ADXL05AH [ADI]
+-1 g to +-5 g Single Chip Accelerometer with Signal Conditioning; ±1克至+ -5克单芯片加速度计信号调理
| 型号： | ADXL05AH |
| 厂家： | 
ADI |
| 描述： | +-1 g to +-5 g Single Chip Accelerometer with Signal Conditioning |
| 文件： | 总20页 (文件大小：284K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
؎1 g to ؎5 g Single Chip Accelerometer
a
with Signal Conditioning
ADXL05*
floor is 500 µg/√Hz, (12× less than the ADXL50), allowing sig-
nals below 5 milli-g to be resolved. T he ADXL05 is a force bal-
anced capacitive accelerometer with the capability to measure
FEATURES
5 m illi-g Resolution
Noise Level 12
؋
Less than the ADXL50 User Selectable Full Scale from ؎1 g to ؎5 g
Output Scale Selectable from 200 m V/ g to 1 V/ g
Com plete Acceleration Measurem ent System on a
Single Chip IC
Self Test on Digital Com m and
+5 V Single Supply Operation
both ac accelerations (typical of vibration) or dc accelerations
(such as inertial force or gravity). T hree external capacitors and
a +5 volt regulated power supply are all that is required to
measure accelerations up to ±5 g. T hree resistors are used to
configure the output buffer amplifier to set scale factors from
200 mV/g to 1 V/g. External capacitors may be added to the
resistor network to provide 1 or 2 poles of filtering. No addi-
tional active components are required to interface directly to
most analog to digital converters (ADCs).
1000 g Shock Survival
APPLICATIONS
Low Cost Sensor for Vibration Measurem ent
Tilt Sensing w ith Faster Response than Electrolytic or
Mercury Sensors
More Sensitive Alarm s and Motion Detectors
Affordable Inertial Sensing of Velocity and Position
T he device features a T T L compatible self-test function that
can electrostatically deflect the sensor beam at any time to verify
that the sensor and its electronics are functioning correctly.
T he ADXL05 is available in a hermetic 10-pin T O-100 metal
can, specified over the 0°C to +70°C commercial, and –40°C to
+85°C industrial temperature ranges. Contact factory for avail-
ability of automotive grade devices.
GENERAL D ESCRIP TIO N
T he ADXL05 is a complete acceleration measurement system
on a single monolithic IC. T he ADXL05 will measure accelera-
tions with full-scale ranges of ±5 g to ±1 g or less. T ypical noise
FUNCTIO NAL BLO CK D IAGRAM
ADXL05
+3.4V
V
REF
REFERENCE
+1.8V
6
OUTPUT
OSCILLATOR
DECOUPLING
CAPACITOR
SENSOR
OSCILLATOR
DEMODULATOR
4
7
C2
PREAMP
SELF-TEST
(ST)
BUFFER
AMP
2
3
10
V
5
1
9
8
V
COM
C1
C1
C3
PR
IN–
R1
R3
V
OUT
DEMODULATOR
CAPACITOR
R2
+5V
*P atents pending.
REV. B
Inform ation furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assum ed by Analog Devices for its
use, nor for any infringem ents of patents or other rights of third parties
which m ay result from its use. No license is granted by im plication or
otherwise under any patent or patent rights of Analog Devices.
© Analog Devices, Inc., 1996
One Technology Way, P.O. Box 9106, Norw ood. MA 02062-9106, U.S.A.
Tel: 617/ 329-4700 Fax: 617/ 326-8703
(T = TMIN to T , T = +25؇C for J Grade Only, V = +5 V, @ Acceleration = 0 g,
A
MAX
A
S
unless otherwise noted)
ADXL05–SPECIFICATIONS
AD XL05J/A
Typ
P aram eter
Conditions
Min
Max
Units
SENSOR INPUT
Measurement Range
Nonlinearity
Guaranteed Full Scale
Best Fit Straight Line, 5 g FS
–5
+5
g
0.2
±1
±2
% of FS
Degrees
%
Alignment Error1
T ransverse Sensitivity2
SENSIT IVIT Y
Initial Sensitivity at VPR
Initial Sensitivity at VOUT
T emperature Drift3
+25°C
+25°C, R3/R1 = 5
175
0.875
200
1.000
±0.5
225
1.125
mV/g
V/g
% of Reading
ZERO g BIAS LEVEL
Initial Offset
at VPR
1.50
1.80
±25/40
10
2.10
32
V
mV
mV/V
vs. T emperature3
vs. Supply
VS = 4.75 V to 5.25 V
NOISE PERFORMANCE
Voltage Noise Density
Noise in 100 Hz Bandwidth
Noise in 10 Hz Bandwidth
at VPR
BW = 4 Hz to 1 kHz
500
5
1.6
1000
µg/√Hz
mg rms
mg rms
FREQUENCY RESPONSE
3 dB Bandwidth4
C1 = 0.022 µF (See Figure 9)
C1 = 0.010 µF
1000
1600
4
12
Hz
kHz
kHz
3 dB Bandwidth4
Sensor Resonant Frequency
SELF T EST INPUT
Output Change at VPR
5
ST Pin from Logic “0” to “1”
T o Common
–0.85
2.0
–1.00
50
–1.15
0.8
V
V
V
kΩ
Logic “1” Voltage
Logic “0” Voltage
Input Resistance
+3.4 V REFERENCE
Output Voltage
3.350
500
3.400
±5
1
3.450
10
V
Output T emperature Drift3
Power Supply Rejection
Output Current
mV
mV/V
µA
DC, VS = +4.75 V to +5.25 V
Sourcing
PREAMPLIFIER OUT PUT
Voltage Swing
Current Output
0.25
30
VS – 1.4
V
µA
pF
Source or Sink
80
100
Capacitive Load Drive
BUFFER AMPLIFIER
Input Offset Voltage6
Input Bias Current
Delta from Nominal 1.800 V
DC
±10
5
80
±25
20
mV
nA
dB
Open-Loop Gain
Unity Gain Bandwidth
Output Voltage Swing
Capacitive Load Drive
Power Supply Rejection
200
kHz
V
pF
IOUT = ±100 µA
0.25
1000
VS – 0.25
10
DC, VS = +4.75 V to +5.25 V
1
mV/V
POWER SUPPLY
Operating Voltage Range
Quiescent Supply Current
4.75
5.25
10.0
V
mA
8.0
T EMPERAT URE RANGE
Operating Range J
Specified Performance A
Automotive Grade*
0
–40
–40
+70
+85
+125
°C
°C
°C
NOT ES
1Alignment error is specified as the angle between the true and indicated axis of sensitivity, (see Figure 2).
2T ransverse sensitivity is measured with an applied acceleration that is 90° from the indicated axis of sensitivity. T ransverse sensitivity is specified as the percent of
transverse acceleration that appears at the VPR output. T his is the algebraic sum of the alignment and the inherent sensor sensitivity errors, (see Figure 2).
3Specification refers to the maximum change in parameter from its initial at +25°C to its worst case value at T MIN to T MAX
.
4Frequency at which response is 3 dB down from dc response assuming an exact C1 value is used. Maximum recommended BW is 6 kHz using a 0.010 µF capacitor, refer to
Figure 9.
5Applying logic high to the self-test input has the effect of applying an acceleration of –5 g to the ADXL05.
6Input offset voltage is defined as the output voltage differential from 1.800 V when the amplifier is connected as a follower. T he voltage at this pin has a temperature drift
proportional to that of the 3.4 V reference.
*Contact factory for availability of automotive grade devices.
All min and max specifications are guaranteed. T ypical specifications are not tested or guaranteed.
Specifications subject to change without notice.
REV. B
–2–
System Performance Specifications–ADXL05
C2
4
1
+5V
C3
ADXL05
0.022µF
1.8V
C1
0.1µF
BUFFER
AMP
PRE-AMP
2
3
0.022µF
9
V
OUT
C1
5
COM
NOMINAL VALUES:
R1 = 49.9kΩ
R3 = 249kΩ
8
6
10
V
V
C4
PR
+3.4V
REF
R1
IN–
R3
R2
R2 = 640kΩ
AC COUPLED CONNECTION (؎1.5 g Full Scale)
(@ V Terminal (Pin 9), unless otherwise noted. 0 g Bias Level = +2.5 V, C1 = 0.022 F, R2 = 2.57 R3
OUT
AD XL05J/A
P aram eter
Conditions
Min
Typ
Max
Units
Buffer Gain
G = R3/R1*
5
FULL-SCALE RANGE
SENSIT IVIT Y
–1.5
875
+1.5
1,125
g
@ +25°C
1000
mV/g
T emperature Drift
T MIN to TMAX
±0.5
% of Reading
ZERO g BIAS LEVEL
T emperature Drift
@ +25°C
+25°C to T MIN or T MAX
2.5
2/5
V
mV
FREQUENCY RESPONSE
C4 = 3.3 µF, R1 = 49.9 kΩ
1
1000
Hz
*Note: Resistor tolerance will affect system accuracy. Use of ±1% (or better) metal film resistors is recommended.
