Sensor module design improves automotive electrical integration, functionality (Part 2)

July 07, 2011 //By Torsten Herz, ZMDI
Sensor module design improves automotive electrical integration, functionality (Part 2)
In order to improve fuel efficiency and reduce CO2 emissions, designers rely on an increasing number of sensors throughout the car. Reliability and space requirements call for higher integration and more intelligence in the sensors. In this second part of the article, readers learn how RF path design and additional capacitors can improve EMC behavior.

Devices must be reliable and robust, and operate with long-term stability and high precision under harsh physical, chemical, and electrical stress conditions. Come up to speed with Part 1 of this feature here.

One effective design concept is based on the choice of non-conducting materials for the example pressure sensor module's case and pressure supply adapter (PSA) (see configuration 1 below).




The non-conducting material ensures maximum values for ZPSA_C and ZC_GND. However, to suppress the influence of the other parasitic capacitances of the PCB's conductive structures relative to GND, the design must take into consideration the conditions of the module’s assembly inside the car. If the connection to the system to be monitored and its case also consists of non-conducting material, then the parasitic impedances are maximums.

During the PCB layout, it is easy to ensure, that the parasitic capacitances of CV+_C and CVOUT_C and CV-_C are almost equal relative to GND in order to make the incoming RF energy acting like a common mode signal for the stand-alone sensor module (SASEM). In other words, at the SASEM there is no proper RF-GND available, which makes the blocking of this RF energy (for instance by capacitors) almost impossible over the wide frequency range tested. Additionally capacitors are not “ideal” parts—they also have parasitics inside—especially their series inductance (ESL) which determines the frequency limit, from which capacitors starts to act like inductances. Typical 0805-cased MLCC-X8R-capacitors have an ESL of 1 to 1.5 nH. Only by a high common mode rejection ratio (CMRR) of the sensor electronics can a high immunity against applied RF energy be achieved.

If conducting material is required for the SASEM’s case and PSA, the resulting parasitic impedances are lower and the induced RF current is higher. Because of the mechanical tolerances of the different parts of a SASEM (i.e. case, PCB, PSA) it would be very difficult to determine these parasitics under the

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