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
. 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 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 manufacturing conditions of a high-volume automotive production.
One solution for this problem is to design a path for the induced RF current from the harness to GND with a very low impedance beside the signal paths of the sensor system. The conductive case of the module can provide this path and shield the module’s PCB from radiated RF signals in the GHz range. Another benefit of this construction concept is that the conditions of the SASEM’s assembly inside the car cannot decrease its electromagnetic immunity because with its design, the worst case conditions (see configuration 10 above) are considered.
A galvanic connection between the SASEMs negative supply voltage and the chassis is not allowed. Adding a case-to-ground capacitor with a sufficient operating DC voltage and transient voltage robustness could provide such a path for the RF current. The huge drawback of this solution is that a robust and long-term-stable electrical connection to the metallic case (i.e. an aluminum case) must be created, which leads to higher cost. Additionally, the case-to-GND capacitor must be specified for relatively high voltages (i.e., 500 or 1,000V), which leads to larger packages and also higher cost.
The mentioned ESL and the resulting limitation of the effective frequency range need to be considered also. The alternative is again to create a PCB layout optimized for the sensitive sensor signal lines to the SSC-IC to provide high RF symmetry and identical impedances for RF energy induced via the floating metallic case. This enables making the incoming RF energy to act like a common mode signal for the SASEM. The required ESD robustness in cases of direct discharge to the metallic case (typical requirement = ±15kV referenced to the GND of the SASEM) can be achieved by potting the application circuit at the PCB or by other proper isolation from the metallic case.