EVB90615 Datasheet, PDF (19/30 Pages) Melexis Microelectronic Systems – Infra Red Thermometer

MLX90615
Infra Red Thermometer
8.7 Principle of operation
The IR sensor consists of series 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 additional sensor is needed for 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 RAM.
The resolution of the calculated Ta is 0.02 ˚C. The sensor is factory calibrated for the range -40 to +85 ˚C. In
RAM cell , 6h, 2D89h corresponds to -40 ˚C and 45F3h corresponds to +85 ˚C. Conversion RAM content to
real Ta is easy:
Ta[°K ] = Tareg × 0.02
8.7.2 Object temperature To
The result has a resolution of 0.02 ˚C and is available in RAM (address 7h). To is derived from RAM as:
To[°K ] = Toreg × 0.02
3901090615
Rev 001
Page 19 of 30
Data Sheet
28/Aug/2008