MPXM2202GS datasheet(138/670 Pages) MOTOROLA

138 / 670 page

AN1640

2–102

Motorola Sensor Device Data

www.motorola.com/semiconductors

along the wiring harness. Once through the chamber wall, the

data is translated to RS232 and fed to a dummy terminal. On

the terminal screen, the A/D codes for the accelerometer can

be monitored for unexpected performance.

Ideally, when the accelerometer is at rest (no acceleration

applied), the output should be at 0

g, regardless of what EMC

testing the system may be subjected to. Depending on the

crash algorithm of the airbag module software, there is some

allowable offset shift that can be tolerated. Higher shift in

output could create errors in the crash analysis software,

perhaps causing the airbags to unnecessarily deploy when

there is not a crash or not deploy when there is a crash.

The Motorola accelerometer must be able to meet the

airbag system requirements throughout BCI exposure. It has

a sensitivity of 40 mV/g and an offset (0

g output) of 2.50 V.

During the BCI test, the accelerometer output should be

2.50 V at 0

g with as little drift as possible. A typical airbag

system may have software that can tolerate from as little as

0.5

g up to 2.0 g. of deviation from the offset. The system would

then expect the accelerometer output to be within 40 mV of the

offset during the entire BCI test. Therefore, at any given

frequency of the BCI test, if the output deviates outside this

expected window of drift, it fails the test.

MMA2202D ACCELEROMETER BCI TEST

RESULTS

If a system has not been well designed for electromagnetic

compatibility, the accelerometer, as well as other devices, can

have performance problems. What has been found for the

accelerometer is that in some system applications, it suffers

from an offset shift when certain frequencies of BCI are

applied. For example, in one airbag system being tested at a

certain frequency, with the desired BCI current applied, the

offset is found to shift down by 60 mV. This would equate to an

error of 1.5

g. See Figure 2. At other frequencies, this shift is

even higher. This DC shift plot was taken with an oscilloscope

using a 20 MHz filter to remove the high frequency component

of the signal. Probes are placed at the accelerometer in the

system application. The plot shows the accelerometer output

before and after BCI was applied (before and after the RF

generator creating the high frequency signal was turned on).

Figure 2. Accelerometer Tested Under High Frequency BCI

ACCELEROMETER VOUT

w/o BCI

ACCELEROMETER VOUT

w/BCI

VCC

This phenomenon has been determined to be system level

related. PCB layout and grounding for the accelerometer will

affect its performance. This was found by testing the

accelerometer outside of the airbag module. The device was

put on a test board by itself with only the supply decoupling

capacitor of 0.1

µF connected to it. To simulate the effect of

BCI on Vcc, a frequency generator was used to inject a known

high frequency sinusoid that caused BCI failure on to the 5.0 V

supply voltage. The device was first tested in small test board

with ground provided by one wire back to the supply. This

grounding reproduced the failure due to BCI seen at the

module level. The test board was then mounted down to a

ground plane provided by a copper plate and the

accelerometer ground was soldered to the plate (providing a

low impedance path to ground). With this setup, the offset shift

did not occur.

If a system does not incorporate a good PCB layout

providing a low impedance to ground, the accelerometer

output may shift at certain high frequencies. This output offset

shift was caused by a shift in the 0–5 V supply window.

Because the accelerometer has a ratiometric output, its offset

is dependent on the supply voltage. Any change in the supply

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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