C2
4
1
+5V
C3
ADXL05
0.022µF
C1
1.8V
0.1µF
BUFFER
AMP
PRE-AMP
2
3
0.022µF
9
V
OUT
C1
5
COM
NOMINAL VALUES:
R1 = 49.9kΩ
8
6
10
V
V
PR
+3.4V
REF
R1
IN–
R3
R3 = 100kΩ (G=2)
R2 = 255kΩ (G=2)
R2
DC COUPLED CONNECTION (؎2 g Full Scale)
(@ V Terminal (Pin 9), unless otherwise noted. 0 g Bias Level = +2.5 V, C1 = 0.022 F, R2 = 2.57 R3)
OUT
AD XL05J/A
P aram eter
Conditions
Min
Typ
Max
Units
Buffer Gain
G = R3/R1*
2
FULL-SCALE RANGE
SENSIT IVIT Y
–2
350
+2
450
g
@ +25°C
400
mV/g
T emperature Drift
TMIN to TMAX
±0.5
% of Reading
ZERO g BIAS LEVEL
@ +25°C
1.75
2.5
3.2
V
T emperature Drift
+25°C to T MIN or TMAX
±50/80
mV
FREQUENCY RESPONSE
dc
1000
Hz
*Note: Resistor tolerance will affect system accuracy. Use of ±1% (or better) metal film resistors is recommended.
REV. B
–3–
ADXL05
ABSO LUTE MAXIMUM RATINGS*
P ackage Char acter istics
Acceleration (Any Axis, Unpowered for 0.5 ms) . . . . . . 1000 g
Acceleration (Any Axis, Powered for 0.5 ms) . . . . . . . . . . 500 g
+VS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7.0 V
Output Short Circuit Duration
P ackage
D evice Weight
JA
JC
10-Pin T O-100
130°C/W
30°C/W
5 Grams
(VPR, VOUT , VREF T erminals to Common) . . . . . . . Indefinite
Operating T emperature . . . . . . . . . . . . . . . . . –55°C to +125°C
Storage T emperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C
O RD ERING GUID E
Tem perature Range
Model
*Stresses above those listed under “Absolute Maximum Ratings” may cause
permanent damage to the device. T his is a stress rating only; the functional
operation of the device at these or any other conditions above those indicated in the
operational sections of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
ADXL05JH
ADXL05AH
0°C to +70°C
–40°C to +85°C
CAUTIO N
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the ADXL05 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. T herefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
Drops onto hard surfaces can cause shocks of greater than 1000 g
and exceed the absolute maximum rating of the device. Care
should be exercised in handling to avoid damage.
CO NNECTIO N D IAGRAM
10-H eader (TO -100)
P IN D ESCRIP TIO N
T he power supply input pin.
+5 V
C2
TOP VIEW
Connection for an external bypass capacitor (nominally 0.022 µF)
used to prevent oscillator switching noise from interfering with
other ADXL05 circuitry. Please see the section on component
selection.
COM
V
C2
REF
7
5
6
9
4
C1
Connections for the demodulator capacitor, nominally 0.022 µF.
See the section on component selection for application information.
ST
C1
C1
3
2
COM
VREF
ST
T he power supply common (or “ground”) connection.
Output of the internal 3.4 V voltage reference.
8
NOTES:
V
PR
AXIS OF SENSITIVITY IS ALONG A LINE
BETWEEN PIN 5 AND THE TAB.
1
10
THE CASE OF THE METAL CAN
PACKAGE IS CONNECTED TO PIN 5
(COMMON).
T he digital self-test input. It is both CMOS and T T L compatible.
+5V
V
OUT
VPR
T he ADXL05 preamplifier output providing an output voltage of
V
ARROW INDICATES DIRECTION OF
POSITIVE ACCELERATION ALONG AXIS
OF SENSITIVITY.
IN–
200 mV per g of acceleration.
VOUT
VIN–
Output of the buffer amplifier.
T he inverting input of the uncommitted buffer amplifier.
REV. B
–4–
ADXL05
GLO SSARY O F TERMS
P olar ity of the Acceler ation O utput
Acceler ation: Change in velocity per unit time.
T he polarity of the ADXL05 output is shown in the Figure 1.
When oriented to the earth’s gravity (and held in place), the
ADXL05 will experience an acceleration of +1 g. T his corre-
sponds to a change of approximately +200 mV at the VPR out-
put pin. Note that the polarity will be reversed to a negative
going signal at the buffer amplifier output VOUT , due to its
inverting configuration.
Acceler ation Vector : Vector describing the net acceleration
acting upon the ADXL05 (AXYZ).
g: A unit of acceleration equal to the average force of gravity
occurring at the earth’s surface. A g is approximately equal to
32.17 feet/s2, or 9.807 meters/s2.
Nonlinear ity: T he maximum deviation of the ADXL05 output
voltage from a best fit straight line fitted to a plot of acceleration
vs. output voltage, calculated as a % of the full-scale output
voltage (@ 5 g).
TAB
+
–
+1g
INDICATED POLARITY IS THAT
OCCURRING AT V
.
PIN 5
PR
Resonant Fr equency: T he natural frequency of vibration of
the ADXL05 sensor’s central plate (or “beam”). At its resonant
frequency of 12 kHz, the ADXL05’s moving center plate has a
peak in its frequency response with a Q of 3 or 4.
Figure 1. Output Polarity at VPR
Sensitivity: T he output voltage change per g unit of accelera-
tion applied, specified at the VPR pin in mV/g.
Acceler ation Vector s in Thr ee D im ensions
T he ADXL05 is a sensor designed to measure accelerations that
result from an applied force. T he ADXL05 responds to the
component of acceleration on its sensitive X axis. Figures 2a
and 2b show the relationship between the sensitive “X” axis and
the transverse “Z” and “Y” axes as they relate to the T O-100
Sensitive Axis (X): T he most sensitive axis of the accelerom-
eter sensor. Defined by a line drawn between the package tab
and Pin 5 in the plane of the pin circle. See Figures 2a and 2b.
Sensor Alignm ent Er r or : Misalignment between the
ADXL05’s on-chip sensor and the package axis, defined by
Pin 5 and the package tab.
Z
Total Alignm ent Er r or : Net misalignment of the ADXL05’s
on-chip sensor and the measurement axis of the application.
T his error includes errors due to sensor die alignment to the
package, and any misalignment due to installation of the sensor
package in a circuit board or module.
TRANSVERSE Z AXIS
SIDE VIEW
X
X
Tr ansver se Acceler ation: Any acceleration applied 90° to the
axis of sensitivity.
PIN 5
SENSITIVE (X) AXIS
TAB
Tr ansver se Sensitivity Er r or : T he percent of a transverse ac-
celeration that appears at the VPR output. For example, if the
transverse sensitivity is 1%, then a +10 g transverse acceleration
will cause a 0.1 g signal to appear at VPR (1% of 10 g). T rans-
verse sensitivity can result from a sensitivity of the sensor to
transverse forces or from misalignment of the internal sensor to
its package.
Z
Figure 2a. Sensitive X and Transverse Z Axis
Y
TRANSVERSE Y AXIS
TOP VIEW
Tr ansver se Y Axis: T he axis perpendicular (90°) to the pack-
age axis of sensitivity in the plane of the package pin circle. See
Figure 2.
X
X
PIN 5
Tr ansver se Z Axis: T he axis perpendicular (90°) to both the
package axis of sensitivity and the plane of the package pin
circle. See Figure 2.
