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Sensor module enhances automotive electrical integration (Part 1)

Posted: 04 Jul 2011     Print Version  Bookmark and Share

Keywords:sensor  Automotive engines  real-time monitoring 

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 fulfil the EMC specification of the SASEM—this leads us to typical automotive EMC requirements.

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 <5MHz (i.e. ZSC31150 for DSP-on-chip typically operates at 3MHz), common ISO- and OEM-standards for electromagnetic emissions of SASEMs can be fulfilled.

Electromagnetic immunity
In terms of electromagnetic immunity against continuous or transient RF energy, there are several standardised test methods for both conducted and radiated modes of RF-energy transfer to the SASEM. Because of the small dimensions of the SASEM itself and of its internal conductive parts, there is no effective RF antenna for radiating RF energy up to 1GHz—all dimensions are smaller than the length la of an equivalent λ/4 dipole. On average, this length is approximately 50 mm at 1GHz as calculated with equation (1).

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Up to 1GHz, the primary effective antenna for RF energy is the SASEM's harness. However, there is a trend to expand the EMC test procedure frequency range to 3GHz (or more). In this case, the effective length la of an equivalent λ/4 dipol decreases to approximately 20mm as calculated by equation (1), and conductive structures on the module's PCB with a length >15mm can be an effective antenna for radiated RF energy. To prevent susceptibility at field strengths up to 600V/m, shielding of sensitive signal paths might be required.

Their susceptibility is measured by EMC test procedures. There are different test configurations for radiated and conducted immunity (i.e. stripline, anechoic chamber, bulk current injection, etc). One of the toughest tests for common automotive SASEMs regarding immunity against continuously applied RF energy is the Bulk Current Injection (BCI) test, which belongs to the radiated immunity EMC test group. Typically the frequency range tested is 1 to 400MHz. The test simulates worst case conditions for RF cross-coupling in a harness for different electric sub-system's wires assembled inside a car. Because of the small distance between RF source (emitting harness or wire) and RF sink (harness of the sensor module), the induced energy can be very high and is measured in "mA" or "dBµA" during the BCI test. To ensure the induced energy can influence only the sensor module during the test, the ECU is replaced by a standardised artificial network and typical circuitry at VSIGNAL, which represents the input impedance of the original ECU used in the car.

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Table: Possible configurations of module construction and automotive assembly.

It is important to note, customising this circuitry for each EMC test before designing the module is strongly recommended because different EMC test circuitries can make different module designs necessary. Typically "universal" solutions are too expensive.

To fulfil the harsh automotive EMC requirements, all relevant electrical parasitics, especially capacitances between the electric sensor circuitry and other conductive parts of the SASEM, need to be considered.

There are a number of different configurations possible for the module's construction and its assembly inside the car. The case and the pressure supply adaptor (PSA) can each be plastic or metal and each can have a galvanic contact with the chassis or no contact.

In the table, configurations 1 and 10 represent the extremes regarding the equivalent RF circuitry at the BCI test. With configuration 1, all parasitic impedances are maximums; with configuration 10, they are minimal or short circuited.

The first consideration is the electromagnetic coupling between the BCI antenna and the harness. If the frequency of the RF current IRF is in the range of the initial resonance frequency of the segment of harness between the RF-emitting BCI antenna and the DUT, then the induced current IRF_sink is maximum. The induced current value is determined by the parasitic impedances, especially by ZC_GND.

As IRF_sink increases, its influence on the DUT becomes stronger. The worst case is configuration 10, because ZC_GND = 0 Ω (galvanic contact between case and the car's chassis) and ZPSA_C = 0 Ω (galvanic contact between PSA and case). In this case IRF_sink is limited by the impedance of the parasitic capacitances of the DUT's signal paths V+, VOUT, and V- relative to the case and of the sensor bridge relative to the PSA.

But there are additional parasitic capacitances (i.e. the internal signal paths relative to the case), which could also decrease the RF susceptibility of the DUT.

An example:

Tolerance allowed for the analogue output voltage of the DUT = ±40 mV (nominal value)

Effective gain "G" of the SSC-IC: G = 400

DC bridge resistance = 4 kΩ / resulting AC bridge impedance at its differential terminals = 2 kΩ

Thus the limit of the differential bridge voltage' s change caused by RF energy: (±40 mV / G) = ±0.1 mV. And the resulting limit of the difference between the bridge's partial currents: ±0.1mV/2kΩ = ±50nA!

This very simplified example illustrates the influence of the mechanical construction and selected materials on the EMC behaviour of the sensor module. It is even more challenging to define parasitics under the conditions of high volume automotive production with consideration for the system's cost

- Torsten Herz
  Field Applications Engineer

To download the PDF version, click here.

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