Thanks to state-of-the-art sensor-based control systems that provide precise real-time monitoring, automotive engines operate more efficiently and with lower environmental impact. One result of this improved performance is that the number of sensor applications in vehicles has realized double-digit growth over the past several years. The other result is a growing trend to add more sensor modules to vehicles. Such modules must be reliable and robust and operate with long-term stability and high precision under harsh physical, chemical, and electrical stress conditions.
Additionally, a set of built-in-diagnostic functions is required for automotive sensor modules to support the “maintenance-on-demand” policy of automotive OEMs as well as special failure-mode-operations required for safety-critical sensor applications like brake pressure sensing.
The chemical (i.e. media/humidity/corrosion resistance) and physical (shock; vibrations) robustness of sensor modules is mainly determined by the materials used, and the assembly and connection technologies. The electrical robustness (i.e. EMC) is determined by the application circuit, the chosen electric components (ICs, discrete parts), and the layout of the electrical connections, according to the application circuit.
This series will describe the latter aspect of the design of an automotive sensor module. The module incorporates a sensor signal conditioner (ZSC31150) to enable the design of highly accurate sensor modules operating at temperatures of -40 to +150C and providing EMC performance and a set of on-chip-protection and diagnostic features addressing safety-critical applications at SIL2-level. By clever electrical design of the sensor module considering all EMC-related parameters (i.e. parasitic capacitances and inductances), high electrical robustness and built-in-diagnostic functionality can be achieved at optimized module cost, together with very high accuracy of the measured signal.
Because the mechanical design and the interconnection between a sensor system and the processing unit have a major influence on their electromagnetic behavior, it is essential to separate “embedded sensing functions” and “stand-alone-sensor modules”.
In case of embedded sensing functions (ESF) the sensor electronics are placed closed to the processing unit—in automotive applications this is an ECU (Electronic Control Unit). The connections between ESF and ECU are typically very short (<< 30 cm) and normally realized as traces on a PCB. Modern ESF provide a digital interface (i.e. SPI), which is connected to the microcontroller of the ECU. Because of this closed placement on the same PCB there are several options in order to fulfill the tough automotive requirements in terms of EMC (i.e. shielding or use of external protection parts). One example for an ESF is barometric pressure sensing.
For stand-alone-sensor modules (SASEM), the situation is completely different. These are typically connected to an ECU via an unshielded harness of up to 2.5 meters in length. The available board space inside the module’s case (made of metal or plastic) is very limited and trends to further miniaturization because lower material consumption equals lower weight, which in turn equals lower cost.
Depending on the mode of power supply (battery-powered or ECU-powered) there are various output interfaces:
PWM output (high-side-load)
PWM output (low-side-load)
Absolute analog voltage output
Ratiometric analog voltage output
SENT interface (fast digital unidirectional point-to-point data transfer)
PSI5 interface (digital 2-wire-current-coded data transfer)
Typical construction of an automotive pressure sensor module
For passenger cars it is still very common to use ECU-powered SASEMs, which provide a ratiometric analog voltage output. The typical supply voltage amounts to 5 VDC ±10% and the current consumption of a SASEM should amount to ≤10 mA. The operational conditions are quite harsh as mentioned at the beginning, which leads to the exclusion of some effective passive protection parts such as ferrite beads, which operate at temperatures only up to +125C.
Depending on the module’s design (i.e. the material of the module’s case), two additional 10nF (maximum) capacitors (shown in green in the figure below) at the differential inputs VINP and VINN to VSSA might be required in order to fulfill the EMC specification of the SASEM—this leads us to typical automotive EMC requirements.
ZSC31150 automotive application circuit
Basically the electromagnetic characteristic of systems like SASEMs is split into areas—electromagnetic emissions (conducted or radiated) and electromagnetic immunity (conducted or radiated). The limitation of electromagnetic emissions ensures that other electrical systems are not disturbed by operation of a SASEM. Thus, the active electronics inside a SASEM determine its “emission performance.” By proper IC design and at digital on-chip-clock frequencies <5 MHz (i.e. ZSC31150 for DSP-on-chip typically operates at 3 MHz), common ISO- and OEM-standards for electromagnetic emissions of SASEMs can be fulfilled.