MLX90614 Grounding and Layout

The MLX90614 is an Infra Red thermometer for non contact temperature measurements. Both the IR sensitive thermopile detector chip and the signal conditioning ASIC are integrated in the same TO-39 can. Integrated into the MLX90614 are a low noise amplifier, 17-bit ADC and powerful DSP unit thus achieving high accuracy and resolution of the thermometer.

Scope
 
Virtually any electronic device needs proper layout, grounding and power supply bypassing. The MLX90614 IR thermometers have simple requirements in this aspect. These are described in the following document.

Decoupling
 
The MLX90614 is a mixed signal device that operates with small sensor signals. The digital part of the device generates switching noise. However, the MLX90614 is a low power device, designed with care for low noise and this noise can be easily nullified with a single capacitor on the power supply pin.

The 5 volt supply voltage version – the MLX90614Axx - uses internal voltage regulators. Their Power Supply Rejection is beneficial in noisy systems.

There are two basic modes of operation of the MLX90614 – SMBus and PWM. (Thermal relay mode uses the same application circuits as PWM.) PWM is free running, with only one output pin and is thus more robust to conducted noise. The application schematic of the MLX90614 in PWM mode is shown on Fig. 2.

Layout
 
General EMC rules apply for the MLX90614 layout. The power supply bypass capacitor needs to be close to both Vdd and Vss pins of the device.

Traces to that capacitor should be short and wide. Supply decoupling needs to be effective for RF and added inductance constrains the performance of the capacitor. For the same reason through hole components are not recommended for decoupling. Electrolytic capacitors are not effective for high frequencies, but paralleling them with appropriate ceramic capacitors is beneficial in wider frequency range.

Conclusion

The MLX90614 IR thermometers are not highly demanding regarding power supply decoupling, grounding and layout. Unlike other mixed and low signal devices all they need is a single ceramic capacitor close to the power and ground pins. The metal can package is beneficial for EMC in a wide frequency range. The MLX90614 is a low power device and generates very little noise.


It is virtually impossible to provide an EMI solution that is appropriate in all cases. Specific EMI can demand more sophisticated solutions, but most applications can adopt the MLX90614 with very little effort.



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