MPXM2102AS datasheet(120/670 Pages) MOTOROLA

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Motorola Sensor Device Data

www.motorola.com/semiconductors

AN1632

MMA1201P Product Overview

and Interface Considerations

Prepared by: Carlos Miranda

Systems and Applications Engineer

and

Gary O’Brien

New Product Development Engineer

INTRODUCTION

Silicon micromachined accelerometers designed for a vari-

ety of applications including automotive airbag deployment

systems must meet stringent performance requirements and

still remain low cost. Achieving the requisite enhanced func-

tionality encompasses overcoming challenges in both trans-

ducer micromachining and subsequent signal conditioning.

Motorola’s accelerometer architecture includes two separate

elements in a single package to achieve overall functionality:

a sensing element (“g–cell”) and a signal conditioning element

(“control ASIC”).

Figure 1 shows a functional block diagram of Motorola’s

new MMA1201P. The transducer is a surface micromachined

differential capacitor with two fixed plates and a third mov-

able plate. The movable plate is attached to an inertial mass.

When acceleration is applied to the device, the inertial mass

is displaced causing a change in capacitance. The second

die is a CMOS control ASIC which acts as a capacitance to

voltage converter and conditions the signal to provide a high

level output. The output signal has an offset voltage nomi-

nally equivalent to VDD/2 so that both positive and negative

acceleration can be measured.

This document describes Motorola’s new MMA1201P

accelerometer, which uses a new control ASIC architecture.

It explains important new features that have been incorpo-

rated into the ASIC, and presents an overview of the key

performance characteristics of the new accelerometer. The

document also details the minimum supporting circuitry

needed to operate a Motorola accelerometer and interface it

to an MCU. Finally, the power supply rejection ratio (PSRR)

characteristics and an aliasing gain model are presented.

MMA1201P FEATURES

Several design enhancements have been implemented into

the new MMA1201P. The oscillator circuit, which is the heart

of the ASIC, has been redesigned to improve stability over

temperature. A filter has been added to the power supply line

for internally generated biases. A new sensing scheme is used

to sample the differential capacitor transducer and condition

the signal. Finally, the temperature compensation stage has

been redesigned to be trimmable. A block diagram represen-

tation of the new accelerometer, in a 16 pin DIP package, is

shown in Figure 1. For simplicity, the EPROM trim and the

self–test circuit blocks have been omitted.

G–Cell

CMOS Control ASIC

Capacitance

to Voltage

Converter

Trimmable

Gain

Stage

Trimmable

Switched

Capacitor

Filter

Trimmable

Temp. Comp.

Output Stage

VSS

VDD

VOUT

VDD

Filter

Oscillator

Figure 1. Block Diagram Representing the MMA1201P

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 2

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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