ADXL210_15 [ADI]
Accelerometers with Digital Output;
| 型号： | ADXL210_15 |
| 厂家： | 
ADI |
| 描述： | Accelerometers with Digital Output |
| 文件： | 总11页 (文件大小：819K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Low Cost ؎2 g/؎10 g Dual Axis
iMEMS® Accelerometers
with Digital Output
a
ADXL202/ADXL210
GENERAL DESCRIPTION
FEATURES
The ADXL202/ADXL210 are low cost, low power, complete
2-axis accelerometers with a measurement range of either
±2 g/±10 g. The ADXL202/ADXL210 can measure both dy-
namic acceleration (e.g., vibration) and static acceleration (e.g.,
gravity).
2-Axis Acceleration Sensor on a Single IC Chip
Measures Static Acceleration as Well as Dynamic
Acceleration
Duty Cycle Output with User Adjustable Period
Low Power <0.6 mA
Faster Response than Electrolytic, Mercury or Thermal
Tilt Sensors
Bandwidth Adjustment with a Single Capacitor Per Axis
5 mg Resolution at 60 Hz Bandwidth
+3 V to +5.25 V Single Supply Operation
1000 g Shock Survival
The outputs are digital signals ose duty cycles (ratio of pulse-
width to period) are proportnal to te acceleration in each of
the 2 sensitive axes. These tputs ay be measured directly
with a microprocessor ounteuiring nA/D converter or
glue logic. The outpperiod is ustafrom 0.5 ms to 10 ms
via a single resisET). If a voe output is desired, a
voltage output roportal to acceleration is available from the
XFILT and YILT pins, or be reconstructed by filtering the
duty cycloutputs.
APPLICATIONS
2-Axis Tilt Sensing
Computer Peripherals
Inertial Navigation
Seismic Monitoring
The dwidtof the ADXL202/ADXL210 may be set from
0.01 Hz kHz via pacitors CX and CY. The typical noise
floor is 50g/√Hz lowing signals below 5 mg to be resolved
for bandwidtow 60 Hz.
Vehicle Security Systems
Battery Powered Motion Sensing
The ADXL202/ADXL210 is available in a hermetic 14-lead
rface Munt CERPAK, specified over the 0°C to +70°C
coial or –40°C to +85°C industrial temperature range.
FUNCK DIAGRAM
+3.0V TO +
C
X
SELF TEST
X
FILT
V
D
D
ADXL202/
ADXL210
X SENSO
R
FILT
32k⍀
X OUT
DEMOD
DEMOD
C
O
U
N
T
E
R
DUTY
CYCLE
MODULATOR
(DCM)
P
ILLATOR
Y SENSOR
R
FILT
32k⍀
Y OUT
Y
T2
FILT
COM
R
SET
C
Y
T2
T1
A(g) = (T1/T2 – 0.5)/12.5%
0g = 50% DUTY CYCLE
T2 = R
/125M⍀
SET
i
MEMS is a registered trademark of Analog Devices, Inc.
