MLX90614_15 Datasheet, PDF (27/51 Pages) Melexis Microelectronic Systems – Single and Dual Zone Infra Red Thermometer in TO-39

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:
( ) V_ir (Ta,To) = A× To⁴ − Ta⁴
Where To is the absolute object temperature (Kelvin), 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 PTAT element. All the sensors conditioning and data
processing is handled on-chip and the linearized sensor die temperature Ta is 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. The linearized die temperature is available in RAM cell 0x06:
- 0x06=0x2DE4 (11748d) corresponds to -38.2˚C (linearization output lower limit)
- 0x06=0x4DC4 (19908d) corresponds to +125˚C. (linearization output higher limit)
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.
Please note that 1LSB corresponds to 0,02° and the MSB bit is error flag (if “1” then error).
Example:
1. 0x27AD
2. 0x27AE
3. 0x3AF7
4. 0x3AF8
5. 0x7FFF
6. 0x8XXX
-70.00˚C (no error)
-69.98˚C (no error)
28.75˚C (no error)
28.77˚C (no error)
382.19˚C (no error) - maximum possible value returned by MLX90614
xxx.xx˚C (flag error)
The result is calculated by following expressions (valid for both To and Ta):
1. Convert it to decimal value i.e. 0x3AF7 = 15095d
2. Divide by 50 (or multiply by 0.02) i.e. ¹⁵⁰⁹⁵ = 301.9 K (result is in Kelvin)
50
3. Convert K -> ˚C i.e. 301.9 - 273.15 = 28.75˚C
3901090614
Rev 009
Page 27 of 52
Data Sheet
June 29, 2015