SENSITIVE (X) AXIS
TAB
Y
Figure 2b. Sensitive X and Transverse Y Axis
REV. B
–5–
ADXL05
package. Figure 2c describes a three dimensional acceleration
vector (AXYZ) which might act on the sensor, where AX is the
component of interest. T o determine AX, first, the component
of acceleration in the XY plane (AXY) is found using the cosine
law:
Table I. Ideal O utput Signals for O ff Axis Applied
Accelerations D isregarding D evice Alignm ent and
Transverse Sensitivity Errors
% of Signal Appearing
at O utput
O utput in gs for a 5 g
Applied Acceleration
θX
A
XY = AXYZ (cosθXY) then
AX = AXY (cosθX)
0
1°
2°
3°
100%
5.000 (On Axis)
99.98%
99.94%
99.86%
99.62%
98.48%
86.60%
70.71%
50.00%
17.36%
8.72%
5.25%
3.49%
1.7%
4.999
4.997
4.993
4.981
4.924
4.330
3.536
2.500
0.868
0.436
0.263
0.175
Therefore: Nominal VPR = 200 mV/g (AXYZ) (cosθXY) cosθX
5°
–Z AXIS
10°
30°
45°
60°
80°
85°
87°
88°
89°
90°
Axyz
θxy
Ax
X AXIS
θx
0.085
0%
0.000 (T ransverse Axis)
Axy
Mounting Fixtur e Resonances
Y AXIS
A common source of error in acceleration sensing is resonance
of the mounting fixture. For example, the circuit board that the
ADXL05 mounts to may have resonant frequencies in the same
range as the signals of interest. T his could cause the signals
measured to be larger than they really are. A common solution
to this problem is to dampen these resonances by mounting the
ADXL05 near a mounting post or by adding extra screws to
hold the board more securely in place.
Figure 2c. A Vector Analysis of an Acceleration Acting
Upon the ADXL05 in Three Dim ensions
Note that an ideal sensor will react to forces along or at angles
to its sensitive axis but will reject signals from its various trans-
verse axes, i.e., those exactly 90° from the sensitive “X” axis.
But even an ideal sensor will produce output signals if the trans-
verse signals are not exactly 90° to the sensitive axis. An accel-
eration that is acting on the sensor from a direction different
from the sensitive axis will show up at the ADXL05 output at a
reduced amplitude.
When testing the accelerometer in your end application, it is
recommended that you test the application at a variety of fre-
quencies in order to ensure that no major resonance problems
exist (refer to Analog Devices Application Note AN-379).
T able I shows the percentage signals resulting from various θX
angles. Note that small errors in alignment have a negligible
effect on the output signal. A 1° error will only cause a 0.02%
error in the signal. Note, however, that a signal coming 1° off of
the transverse axis (i.e., 89° off the sensitive axis) will still con-
tribute 1.7% of its signal to the output. T hus large transverse
signals could cause output signals as large as the signals of
interest. T able I may also be used to approximate the effect of
the ADXL05’s internal errors due to misalignment of the die to
the package. For example: a 1 degree sensor alignment error will
allow 1.7% of a transverse signal to appear at the output.
100
90
10
0%
0.5ms
Figure 3. 500 g Shock Overload Recovery. Top Trace, PCB
Reference Accelerom eter Output: 500 g/Vertical Division.
Bottom Trace, ADXL05 Output at VPR
REV. B
–6–
ADXL05
Typical Characteristics (@ +25؇C, C1 = C2 = 0.022 F, V = +5 V unless otherwise noted)
S
9
0.2
6
3
0.1
0
–3
0
–6
–9
–0.1
–12
–15
–0.2
–18
–21
0
1
2
3
4
5
1
10
100
1k
10k
g LEVEL APPLIED
FREQUENCY – Hz
Figure 7. % Nonlinearity vs. g Level Applied
Figure 4. Norm alized Sensitivity vs. Frequency
40
30
2
1
20
0
10
–1
–2
0
–10
–20
–3
–4
–50
0
50
100
150
–50
0
50
100
150
TEMPERATURE – °C
TEMPERATURE – °C
Figure 8. % Change in Noise from +25°C vs. Tem perature
Figure 5. –3 dB Bandwidth vs. Tem perature
10k
0.30
0.20
0.10
1k
0.00
–0.10
–0.20
–0.30
–0.40
–0.50
100
10
0.01
0.1
1
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
DEMODULATOR CAPACITANCE – µF
SUPPLY VOLTAGE – Volts
Figure 9. –3 dB Bandwidth vs. Dem odulator
Capacitance
Figure 6. Sensitivity Change at VPR vs. Supply Voltage
REV. B
–7–
ADXL05
Typical Characteristics (@ +25؇C, C1 = C2 = 0.022 F, V = +5 V unless otherwise noted)
S
80
60
40
20
80
60
T
= +25°C
A
V
= +5V + (0.5Vp-p)
S
E
B
A
40
20
C
B
D
A
0
C
–20
–40
–60
D
E
–40
0
40
TEMPERATURE – °C
80
120
1
10
100
1k
10k
100k
FREQUENCY – Hz
Figure 10. Change in 0 g Bias Level vs. Tem perature
(Characteristic Curves from Five Typical Units)
Figure 13. +3.4 V VREF PSRR vs. Frequency
40
30
20
10
100
T
= +25°C, ACL = 2
A
80
60
40
20
0
NOTE: AT THIS FREQUENCY, THE SIGNAL ON THE
POWER SUPPLY IS IN SYNCHRONISM WITH THE
ACCELEROMETER'S INTERNAL CLOCK OSCILLATOR
(SEE EMI/RFI SECTION)
0
–10
–20
10
100
1k
10k
100k
1M
10
100
1k
10k
FREQUENCY – Hz
FREQUENCY – Hz
Figure 11. 0 g PSRR vs. Frequency
Figure 14. Buffer Am plifier Output Im pedance vs.
Frequency
30
0.05
0
25
20
T = +25°C
A
G =10
G = 2
15
–0.05
–0.1
–0.15
–0.2
–0.25
10
5
0
–5
–10
–15
–20
–50
0
50
100
150
10
100
1k
10k
100k
1M
FREQUENCY – Hz
TEMPERATURE – °C
Figure 15. Buffer Am plifier Closed-Loop Gain vs.
Frequency
Figure 12. % VREF Drift vs. Tem perature
REV. B
–8–
ADXL05
TH EO RY O F O P ERATIO N
Figure 17 shows the sensor responding to an applied accelera-
tion. When this occurs, the common central plate or “beam”
moves closer to one of the fixed plates while moving further
from the other. T his creates a mismatch in the two capacitances,
resulting in an output signal at the central plate. T he output
amplitude of the signal varies directly with the amount of accel-
eration experienced by the sensor.
T he ADXL05 is a complete acceleration measurement system
on a single monolithic IC. It contains a polysilicon surface-
micro machined sensor and signal conditioning circuitry which
implements a force-balance control loop. T he ADXL05 is ca-
pable of measuring both positive and negative acceleration to a
maximum level of ±5 g.
Figure 16 is a simplified view of the ADXL05’s acceleration
sensor at rest. T he actual structure of the sensor consists of 46
unit cells and a common beam. T he differential capacitor sensor
consists of independent fixed plates and central plates attached
to the main beam that moves in response to an applied accelera-
tion. T he two capacitors are series connected, forming a
capacitive divider with a common movable central plate. T he
sensor’s fixed capacitor plates are driven differentially by a
1 MHz square wave: the two square wave amplitudes are equal
but are 180° out of phase from one another. When at rest, the
values of the two capacitors are the same, and therefore, the
voltage output at their electrical center (i.e., at the center plate)
is zero.
TOP VIEW
APPLIED
BEAM
ACCELERATION
CENTER PLATE
FIXED
OUTER
PLATES
UNIT CELL
CS1
< CS2
CS1
DENOTES ANCHOR
CS2
Figure 17. The ADXL05 Sensor Mom entarily Responding
to an Externally Applied Acceleration
BEAM
Figure 18 shows a block diagram of the ADXL05. T he voltage
output from the central plate of the sensor is buffered and then
applied to a synchronous demodulator which is clocked, in
phase, with the same oscillator that drives the fixed plates
of the sensor. If the applied voltage is in sync and in phase
with the clock, a positive output will result. If the applied volt-
age is in sync but 180° out of phase with the clock, then the
demodulator’s output will be negative. All other signals will be
rejected. An external capacitor, C1, sets the bandwidth of the
demodulator.
