EVB90615 Datasheet, PDF (8/30 Pages) Melexis Microelectronic Systems – Infra Red Thermometer

MLX90615
Infra Red Thermometer
8 Detailed description
8.1 Block diagram
IR sensor
OPA
ADC
FIR/IIR
filters
SMBus /
PWM
t°
DSP
MLX90615
MLX90325
Voltage
Reference
Figure 3: block diagram
8.2 Signal processing principle
A DSP embedded in the MLX90615 controls the measurements, calculates object and ambient temperatures
and does the post-processing of the temperatures to output them through SMBus compatible interface or
PWM (whichever activated).
The output of the IR sensor is amplified by a low noise, low offset chopper amplifier with programmable gain,
then converted by a Sigma Delta modulator to a single bit stream and fed to the DSP for further processing.
The signal passes a FIR low pass filter. The output of the FIR filter is the measurement result and is available
in the internal RAM. Based on results of the above measurements, the corresponding ambient temperature
T_a and object temperatures T_o are calculated. Both calculated temperatures have a resolution of 0.02 ˚C.
An additional IIR LPF is programmable in EEPROM and allows customization of the thermometer output in
order to trade-off noise versus measurement speed . The IIR filter can also limit effect of spurious objects that
may appear in the FOV in some applications.
The PWM output can be enabled in EEPROM as the POR default. Linearized temperatures (To or Ta,
selectable in EEPROM) are available through the free-running PWM output.
8.3 Block description
8.3.1 Amplifier
A low noise low offset amplifier with programmable gain is implemented for amplification of the IR sensor
voltage. With a carefully designed input modulator and balanced input impedance, an offset as low as 0.5µV
is achieved.
3901090615
Rev 001
Page 8 of 30
Data Sheet
28/Aug/2008