MPXM2202GS datasheet(139/670 Pages) MOTOROLA

139 / 670 page

AN1640

2–103

Motorola Sensor Device Data

www.motorola.com/semiconductors

voltage will result in the same proportional change in the

output. For example, if the 5 V supply were to change by 10%,

from 5.0 V to 5.5 V, the accelerometer offset will change by

10% also, from 2.5 V to 2.75 V. This phenomena would also

occur if the ground were to shift. A 100 mV change in ground

would result in a 50 mV change in the output. If the

accelerometer does not have low impedance path to ground

and parasitics from a poor ground are present as a result, the

ground seen by the accelerometer may change over

frequency. So, during a BCI test, if the 5.0 V supply does not

shift but the output of the accelerometer does, the ground to

the accelerometer may be moving.

It was found with some experimentation that the offset shift

can be eliminated with proper board layout techniques as

described below.

PROPER LAYOUT TECHNIQUES

Since the Motorola accelerometer is a sensitive analog

device that relies on a clean supply to function within

established parameters, there are some techniques that can

be employed to minimize the effects of BCI on the

accelerometer performance. PCB layout is paramount to

reducing susceptibility to BCI.

• A low impedance path to ground will provide shunting of the

high frequency interference and minimize its effect on the

accelerometer. The best way to provide a good path is by

putting a solid, unbroken ground plane in the PCB. This

ground plane should be shunted to chassis ground at the

module connector. This will ensure that the high frequency

BCI will be shunted before interfering with accelerometer

performance.

• All accelerometer pins that require ground connection

should be tied together to a common ground.

• Traces attached directly to the connector pins can receive

high RF noise, which can couple to nearby traces and com-

ponents. Increasing series impedance of the traces helps

reduce the couple or conducted noise. High frequency fil-

ters on the supply line and other susceptible lines may be

required to filter out high frequency interference introduced

by the BCI test. Signal lines that carry low current can toler-

ate series resistances of 100–200

Ω.

• Decoupling capacitors on every input line to the common

ground plane will help shunt the high frequency away from

the system. These should be placed near the connector.

• Signal trace lengths to and from the accelerometer should

be kept at a minimum. The shorter the trace, the less

chance it has of picking up high frequency BCI signals as

it crosses the board. Trace lengths can be reduced by plac-

ing the accelerometer and the microcontroller as close

together as possible. Signal and ground traces looping

should be minimized.

• A decoupling capacitor on the accelerometer Vcc pin will

also help minimize BCI effects. The recommended value is

0.1

µF. This capacitor should be placed as close as pos-

sible to the accelerometer to achieve the best results.

• To maximize ratiometricity, the accelerometer Vcc and the

microcontroller A/D reference pin should be on the same

trace. The accelerometer ground and the microcontroller

ground should also share the same ground point. There-

fore, when there is signal interference due to BCI, the A/D

converter and the accelerometer will see the interference

at the same level. This will result in the same digital code

representation of acceleration without signal interference.

• A clean power supply to both the accelerometer and the

microcontroller should be provided. Supply traces should

avoid high current traces that might carry high RF currents

during the BCI test. The traces should be as short as

possible.

• The accelerometer should be placed on the opposite end

of the PCB away from the connector. The farther the dis-

tance, the lower the chance high frequency RF from BCI

will interfere with the accelerometer.

• The accelerometer should be placed away from high cur-

rent paths that may carry high RF currents during the BCI

test.

Automotive customers will continue to require airbag

systems to have high standards for EMC. One way to test for

EMC is perform the Bulk Current Injection test. Because of the

high current involved, BCI is one of the most difficult EMC tests

to pass. Being part of the airbag system, the accelerometer

must continue to function normally under application of high

frequency BCI. The accelerometer is highly sensitive to

placement on the board and its connection to ground. Poor

design will caused the device to fail the BCI test. The practice

of good PCB layout, device placement and good grounding

will allow the accelerometer to function within specification

and pass the BCI test.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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