CENTER PLATE
FIXED
OUTER
PLATES
UNIT CELL
CS1 = CS2
CS1
CS2
DENOTES ANCHOR
Figure 16. A Sim plified Diagram of the ADXL05
Sensor at Rest
DENOTES EXTERNAL
PIN CONNECTION
+3.4V
+3.4V
75Ω
+5V
33kΩ
33kΩ
PREAMP
+1.8V
C2
C1
C1
V
PR
EXTERNAL
DEMODULATION
CAPACITOR
EXTERNAL
OSCILLATOR
DECOUPLING
CAPACITOR
0
°
CS1
CS2
+1.8V
1MHz
OSCILLATOR
BEAM
SYNCHRONOUS
DEMODULATOR
180
°
LOOP GAIN = 10
RST
INTERNAL
FEEDBACK
LOOP
SYNC
+5V
+5V
3MΩ
+5V
COMMON
+0.2V
INTERNAL
REFERENCE
V
REF
BUFFER
AMPLIFIER
+3.4V
+3.4V
+1.8V
50kΩ
V
OUT
V
IN–
+3.4V
+1.8V +0.2V
SELF–TEST
(ST)
COM
Figure 18. Functional Block Diagram
REV. B
–9–
ADXL05
amount equal to –5 g (the negative full-scale output of the de-
vice). Note that the ±15% tolerance of the self-test circuit is not
proportional to the sensitivity error, see Self-T est section.
T he output of the synchronous demodulator drives the
preamp—an instrumentation amplifier buffer which is refer-
enced to +1.8 volts. T he output of the preamp, VPR, is fed back
to the outer plate of the sensor through a 3 MΩ isolation resis-
tor. T he VPR voltage electrostatically resets the sensor back to its
0 g position and is a direct measure of the applied acceleration.
BASIC CO NNECTIO NS FO R TH E AD XL05
Figure 19 shows the basic connections needed for the ADXL05
to measure accelerations in the ±5 g range with an output scale
factor 400 mV/g, a 2.5 V 0 g level, a ±2.0 V full-scale swing
around 0 g, and a 3 dB bandwidth of approximately 1.6 kHz.
T he output of the ADXL05’s preamplifier is 1.8 V ± 200 mV/g,
with an output range of ±1 V for a ±5 g input. An uncommit-
ted buffer amplifier provides the capability to adjust the scale
factor and 0 g offset level over a wide range. An internal refer-
ence supplies the necessary regulated voltages for powering the
chip and +3.4 volts for external use.
Using the circuit of Figure 19, the overall transfer function is:
R3
R1
R3
R2
VOUT
=
1. 8 V – V
(
+
(1.8) +1. 8 V
)
PR
(
)
(
)
A self-test is initiated by applying a T T L “high” level voltage
(> +2.0 V dc) to the ADXL05’s self-test pin, which causes the
chip to apply a deflection voltage to the beam which moves it an
ADXL05
+3.4V
V
REF
6
REFERENCE
+1.8V
OUTPUT
OSCILLATOR
DECOUPLING
CAPACITOR
4
7
SENSOR
OSCILLATOR
DEMODULATOR
C2
0.022µF
PREAMP
SELF-TEST
(ST)
BUFFER
AMP
2
5
10
1
3
8
9
C3
0.1µF
R1
49.9kΩ
R3
100kΩ
V
PR
V
IN–
COM
C1
C1
V
OUT
0.022µF
DEMODULATOR
CAPACITOR
+5V
R2
274kΩ
Figure 19. ADXL05 Application Providing an Output Sensitivity of 400 m V/g,
a +2.5 V 0 g Level and a Bandwidth of 1 kHz
REV. B
–10–
ADXL05
USING TH E INTERNAL BUFFER AMP LIFIER TO VARY
TH E ACCELERO METER’S O UTP UT SCALE FACTO R
AND 0 g BIAS LEVEL
T he ADXL05 accelerometer has an onboard buffer amplifier
that allows the user to change the output scale factor and 0 g
bias level.
C2
4
1
+5V
ADXL05
0.022µF
C1
1.8V
0.1µF
BUFFER
AMP
PRE-AMP
2
3
0.022µF
9
V
OUT
C1
5
T he output scale factor of an accelerometer is simply how many
volts output are provided per g of applied acceleration. T his
should not be confused with its resolution. T he resolution of the
device is the lowest g level the accelerometer is capable of mea-
suring. Resolution is principally determined by the device noise
and the measurement bandwidth.
COM
8
6
10
V
V
+3.4V
REF
PR
IN–
R3
R1a
R1b
R3
(R1a + R1b)
OUTPUT SCALE FACTOR =
OUTPUT: 200mV/g
x V OUTPUT
PR
V
PR
T he 0 g bias level is simply the dc output level of the accelerom-
eter when it is not in motion or being acted upon by the Earth’s
gravity.
Figure 21. External Scale Factor Trim m ing
Setting the Acceler om eter ’s 0 g Bias Level, AC Coupled
Response
Setting the Acceler om eter ’s Scale Factor
If a dc (gravity) response is not required—for example in motion
sensing or vibration measurement applications—ac coupling can
be used between the preamplifier output and the buffer input as
shown in Figure 22. T he use of ac coupling between VPR and
the buffer input virtually eliminates any 0 g drift and allows the
maximum buffer gain without clipping.
Figure 20 shows the basic connections for using the onboard
buffer amplifier to increase the output scale factor. T he nominal
output level in volts from VPR (the preamplifier output) is equal
to the g forces applied to the sensor (along its sensitive axis)
times 200 mV/g. T he use of the buffer is always recommended,
even if the preset scale factor is adequate, as the buffer helps
prevent any following circuitry from loading down the VPR
output.
Resistor R1 and capacitor C4 together form a high pass filter
whose corner frequency is 1/(2 π R1 C4). T his means that this
simple filter will reduce the signal from VPR by 3 dB at the
corner frequency, and it will continue to reduce it at a rate of
6 dB/octave (20 dB per decade) for signals below the corner
frequency.
C2
4
1
+5V
C3
0.022µF
C1
ADXL05
1.8V
0.1µF
BUFFER
AMP
PRE-AMP
2
3
Note that capacitor C4 should be a nonpolarized, low leakage
type. If a polarized capacitor is used, tantalum types are pre-
ferred, rather than electrolytic. With polarized capacitors, VPR
should be measured on each device and the positive terminal of
the capacitor connected toward either VPR or VIN—whichever is
more positive
0.022µF
9
V
OUT
C1
5
COM
8
6
10
V
V
+3.4V
REF
PR
R1
IN–
R3
R3
R1
OUTPUT SCALE FACTOR =
x V OUTPUT
PR
T he 0 g offset level of the ADXL05 accelerometer is preset at
+1.8 V. T his can easily be changed to a more convenient level,
such as +2.5 V which, being at the middle of the supply voltage,
provides the greatest output voltage swing.
V
OUTPUT: 200mV/g
PR
Figure 20. Basic Buffer Connections
In Figure 20, the output scale factor at Pin 9 (VOUT ) is the out-
put at VPR times the gain of the buffer, which is simply the value
of resistor R3 divided by R1. Choose a convenient scale factor,
keeping in mind that the buffer gain not only amplifies the sig-
nal but any noise or drift as well. T oo much gain can also cause
the buffer to saturate and clip the output wave form.
When using the ac coupled circuit of Figure 22, only a single re-
sistor, R2, is required to swing the buffer output to +2.5 V.
Since the “+” input of the buffer is referenced at +1.8 V, its
summing junction, Pin 10, is also held constant at +1.8 V.
T herefore, to swing the buffer’s output to the desired +2.5 V
0 g bias level, its output must move up +0.7 V (2.5 V – 1.8 V =
0.7 V). T herefore, the current needed to flow through R3 to
cause this change, IR3, is equal to:
T he circuit of Figure 20 is entirely adequate for many applica-
tions, but its accuracy is dependent on the pretrimmed accuracy
of the accelerometer and this will vary by product type and
grade. For the highest possible accuracy, an external trim is rec-
ommended. As shown by Figure 21, this consists of a potenti-
ometer, R1a, in series with a fixed resistor, R1b.
0.7 Volts
IR3 =
R3 in Ohms
REV. B
–11–
ADXL05
With a dc coupled connection, any difference between a non-
ideal +1.8 V 0 g level at VPR and the fixed +1.8 V level at the
buffer’s summing junction will be amplified by the gain of the
buffer. If the 0 g level only needs to be approximate and the
buffer is operated a low gain, a single fixed resistor, R2, can still
be used. But to obtain the exact 0 g output desired or to allow
the maximum output voltage swing from the buffer, the 0 g
offset will need to be externally trimmed using the circuit of Fig-
ure 23. Normally, a value of 100 kΩ is typical for R2.
