MPXM2102AS datasheet(137/670 Pages) MOTOROLA

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Motorola Sensor Device Data

www.motorola.com/semiconductors

AN1640

Reducing Accelerometer Susceptibilityto BCI

Prepared by Brandon Loggins

Automobile manufacturers require all system electronics to

pass stringent electromagnetic compatibility (EMC) tests.

Airbag systems are one of the systems that must perform

adequately under EMC tests. There are different types of tests

for EMC, one of which is testing the tolerance of the system

to high frequency conducted emissions. One of the most

stringent methods for EMC evaluation is the Bulk Current

Injection (BCI) test. The entire airbag system must continue to

function normally throughout the BCI test. This application

note will discuss how to reduce susceptibility to BCI for the

Motorola accelerometer but the information presented here

can be applied to other electronic components in the system.

BCI TEST SETUP

The BCI test procedure follows a published SAE engineering

standard, “Immunity to Radiated Electric Fields ~ Bulk Current

Injection (BCI)”, or SAE J 1113/401. For an airbag module, this

involves injecting the desired current into the wiring harness by

controlling current in the injection probe. The test frequency can

vary from one to several hundred MHz. There are at least 20

frequency steps per octave required for the test, but as many

as 50 steps per octave can be used. The injection probe is

placed on the harness in one of three distances from the airbag

module connector: 120, 450 and 750 mm. There is a monitor

pickup probe present to measure the amount of current being

injected. It is placed 50 mm from the airbag module. This feeds

back to the system to ensure that the desired test current is

being injected on to the wiring harness. Figure 1 shows the

setup for the BCI test. (For more details, see the SAE J

1113/401 Test Procedure).

Figure 1. BCI Test Setup

ANECHOIC CHAMBER WALL

LOAD BOX

INJECTION

PROBE

PICKUP

PROBE

WIRING HARNESS

AIRBAG

MODULE

70, 450, or 750 mm

50 mm

BASEPLATE CONNECTED

TO GROUND

The harness connects the airbag module to a load box. This

load box provides simulated loads for terminating the

remainder of the airbag system (firing ignitors, etc.). The data

coming back is translated from J1850 to RS232 to be

communicated to a dummy terminal on a PC. For safety

reasons, this test is typically performed inside an anechoic

chamber to shield high frequency emissions from equipment

and humans.

BCI TEST PROCEDURE FOR THE

MMA2202D ACCELEROMETER

The accelerometer is evaluated in the following manner. In

an airbag system, the microcontroller’s A/D converter digitizes

the accelerometer output. The microcontroller sends this

value to the communication ASIC which translates the logic

from board level logic to RS232, then sends the value back

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 2

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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