Application Notes

MOTOROLA
Freescale Semiconductor, Inc.
SEMICONDUCTOR APPLICATION NOTE
ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005
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by AN4004/D
AN4004
±2g
Acceleration Sensing Module Based on a ±40g
Integrated Accelerometer
INTRODUCTION
ARCHIVED BY FREESCALE SEMICONDUCTOR, INC. 2005
Freescale Semiconductor, Inc...
by: Arnaud Delpoux
Application Engineer, Toulouse, France
and
Brandon Loggins
Application Engineer, Phoenix, USA
Micromachined accelerometers, with built–in signal
conditioning and calibration are widely used in automotive
safety devices such as airbag modules. Automotive
applications for accelerometers also include comfort
features such as active suspension. For such an application,
a ±2g accelerometer is required. But most accelerometers
offered today that are able to provide sufficient sensitivity are
expensive and/or unavailable in production volumes. With
the circuitry described herein, the accelerometer can be
used for sensing acceleration in the range of ±2g, with
performance in line with the technical requirements of this
application.
Figure 1. 40G–2G ±2g Sensing Module Evaluation Board
REV 5
Motorola Sensor Device Data
 Motorola, Inc. 1999
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EVALUATION BOARD DESCRIPTION
The information required to use evaluation board “40G–2G”
follows and a discussion of the design appears in the Design
Considerations section.
Electrical Characteristics
The following electrical characteristics are included as a
guide to operation.
Designator
Qty.
Description
Value/Part
Number
R1
1
1/4 Watt Resistor
90.9 K
R2, R8
2
1/4 Watt Resistor
162 K
R5, R6,
R7, R9,
R10
5
1/4 Watt Resistor
11 K
R3, R4
2
1/4 Watt Resistor
432 K
C1–C6
6
Ceramic Capacitor
0.1 mF
U1
1
Quad Rail–to–Rail
Op Amp
MC33204P
U2
1
40g Accelerometer
MMA2200W
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Table 2. Parts List
Characteristics
Symbol
Min
Typ
Max
Units
Supply Voltage
VCC
4.75
5.0
5.25
Volts
Supply Current
IO
—
7.0
—
mA
Acceleration Range
G
–2.0
—
+2.0
g
DESIGN CONSIDERATIONS
25
Using a 40g accelerometer to measure ±2g of acceleration
can be tricky. Motorola’s accelerometer provides a wide
bandwidth of 400 Hz for acceleration sensing. In many low g
applications, such as active suspension, the bandwidth
requirements are considerably lower. Limiting the bandwidth
of the accelerometer can reduce noise. After reducing the
bandwidth, gain can be applied to provide higher sensitivity for
low g acceleration measurements.
The design challenge is how to bandpass filter the
accelerometer’s output with gain using a few low cost
components. To accommodate the accelerometer’s wide
dynamic range, since the supply voltage is limited to 5 volts,
it is necessary to use rail to rail operational amplifiers, such as
Motorola’s MC33204P.
In this design, the output signal passes through a biquad
filter stage, then through an additional low–pass gain stage to
provide ±2g sensing capability. The biquad filter, shown in
Figure 3, with some gain, is a good choice for bandpass
filtering the accelerometer output when a high quotient factor,
Q, is desired. The gain is set by Rg and Rb with the high cutoff
frequency being set by Rb and C1. The low cutoff frequency
is set by Rf and C2.
20
C2
Zero G Output
Voff
2.0
2.1
2.2
Volts
∆V/∆G
850
1000
1150
mV/g
Low Cutoff Frequency
—
0.8
0.9
1.0
Hz
High Cutoff Frequency
—
4.0
5.0
6.0
Hz
Operating Temperature
Ta
–40
—
+85
°C
Sensitivity
Table 1. Electrical Characteristics
Frequency Response
Below is the simulated typical output frequency response
using the devices listed in the parts list below. The system
provides a gain of 25 over a 5 Hz bandwidth.
30
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Function
The evaluation board shown in Figure 1, when supplied with
an accelerometer, provides a ±2g full scale acceleration
measurement.
The output is an analog signal. It nominally supplies 2.1
volts at zero g and has a sensitivity of 1000 mV/g. It is easily
interfaced with a microcontroller’s A/D input.
A through–hole area is provided on the PCB for the designer
to add other circuitry as needed.
Evaluation Board Content
Board contents are described in the parts list shown in Table 2.
A schematic and silk screen plot are shown in Figures 4 and 5.
15
Rf
10
C1
5.0
Rb
Rg
Vin
0
0.1
1.0
10
Hz
100
1000
–
+
Rf
–
+
R
R
–
+
BANDPASS
Figure 2. ±2g Biquad Filter with Additional Gain
Stage Frequency Response
Figure 3. Biquad Active Filter
2
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AN4004
APPLICATION
Integrating the accelerometer with the biquad amplifier is
quite simple. First, in order to minimize the noise level, the
accelerometer is properly bypassed, as indicated in the data
sheet, with one 0.1 mF ceramic capacitor between VCC (pin 8)
and Ground (pin 7). The self–test feature remains unused in
this design. Pin 11 is tied to VCC, as indicated in the data sheet.
The output signal is taken on pin 5. The remaining pins of the
accelerometer are unused.
C4
C5
U2
R4
R3
ACCELEROMETER
4
8
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7
U1
C3
ST
R1
GND
OUT
13
–
+ 12
VOFF
C6
R2
5
R5
VS
VCC
VCC
R10
C1
14
R8
4
2
–
3 +
R6
U1
1
R9
VOFF
VOFF
6
–
5 +
R7
7
9
10
U1
–
+
8
VOUT
U1
VOFF
Figure 4. ±2g Acceleration Sensing Module EVB Schematic
In this design, the biquad filter stage gain is set to 1.76 with
R2 (160 kW) and R1 (91 kW). A higher gain cannot be set for
this stage, otherwise some operational amplifiers saturate
when the accelerometer output drifts over temperature. The
upper cutoff frequency is set with R2 (160 kW) and C3 (0.1 mF).
Two capacitors, C4 (0.1 mF) and C5 (0.1 mF), are needed to set
the lower cutoff frequency.
A final low pass filter stage, with a gain of 14.55, brings the
overall gain to 25. Resistors R8 (160 kW) and R7 (11 kW) set
the gain. The amplifier is low passed with C6 (0.1 mF) to
remove any high frequency noise in the signal. The output can
then be connected to a microcontroller’s A/D converter by a
simple direct connection from the evaluation board analog
Motorola Sensor Device Data
output VOUT to the A/D input. Using the MC68HC11 as an
example, the output is connected to any of the E ports.
Since the accelerometer signal passes through 3 inverter
stages, the positive direction of acceleration is reversed.
CONCLUSION
Perhaps the most noteworthy aspect to the ±2g sensing
module described here is the ease with which it can be
designed. Only two dual, or one quad, operational amplifier
and a few resistors and capacitors are required. The result is
a simple and inexpensive circuit that is capable to measure
acceleration within the range of ±2g with an analog output that
can be directly interfaced to a microcontroller.
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40G–2G
U2
MMAS40G10D
XL
C2
C1
C5 C4
R3
+5 V
+
U1
R1
OUT
VOUT
–
R2
–
GND
C3
+
C7
GND
R5
R4
R9
R7
R10
R8
MOTOROLA
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R6
C6
08/96
Figure 5. Silk Screen
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