C2
4
1
9
+5V
C3
ADXL05
0.022µF
1.8V
C1
2
0.1µF
BUFFER
AMP
PRE-AMP
0.022µF
3
V
OUT
C1
R3
R1
5
SCALE FACTOR =
COM
8
6
10
V
1
C4 =
V
C4
PR
+3.4V
REF
R1
IN–
R3
2πR1 FL
FOR A +2.5V 0g LEVEL,
IN AN AC COUPLED
CONFIGURATION,
R2 = 2.57 R3
T he buffer’s maximum output swing should be limited to
±2 volts, which provides a safety margin of ±0.25 volts before
clipping. With a +2.5 volt 0 g level, the maximum gain the
buffer should be set to (R3/R1) equals:
R2
RECOMMENDED COMPONENT VALUES
SCALE
FACTOR
IN
R2 VALUE
IN kΩ FOR
+2.5V 0g
LEVEL
DESIRED
LOW
R1
IN
kΩ
CLOSEST
C4
VALUE
R3
IN
kΩ
FULL
SCALE
RANGE
2Volts
FREQUENCY
LIMIT, F
L
mV/g
200 mV/g Times the Max Applied Acceleration in gs
49.9
127
49.9
127
127
0.10µF
0.039µF
1.0µF
249
249
249
249
249
±2g
±5g
±2g
±5g
±5g
1000
400
30Hz
30Hz
3Hz
640
640
640
640
640
Note that the value of R1 should be kept as large as possible,
20 kΩ or greater, to avoid loading down the VPR output.
1000
400
T he device scale factor and 0 g offset levels can be calibrated us-
ing the Earth’s gravity as explained in the section “calibrating
the ADXL05.”
1.5µF
1Hz
15µF
400
0.1Hz
Figure 22. Typical Com ponent Values for AC Coupled
Circuit
C2
4
1
+5V
ADXL05
0.022µF
C1
1.8V
0.1µF
In order to force this current through R3, the same current
needs to flow from Pin 10 to ground through resistor R2. Since
Pin 10 is always held at +1.8 V, R2 is equal to:
BUFFER
AMP
PRE-AMP
2
3
0.022µF
9
V
OUT
C1
5
1. 8 Volts
R2 =
COM
8
6
10
IR3
V
+3.4V
REF
V
PR
R1
IN–
R3
V
T herefore, for an ac coupled connection and a +2.5 V 0 g
X
output:
50kΩ
R3
R1
0g
LEVEL
TRIM
S.F. =
R2
100kΩ
1. 8 Volts × R3
R1 ≥ 20kΩ
R2 =
= 2. 57 × R 3
0.7Volts
RECOMMENDED COMPONENT VALUES FOR
VARIOUS OUTPUT SCALE FACTORS
If ac coupling is used, the self-test feature must be monitored at
VPR, rather than at the buffer output (since the self test output is
FULL
mV
per g
R1
R3
a dc voltage).
SCALE
kΩ
kΩ
±1g
±2g
±4g
±5g
2000
1000
500
30.1
40.2
40.2
49.2
301
200
100
100
Setting the Acceler om eter ’s 0 g Bias Level, D C Coupled
Response
When a true dc (gravity) response is needed, the output from
the preamplifier, VPR, must be dc coupled to the buffer input.
For high gain applications, a 0 g offset trim will also be needed.
T he external offset trim permits the user to set the 0 g offset
voltage to exactly +2.5 volts, since this is at the center of the +5
volt power supply it will allow the maximum output swing from
the buffer without clipping.
400
Figure 23. Typical Com ponent Values for Circuit with
External 0 g Trim m ing
REV. B
–12–
ADXL05
T he equivalent rms noise of the bandpass filter is equal to
D EVICE BAND WID TH VS. MEASUREMENT
RESO LUTIO N
Although an accelerometer is usually specified according to its
full scale (clipping) g level, the limiting resolution of the device,
i.e., its minimum discernible input level, is extremely important
when measuring low g accelerations.
500 µg/ Hz × (1.5 FH ) – (FL /1.5) .
For example, the typical rms noise of the ADXL05 using 1 pole
ac coupling with a bandwidth of 10 Hz and 1 pole low-pass
filter of 100 Hz is:
Noise (rms) = 500 µg / Hz × 1. 5(100 ) – (10 / 1. 5)
= 5,987 µg rms or ≈ 5.9 mg rms
T he limiting resolution is predominantly set by the measure-
ment noise “floor” which includes the ambient background
noise and the noise of the ADXL05 itself. T he level of the noise
floor varies directly with the bandwidth of the measurement. As
the measurement bandwidth is reduced, the noise floor drops,
improving the signal-to-noise ratio of the measurement and in-
creasing its resolution.
Because the ADXL05’s noise is for all practical purposes
Gaussian in amplitude distribution, the highest noise amplitudes
have the smallest (yet nonzero) probability. Peak-to-peak noise
is therefore difficult to measure and can only be estimated due
to its statistical nature. T able II is useful for estimating the
probabilities of exceeding various peak values, given the rms
value.
T he bandwidth of the accelerometer can be easily reduced by
adding low-pass or bandpass filtering. Figure 24 shows the typi-
cal noise vs. bandwidth characteristic of the ADXL05.
Table II.
100mg
660mg
Nom inal P eak-to-
P eak Value
% of Tim e that Noise Will Exceed
Nom inal P eak-to-P eak Value
2.0 × rms
4.0 × rms
6.0 × rms
6.6 × rms
8.0 × rms
32%
4.6%
0.27%
0.1%
0.006%
10mg
66mg
RMS and peak-to-peak noise (for 0.1% uncertainty) for various
bandwidths is estimated in Figure 24. As shown by the figure,
device noise drops dramatically as the operating bandwidth is
reduced. For example, when operated in a 1 kHz bandwidth,
the ADXL05 typically has an rms noise level of 19 mg. With
±5 g applied accelerations, this 19 mg resolution limit is nor-
mally quite satisfactory; but for smaller acceleration levels the
noise is now a much greater percentage of the signal. As shown
by the figure, when the device bandwidth is rolled off to 100 Hz,
the noise level is reduced to approximately 6 mg, and at 10 Hz it
is down to less than 2 mg.
6.6mg
1k
1mg
10
100
3dB BANDWIDTH – Hz
Figure 24. Noise Level vs. 3 dB Bandwidth
T he output noise of the ADXL05 scales with the square root of
the measurement bandwidth. With a single pole roll-off, the
equivalent rms noise bandwidth is π divided by 2 or approxi-
mately 1.5 times the 3 dB bandwidth. For example, the typical
rms noise of the ADXL05J using a 100 Hz one pole post filter is:
Alternatively, the signal-to-noise ratio may be improved consid-
erably by using a microprocessor to perform multiple measure-
ments and then compute the average signal level. When using
this technique, with 100 measurements, the signal-to-noise ratio
will be increased by a factor of 10 (20 dB).
Noise (rms ) = 500 µg/ Hz × 100(1.5) = 6,124 µg or 6.1 mg rms
For the bandpass filter of Figure 27 where both ac coupling and
low pass filtering are used, the low frequency roll-off, FL, is de-
termined by C4 and R1 and the high frequency roll-off, FH, is
determined by the 1-pole post filter R3, C5.
