresponds to 125 ˚C. The conversions from RAM content to real Ta is easy using the
following relation:
Ta[K]  Tareg  0.02 , or 0.02 °K / LSB.
8.7.2 Object temperature To
The result has a resolution of 0.02 ˚C and is available in RAM. To is derived from RAM as:
To[K]  Toreg  0.02 , or 0.02 °K / LSB.
Example:
1. 0x0000 => -273.15˚C (- min possible value returned by MLX90616
2. 0x0001 => -273.13˚C
3. 0x0002 => -273.11˚C and so on
4. 0x3AF7 => 28.75˚C
5. 0x7FFF => 382.19˚C - max possible value returned by MLX90616
6. 0xFFFF => 1037.55 - max possible value returned by MLX90616
The result is calculated by using the following expressions:
1. Convert it to decimal value i.e 0x3AF7 = 15095d
2. Divide by 50 (or multiply by 0.02) i.e. 15095/50=301.9K (result is in Kelvin)
3. Convert K -> ˚C i.e. 301.9-273.15=28.75˚C
8.7.3 Calculation flow
The measurement, calculation and linearization are held by core, which executes a program form ROM.
After POR the chip is initialized with calibration data from EEPROM. Measurements, compensation and
linearization routines run in a closed loop afterwards.
3901090616
Rev 002
Page 24 of 36
Data Sheet
August 20, 2012