MLX90614 family
Single and Dual Zone
Infra Red Thermometer in TO-39
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:
Vir
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 to 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 contend 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.
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. During this phase the number of IR
sensors is selected and it is decided which temperature sensor will be used. Measurements, compensation
and linearization routines run in a closed loop afterwards.
Processing ambient temperature includes:
Offset measurement with fixed length FIR filter
Additional filtering with fixed length IIR filter. The result is stored into RAM as TOS
Temperature sensor measurement using programmable length FIR *.
Offset compensation
Additional processing with programmable length IIR **. The result is stored into RAM as TD.
Calculation of the ambient temperature. The result is stored into RAM as TA
Processing of the object temperature consists of three parts. The first one is common for both IR sensors, the
third part can be skipped if only one IR sensor is used.
IR offset:
Offset measurement with a fixed length FIR
Additional filtering with a fixed length IIR. The result is stored into RAM as IROS.
3901090614
Rev 003
Page 22 of 40
Data Sheet
03/Oct/2007
MELEXIS [ Melexis Microelectronic Systems ]