REV. B
–13–
ADXL05
Low-P ass Filter ing
T he bandwidth of the accelerometer can easily be reduced by
using post filtering. Figure 25 shows how the buffer amplifier
can be connected to provide 1-pole post filtering, 0 g offset trim-
ming, and output scaling. T he table provides practical compo-
nent values for various full-scale g levels and approximate circuit
bandwidths. For bandwidths other than those listed, use the
formula:
ADXL05
OPTIONAL CAPACITOR
FOR 3-POLE FILTERING
1.8V
BUFFER
AMP
PRE-AMP
V
OUT
9
R5
8
6
10
V
C4
+5V
V
V
PR
REF
IN–
0.01µF
1
R5
42.2kΩ
C4 =
(2 π R3)Desired 3dB Bandwidth in Hz
R4a
R4b
7
2
OUTPUT
R3
82.5kΩ
R1
82.5kΩ
C3
6
AD820
or simply scale the value of capacitor C4 accordingly, i.e., for an
application with a 50 Hz bandwidth, the value of C4 will need
to be twice as large as its 100 Hz value. If further noise reduc-
tion is needed while maintaining the maximum possible band-
width, then a 2- or 3-pole post filter is recommended. T hese
provide a much steeper roll-off of noise above the pole fre-
quency. Figure 26 shows a circuit that uses the buffer amplifier
to provide 2-pole post filtering. Component values for the 2-
pole filter were selected to operate the buffer at unity gain. Ca-
pacitors C3 and C4 were chosen to provide 3 dB bandwidths of
10 Hz, 30 Hz, 100 Hz, and 300 Hz.
SCALE
FACTOR
TRIM
3
4
OFFSET AND
SCALING
AMPLIFIER
2-POLE FILTER
+3.4V
R6
40.2kΩ
20kΩ
0g
LEVEL
TRIM
R7
71.5kΩ
OFFSET AND SCALING
AMPLIFIER COMPONENT VALUES
2-POLE FILTER
COMPONENT VALUES
mV
per g
R4a
kΩ
R5
kΩ
R4b
kΩ
3dB
FULL
SCALE
GAIN
10.00
4.98
C3µF
0.027
0.082
0.27
C4µF
0.0033
0.01
BW (Hz)
300
In this configuration, the nominal buffer amplifier output will be
+1.8 V ± the 200 mV/g scale factor of the accelerometer. An
AD820 external op amp allows noninteractive adjustment of 0 g
offset and scale factor. T he external op amp offsets and scales
the output to provide a +2.5 V ± 2 V output over a wide range
of full-scale g levels.
2000
1000
500
10
10
10
10
301
200
100
100
24.9
35.7
35.7
45.3
±1g
±2g
±4g
±5g
100
2.50
0.033
0.1
30
400
2.00
0.82
10
Figure 26. Two-Pole Filtering Circuit with Gain and 0 g
Offset Adjustm ent
C2
4
1
+5V
ADXL05
0.022µF
C1
1.8V
Bandpass Filter ing
0.1µF
BUFFER
AMP
PRE-AMP
2
3
Figure 27 shows how the combination of ac coupling and low-
pass filtering together form a bandpass filter that provides an
even greater improvement in noise reduction.
0.022µF
9
V
OUT
C1
5
COM
8
6
10
V
R1a
V
+3.4V
REF
PR
IN–
R1b
R3
C4
ADXL05
1.8V
BUFFER
OPTIONAL SCALE
FACTOR TRIM
PRE-AMP
AMP
*
V
V
OUT
PR
0g
LEVEL
TRIM
9
50kΩ
*TO OMIT THE OPTIONAL SCALE FACTOR
TRIM, REPLACE R1a AND R1b WITH A FIXED
VALUE 1% METAL FILM RESISTOR.
SEE VALUES SPECIFIED IN TABLES BELOW.
NOTE: FOR NONINTERACTIVE TRIMS,
R2
8
10
V
PR
V
IN–
C4
R1
R3
COMPONENT
VALUES ARE
APPROXIMATE.
SET SCALE FACTOR FIRST, THEN OFFSET.
R2
COMPONENT VALUES FOR VARIOUS
FULL-SCALE RANGES AND BANDWIDTHS
C5
R2
kΩ
FULL
SCALE per g
mV
R1a
kΩ
R3
kΩ
C4
µF
3dB
BW (Hz)
R1b
kΩ
DESIRED
LOW
FREQUENCY
DESIRED
HIGH
FREQUENCY
SCALE
FACTOR
IN
VALUE
OF R2
FOR +2.5V
0g LEVEL
R1
VALUE
IN kΩ
CLOSEST
C4
VALUE
CLOSEST
C5
VALUE
R3
IN
kΩ
100
100
100
100
±1g
±2g
±4g
±5g
2000
1000
500
10
10
10
10
301
200
100
100
0.056
0.0082
0.0082
0.0056
10
24.9
35.7
35.7
45.3
LIMIT, F
LIMIT, F
mV/g
L
H
100
200
300
1000
200
30
30
3
49.9
249
49.9
249
249
0.10µF
0.022µF
1.0µF
300
300
100
100
10
249 0.002µF
249 0.002µF
249 0.0068µF
249 0.0068µF
249 0.068µF
640kΩ
640kΩ
640kΩ
640kΩ
640kΩ
400
1000
200
1
3dB BW =
1
0.68µF
6.8µF
2π R3 C4
200
0.1
Figure 25. Using the Buffer Am plifier to Provide 1-Pole
Post Filtering Plus Scale Factor and 0 g Level Trim m ing
Figure 27 AC Coupling and Low-Pass Filtering Used
Together to Provide a Bandpass Function
REV. B
–14–
ADXL05
Additional Noise Reduction Techniques
Shielded wire should be used for connecting the accelerometer to
any circuitry that is more than a few inches away—to avoid 60 Hz
pickup from ac line voltage. Ground the cable’s shield at only one
end and connect a separate common lead between the circuits;
this will help to prevent ground loops. Also, if the accelerometer
is inside a metal enclosure, this should be grounded as well.
θ
θ
1g
1g
Methods for Reducing 0 g O ffset D r ift
When using any accelerometer with a dc (gravity sensing) re-
sponse, the 0 g offset level will exhibit some temperature drift.
For very high accuracy applications, one very straightforward
approach is to use a low cost crystal oven to maintain the accel-
erometer at a constant temperature. T hese ovens are available in
a variety of different temperatures. After the circuit has been built
and is operating correctly, the crystal oven can be mounted over
the accelerometer and powered off the same +5 V power supply.
Figure 28. Two Possible Orientations for Tilt Measurem ent
Conversely, for a given acceleration signal and assuming no
other changes in the axis or interfering signals, the tilt angle is
proportional to the voltage output as shown in Figure 29. T he
angle, θ can be calculated using:
VOUT – zero g output(V )
θ
= arcsin 1g ×
T he ovens may be purchased from Isotemp Research, Inc., P.O.
Box 3389, Charlottesville, VA 22903, phone 804-295-3101. For
more details on crystal oven compensation, refer to application
note AN-385.
Scale Factor (V/g)
500
400
Other methods for 0 g drift compensation include using a low
cost temperature sensor such as the AD22100 to supply a mi-
croprocessor with the device temperature. If the drift curve of
the accelerometer is stored in the µP, then a software program
can be used to subtract out the drift. Alternatively, a simple 1st
order (straight line) correction circuit can be used to subtract
out the linear portion of the accelerometer’s drift by using a
temperature sensor and op amp to supply a small compensation
current. For more details on software and hardware drift com-
pensation, refer to application note AN-380.
300
200
g
100
0
–100
–200
–300
–400
–500
–90
–70
–50 –30
–10
10
30
50
70
90
ACCELERO METER AP P LICATIO NS
ANGLE OF TILT
Popular applications for low g accelerometers tend to fall into
three categories: measurement of tilt and orientation, inertial
measurement of acceleration, velocity and distance, and vibra-
tion or shock measurement.
Figure 29. VOUT vs. Tilt Angle
T he use of an accelerometer in tilt applications has several ad-
vantages over the use of a traditional tilt sensor. A traditional tilt
sensor consists of glass vial filled with a conductive liquid, typi-
cally a mercury or electrolytic solution. Besides being larger
than an XL05, it requires additional signal conditioning cir-
cuitry. T he settling time and frequency response is limited by
the amount of time required for the liquid to stop sloshing
around in the vial. In high vibration environments, or where
high lateral accelerations may be present, it may not be possible
to resolve the tilt signal above the “slosh” noise. T he acceler-
ometer has faster frequency (up to 50 ×) response and set-
tling time. Interfering vibrations may be filtered out if
T he ADXL05 is a “dc” accelerometer, meaning that it is ca-
pable of measuring static accelerations such as the Earth’s grav-
ity. T he ADXL05 differs from other acceleration measurement
technologies such as piezoelectric and piezofilm sensors which
can only respond to ac signals greater than approximately 1 Hz.
T his dc capability is required for tilt and inertial measurement.
For ac shock or vibration the ADXL05 can measure frequencies
of up to 4 kHz and has the added benefit of measuring all the
way down to dc.
