Datasheet

MLX90640 32x24 IR array

Datasheet

Page 54 of 60

REVISION 11 – 4 MAY 2018

Figure 27 Calculation flow in thermal image mode

14. Application Comments

Significant contamination at the optical input side (sensor filter) might cause unknown additional filtering/distortion of the

optical signal and therefore result in unspecified errors.

IR sensors are inherently susceptible to errors caused by thermal gradients. There are physical reasons for these phenomena

and, in spite of the careful design of the MLX90640Bxx, it is recommended not to subject the MLX90640Bxx to heat transfer

and especially transient conditions.

The MLX90640Bxx is designed and calibrated to operate as a non-contact thermometer in settled conditions. Using the

thermometer in a very different way will result in unknown results.

Capacitive loading on an I

C can degrade the communication. Some improvement is possible with use of current sources

compared to resistors in pull-up circuitry. Further improvement is possible with specialized commercially available bus

accelerators. With the MLX90640Bxx additional improvement is possible by increasing the pull-up current (decreasing the

pull-up resistor values). Input levels for I

C compatible mode have higher overall tolerance than the I

C specification, but the

output low level is rather low even with the high-power I

C specification for pull-up currents. Another option might be to go

for a slower communication (clock speed), as the MLX90640Bxx implements Schmidt triggers on its inputs in I

C compatible

mode and is therefore not really sensitive to rise time of the bus (it is more likely the rise time to be an issue than the fall

time, as far as the I

C systems are open drain with pull-up).

Power dissipation within the package may affect performance in two ways: by heating the “ambient” sensitive element

significantly beyond the actual ambient temperature, as well as by causing gradients over the package that will inherently

cause thermal gradient over the cap

Power supply decoupling capacitor is needed as with most integrated circuits. MLX90640Bxx is a mixed-signal device with

sensors, small signal analog part, digital part and I/O circuitry. In order to keep the noise low power supply switching noise

needs to be decoupled. High noise from external circuitry can also affect noise performance of the device. In many

applications a 100nF SMD plus 1µF ceramic capacitors close to the Vdd and Vss pins would be a good choice. It should be

noted that not only the trace to the Vdd pin needs to be short, but also the one to the Vss pin. Using MLX90640Bxx with

short pins improves the effect of the power supply decoupling.

Ambient temperature calculation

(common for all pixels)

11.2.2.3

Gain compensation - 11.2.2.5.1

IR data compensation

–

offset, VDD and Ta

11.2.2.5.3

IR data gradient compensation - 11.2.2.7

Normalizing to sensitivity - 11.2.2.8

Image (data) processing