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Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
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
ST
VDD
Filter
Oscillator
Figure 1. Block Diagram Representing the MMA1201P
SEMICONDUCTOR APPLICATION NOTE
REV 2
F
Freescale Semiconductor, Inc.
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