Using the AD XL05 in Tilt Applications
T he ADXL05’s precision dc characteristics make it suitable for
tilt measurement. It can directly measure the Earth’s gravity and
use this constant force as a position reference to determine incli-
nation. As shown in Figure 28, the accelerometer should be
mounted so that its sensitive axis is perpendicular to the force of
gravity, i.e., parallel to the Earth’s surface. In this manner, it
will be most sensitive to changes in orientation (when it is orien-
tated 90° to the force of gravity). Its output can be then de-
scribed by the sine function; a tilt occurring at an angle θ will
cause a voltage output equal to:
necessary, an impossibility with a liquid tilt sensor, since one
cannot filter the liquid. Finally, in the presence of lateral accel-
erations, an accelerometer provides more useful information,
i.e., an acceleration signal, which if cleverly signal processed,
can provide both a tilt and an acceleration output. A single ac-
celerometer can be used to measure tilt over a 180° range; two
accelerometers gives a complete 360° of measurement.
An important characteristic for an accelerometer used in a tilt
application is its 0 g offset stability over temperature. T he
ADXL05 typically exhibits offsets that deviate no more than
0.1 g over the 0°C to +70°C temperature range, corresponding
V
VOUT
=
Accelerometer Scale Factor
× sin
θ
( )
×1g + zero g output(V )
g
REV. B
–15–
ADXL05
control of system and mounting resonances are critical to proper
measurement. Refer to the application note AN-379, available
from Analog Devices.
to a 5° tilt error over the entire temperature range. Straight-
forward calibration schemes discussed in this data sheet may be
used to reduce or compensate for temperature drift to improve
the absolute accuracy of the measurement.
CALIBRATING TH E AD XL05
Using the AD XL05 in Iner tial Measur em ent Applications
Inertial measurement refers to the practice of measuring accel-
eration for the purpose of determining the velocity of an object
and its change in position, or distance traveled. T his technique
has previously required expensive inertial guidance systems of
the type used in commercial aircraft and military systems. T he
availability of a low cost precision dc accelerometer such as the
ADXL05 enables the use of inertial measurement for more cost
sensitive industrial and commercial applications.
If a calibrated shaker is not available, both the 0 g level and scale
factor of the ADXL05 may be easily set to fair accuracy by using
a self-calibration technique based on the 1 g (average) accelera-
tion of the earth’s gravity. Figure 30 shows how gravity and
package orientation affect the ADXL05’s output. Note that the
output polarity is that which appears at VPR; the output at VOUT
will have the opposite sign. With its axis of sensitivity in the
vertical plane, the ADXL05 should register a 1 g acceleration,
either positive or negative, depending on orientation. With the
axis of sensitivity in the horizontal plane, no acceleration (the
0 g bias level) should be indicated.
Inertial measurement makes use of the fact that the integral of
acceleration is velocity and the integral of velocity is distance.
By making careful measurements of acceleration and math-
ematically integrating the signals, one can determine both veloc-
ity and the distance traveled. T he technique is useful for
applications where a traditional speed and distance measure-
ment is impractical, or where a non-contact, relative position
measurement must be made.
+1g
(d)
0g
(a)
0g
(b)
–1g
(c)
A practical inertial measurement system uses multiple acceler-
ometers to measure acceleration in three axes, and gyroscopes to
measure rotation in three axes, the requirement for a 6 degree of
freedom system. For simpler systems where one or more of the
axes can be constrained, it is possible to build a system with
fewer accelerometers and gyros.
INDICATED POLARITY IS THAT
OCCURRING AT V
.
PR
T he measurement system must take the acceleration sensor and
calibrate out all static errors including any initial inaccuracy or
temperature drift. A mathematical model is used to describe the
performance of the sensor in order to calibrate it. If these errors
are not removed, then the process of double integration will
quickly cause any small error to dominate the result. Most prac-
tical systems use microprocessors for error correction and a tem-
perature sensor for temperature drift compensation. Another
approach is to maintain all of the sensors at a controlled tem-
perature. T he microprocessors have the additional advantage of
providing a low cost method of performing the single and
double integration of the acceleration signal.
Figure 30. Using the Earth’s Gravity to Self-Calibrate the
ADXL05
T o self-calibrate the ADXL05, place the accelerometer on its
side with its axis of sensitivity oriented as shown in “a.” T he 0 g
offset potentiometer, Rt, is then roughly adjusted for midscale:
+2.5 V at the buffer output (see Figure 25).
Next, the package axis should be oriented as in “c” (pointing
down) and the output reading noted. T he package axis should
then be rotated 180° to position “d” and the scale factor poten-
tiometer, R1a, adjusted so that the output voltage indicates a
change of 2 gs in acceleration. For example, if the circuit scale
factor at the buffer output is 400 mV per g, then the scale factor
trim should be adjusted so that an output change of 800 mV is
indicated.
T he stability and repeatability of the accelerometer is the most
important specification in an inertial system. T he ADXL05 is
“well behaved” that is, its response and temperature characteris-
tics are easy to model and correct, and once modeled they are
very repeatable. For example, temperature performance can be
adequately modeled using first order, (straight line) approxima-
tions for most applications, and other errors such as on-axis and
pendulous rectification are minimal. T his greatly simplifies the
math required to correct the sensor.
Adjusting the circuit’s scale factor will have some effect on its
0 g level so this should be readjusted, as before, but this time
checked in both positions “a” and “b.” If there is a difference in
the 0 g reading, a compromise setting should be selected so that
the reading in each direction is equidistant from +2.5 V. Scale
factor and 0 g offset adjustments should be repeated until both
are correct.
Vibr ation and Shock Measur em ent Applications
T he ADXL05 can measure shocks and vibrations from dc to
4 kHz. T ypical signal processing for vibration signals includes
fast Fourier transforms, and single and double integration for
velocity and displacement. It is possible to build a single inte-
grator stage using the ADXL05’s output buffer amplifier in
order to provide a velocity output.
RED UCING P O WER CO NSUMP TIO N
T he use of a simple power cycling circuit provides a dramatic
reduction in the ADXL05’s average current consumption. In
low bandwidth applications such as shipping recorders, this
simple, low cost circuit can provide substantial power reduction.
T he sensitivity of the accelerometer will typically vary only
±0.5% over the full industrial temperature range, making it one
of the most stable vibration measurement devices available. In
vibration measurement applications, mechanical mounting and
If a microprocessor is available, only the circuit of Figure 31 is
needed. T he microprocessor supplies a T T L clock pulse to gate
buffer transistor Q1 which inverts the output pulse from the µP
REV. B
–16–
ADXL05
so that the duty cycle is correct when the pulse is re-inverted
again by transistor, Q2, which cycles the accelerometer’s supply
voltage on and off.
2V
200µs
100
90
+5V
0.1µF
100kΩ
BUFFER
10kΩ
10
2N3906
1
0%
Q1
10kΩ
FROM µP
OR
FIGURE 1b
Q2
2N2222
500mV
ADXL05
Figure 33. Top Trace: Voltage at Pin 1; Bottom Trace:
Buffer Output With R1 = R3 = 100 kΩ, CF = 0.01 µF
V
8
V
V
OUT
PR
IN–
10
5
9
R1
R3
CO MP O NENT SELECTIO N
V
OUT
LO AD D RIVE CAP ABILITIES O F TH E VP R AND BUFFER
O UTP UTS
T he VPR and the buffer amplifier outputs are both capable of
driving a load to voltage levels approaching that of the supply
rail. However, both outputs are limited in how much current
they can supply, affecting component selection.
C
F
Figure 31. Basic Power Cycling Circuit
Figures 32 and 33 show typical waveforms of the accelerometer
being operated with a 10% duty cycle: 1 ms on, 9 ms off. T his
reduces the average current consumption of the accelerometer
from 8 mA to 800 µA, providing a power reduction of 90%. T he
µP should sample acceleration during the interval between the
time the 0 g level has stabilized (≈ 400 µs using a 0.022 µF
demod cap) and the end of the pulse duration. T he measure-
ment bandwidth of a power-cycled circuit will be set by the
clock pulse rate and duty cycle. In this example, 1 sample can
be taken every 10 ms which is 100 samples per second or 100
Hz. As defined by the “Nyquist criteria,” the best case measure-
ment bandwidth is FS/2 or half the clock frequency. T herefore
50 Hz signals can be processed if adequate filtering is provided.
VP R O utput
T he VPR pin has the ability to source current up to 500 µA but
only has a sinking capability of 30 µA which limits its ability to
drive loads. It is recommended that the buffer amplifier be used
in most applications, to avoid loading down VPR. In standard
±5 g applications, the resistor R1 from VPR to VIN– is recom-
mended to have a value greater than 20 kΩ to reduce loading
effects.
Capacitive loading of the VPR pin should be minimized. A load
capacitance between the VPR pin and common will introduce an
offset of approximately 1 mV for every 10 pF of load. T he VPR
pin may be used to directly drive an A/D input or other source
as long as these sensitivities are taken into account. It is always
preferable to drive A/D converters or other sources using the
buffer amplifier (or an external op amp) instead of the VPR pin.
2V
200µs
100
90
Buffer Am plifier O utput
T he buffer output can drive a load to within 0.25 V of either
power supply rail and is capable of driving 1000 pF capacitive
loads. Note that a capacitance connected across the buffer
feedback resistor for low-pass filtering does not appear as a
capacitive load to the buffer. T he buffer amplifier is limited to
sourcing or sinking a maximum of 100 µA. Component values
for the resistor network should be selected to ensure that the
buffer amplifier can drive the filter under worst case transient
conditions.
10
0%
1V
Figure 32. Top Trace: Voltage at Pin 1;
Bottom Trace: Output at VPR
Higher measurement bandwidths can be achieved by reducing
the size of the demodulation capacitor below 0.022 µF and in-
creasing the pulse frequency. A 0.01 µF capacitor was con-
nected across the feedback resistor of the ADXL05 buffer to
improve its transient characteristics. T he optimum value for this
capacitor will change with buffer gain and the cycling pulse rate.
For more details, refer to application note AN-378.
Self-Test Function
T he digital self-test input is compatible with both CMOS and
T T L signals. A Logic “l” applied to the self-test (ST ) input will
cause an electrostatic force to be applied to the sensor which
will cause it to deflect to the approximate negative full-scale
output of the device. Accordingly, a correctly functioning accel-
erometer will respond by initiating an approximate –1 volt
REV. B
–17–
ADXL05
output change at VPR. If the ADXL05 is experiencing an
acceleration when the self-test is initiated, the VPR output will
equal the algebraic sum of the two inputs. T he output will stay
at the self-test level as long as the ST input remains high and
will return to the 0 g level when the ST voltage is removed.
low-pass filtering generally results in smaller capacitance values
and better overall performance. It is also a convenient and more
precise way to set the system bandwidth. Post filtering allows
bandwidth to be controlled accurately by component selection
and avoids the ±40% demodulation tolerance. Note that signal
noise is proportional to the square root of the bandwidth of the
ADXL05 and may be a consideration in component selection—
see section on noise.
A self-test output that varies more than ±15% from the nominal
–1.0 V change indicates a defective beam or a circuit problem
such as an open or shorted pin or component.
Care should be taken to reduce or eliminate any leakage paths
from the demodulator capacitor pins to common or to the +5 V
pin. Even a small imbalance in the leakage paths from these pins
will result in offset shifts in the zero-g bias level. As an example,
an unbalanced parasitic resistance of 30 MΩ from either de-
modulator pin to ground will result in an offset shift at VPR of
approximately 50 mV. Conformal coating of PC boards with a
high impedance material is recommended to avoid leakage prob-
lems due to aging or moisture.
Operating the ADXL05’s buffer amplifier at Gains > 2, to pro-
vide full-scale outputs of less than ±5 g, may cause the self-test
output to overdrive the buffer into saturation. T he self-test may
still be used in the case, but the change in the output must then
be monitored at the VPR pin instead of the buffer output.
Note that the value of the self-test delta is not an exact indica-
tion of the sensitivity (mV/g) of the ADXL05 and, therefore,
may not be used to calibrate the device for sensitivity error.
In critical applications, it may be desirable to monitor shifts in
the zero-g bias voltage from its initial value. A shift in the 0 g
bias level may indicate that the 0 g level has shifted which may
warrant an alarm.
MINIMIZING EMI/RFI
T he architecture of the ADXL05 and its use of synchronous de-
modulation make the device immune to most electromagnetic
(EMI) and radio frequency (RFI) interference. T he use of syn-
chronous demodulation allows the circuit to reject all signals ex-
cept those at the frequency of the oscillator driving the sensor
element. However, the ADXL05 does have a sensitivity to RFI
that is within ±5 kHz of the internal oscillator’s nominal fre-
quency of 1 MHz and also to any odd harmonics of this fre-
quency. T he internal oscillator frequency will exhibit part to
part variation in the range of 0.5 MHz to 1.4 MHz.
P ower Supply D ecoupling
T he ADXL05 power supply should be decoupled with a 0.1 µF
ceramic capacitor from +5 V pin of the ADXL05 to common
using very short component leads. For other decoupling consid-
erations, see EMI/RFI section.
O scillator D ecoupling Capacitor , C2
An oscillator decoupling capacitor, C2, is used to remove
1 MHz switching transients in the sensor excitation signal, and
is required for proper operation of the ADXL05. A ceramic
capacitor with a minimum value of 0.022 µF is recommended
from the oscillator decoupling capacitor pin to common. Small
amounts of capacitor leakage due to a dc resistance greater than
1 MΩ will not affect operation (i.e., a high quality capacitor is
not needed here). As with the power supply bypass capacitor,
very short component leads are recommended. Although
0.022 µF is a good typical value, it may be increased for reasons
of convenience, but doing this will not improve the noise perfor-
mance of the ADXL05.
In general the effect is difficult to notice as the interference must
match the internal oscillator within ±5 kHz and must be large
in amplitude. For example: a 1 MHz interference signal of
20 mV p-p applied to the +5 V power supply pin will produce a
200 mV p-p signal at the VPR pin if the internal oscillator and
interference signals are matched exactly or at odd harmonics. If
the same 20 mV interference is applied but 5 kHz above or be-
low the internal oscillator’s frequency, the signal level at VPR will
only be 20 mV p-p in amplitude.
Power supply decoupling, short component leads (especially for
capacitors C1 and C2), physically small (surface mount, etc.)
components and attention to good grounding practices all help
to prevent RFI and EMI problems. Good grounding practices
include having separate analog and digital grounds (as well as
separate power supplies or very good decoupling) on the printed
circuit boards. A single ground line shared by both the digital
and analog circuitry can lead to digital pulses (and clock signals)
interfering with the sensor’s onboard oscillator. In extreme
cases, a low cost radio frequency choke (≈10 µH) may be
needed in series with the accelerometer’s power supply pin.
T his, together with the recommended 0.1 µF power supply by-
pass capacitor, will form an effective RF filter. T he use of an RF
choke is preferred over a resistor since any series resistance in
the power supply will “unregulate” the device from the supply,
degrade its power supply rejection and reduce its supply voltage.
D em odulator Capacitor , C1
T he demodulator capacitor is connected across Pins 2 and 3 to
set the bandwidth of the force balance control loop. T his capaci-
tor may be used to approximately set the bandwidth of the ac-
celerometer. A capacitor is always required for proper operation.
T he frequency response of the ADXL05 exhibits a single pole
roll-off response, see Figure 4.
A nominal value of 0.022 µF is recommended for C1. In gen-
eral, the design bandwidth should be set 40% higher than the
minimum desired system bandwidth due to the ±40% tolerance,
to preserve stability C1 should be kept > 0.01 µF.
T he demodulation capacitor should be a low leakage, low drift
ceramic type with an NPO (best) or X7R (good) dielectric.
In general, it’s best to use the recommended 0.022 µF capacitor
across the demodulator pins and perform any additional low-
pass filtering using the buffer amplifier. T he use of the buffer for
REV. B
–18–
ADXL05
O UTLINE D IMENSIO NS
D imensions shown in inches and (mm).
REFERENCE PLANE
0.750 (19.05)
0.500 (12.70)
0.160 (4.06)
0.185 (4.70)
0.165 (4.19)
0.110 (2.79)
6
7
5
0.335 (8.51)
0.305 (7.75)
8
0.045 (1.14)
0.027 (0.69)
0.115
(2.92)
BSC
4
9
0.370 (9.40)
0.335 (8.51)
3
10
2
1
0.230 (5.84)
BSC
0.040 (1.02) MAX
0.034 (0.86)
0.027 (0.69)
0.045 (1.14)
0.010 (0.25)
SEATING PLANE
REV. B
–19–
–20–
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