REV. B
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
AIN2 =
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
Fax: 781/326-8703
World Wide Web Site: http://www.analog.com
© Analog Devices, Inc., 1999
(TA = TMIN to TMAX, TA = +25؇C for J Grade only, VDD = +5 V,
SET = 125 k⍀, Acceleration = 0 g, unless otherwise noted)
ADXL202/ADXL210–SPECIFICATIONS R
ADXL202/JQC/AQC
ADXL210/JQC/AQC
Parameter
Conditions
Min
Typ Max
Min
Typ
Max
Units
SENSOR INPUT
Measurement Range1
Nonlinearity
Each Axis
±1.5
±2
0.2
±1
±0.01
±2
±8
±10
0.2
±1
±0.01
±2
g
Best Fit Straight Line
X Sensor to Y Sensor
% of FS
Degrees
Degrees
%
Alignment Error2
Alignment Error
Transverse Sensitivity3
SENSITIVITY
Each Axis
Duty Cycle per g
T1/T2 @ +25°C
At Pins XFILT, YFILT
∆ from +25°C
10
25
12.5 15
312
±0.5
3.2
42
4.0
100
±0.5
4.8
%/g
mV/g
% Rdg
Sensitivity, Analog Output
Temperature Drift4
ZERO g BIAS LEVEL
0 g Duty Cycle
Each Axis
T1/T2
50
75
50
58
%
Initial Offset
±2
1.0
2.0
0
g
0 g Duty Cycle vs. Supply
4.0
0
%/V
mg/°C
0 g Offset vs. Temperature4
∆ from +25°C
@ +25°C
NOISE PERFORMANCE
Noise Density5
500
00
500
1000
µg/√Hz
FREQUENCY RESPONSE
3 dB Bandwidth
3 dB Bandwidth
Duty Cycle Output
At Pins XFILT, YFILT
5
5
10
500
5
14
Hz
kHz
kHz
Sensor Resonant Frequency
FILTER
RFILT Tolerance
Minimum Capacitance
32 kΩ Nominal
At XFILT, YFILT
±15
±15
%
pF
1000
1000
0.7
SELF TEST
Duty Cycle Change
Self-Test “0” to “1”
10
10
%
DUTY CYCLE OUTPUT STAGE
FSET
FSET Tolerance
Output High Voltage
Output Low Voltage
T2 Drift vs. Temperature
Rise/Fall Time
25 MΩ/RSET
125 MΩ/RSET
RSET = 5 kΩ
I = 25 µ
I = 25 µA
0.7
1.3
1.3
kHz
mV
mV
ppm/°C
ns
VS – 200 mV
VS – 200 mV
200
200
35
200
35
200
POWER SUPPLY
Operating Voltage Range
Specified Performance
Quiescent Supply Cu
Turn-On Time6
3.0
4.75
5.25
5.25
1.0
2.7
4.75
5.25
5.25
1.0
V
V
mA
ms
0.6
0.6
To 99%
160 CFILT + 0.3
160 CFILT + 0.3
TEMPERATURE RAN
Operating Range
Specified Performance
JQC
AQC
0
–40
+70
+85
0
–40
+70
+85
°C
°C
NOTES
1For all combinations of offset and sensitivity variation.
2Alignment error is specified as the angle between the true and indicated axis of sensitivity.
3Transverse sensitivity is the algebraic sum of the alignment and the inherent sensitivity errors.
4Specification refers to the maximum change in parameter from its initial at +25°C to its worst case value at TMIN to TMAX
5Noise density (µg/√Hz) is the average noise at any frequency in the bandwidth of the part.
.
6CFILT in µF. Addition of filter capacitor will increase turn on time. Please see the Application section on power cycling.
All min and max specifications are guaranteed. Typical specifications are not tested or guaranteed.
Specifications subject to change without notice.
REV. B
–2–
ADXL202/ADXL210
PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGS*
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
(Any Pin to Common) . . . . . . . . . . . . . . . . . . . . . . Indefinite
Operating Temperature . . . . . . . . . . . . . . . . . –55°C to +125°C
Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C
1
2
3
4
5
6
7
14
13
12
11
10
9
NC
V
V
X
Y
X
Y
DD
ADXL202/
ADXL210
V
TP
DD
ST
FILT
FILT
OUT
OUT
TOP VIEW
(Not to Scale)
COM
T2
A
X
NC
*Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent damage to the device. This 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.
NC
COM
8
A
Y
NC = NO CONNECT
Figure 1 shows the response ohe ADXL202 to the Earth’s
gravitational field. The outpvalues hown are nominal. They
are presented to show the uwhtype of response to expect
from each of the outppins do chanin orientation with
respect to the Earthe ADXLrets similarly with out-
put changes appprito its scale.
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.
PIN FUNCTION DESCRIPTIONS
TYOUTPUT AT PIN:
9 = UTY CYCLE
10 = 6% DUTY CYCLE
11 = 2.5V
Pin
Name
Description
1
NC
No Connect
12 = 2.188V
2
3
VTP
ST
Test Point, Do Not Connect
Self Test
4
5
6
COM
T2
NC
Common
Connect RSET to Set T2 Period
No Connect
TYPICAL OUTPUT AT PIN:
9 = 62.5% DUTY CYCLE
10 = 50% DUTY CYCLE
11 = 2.188V
YPICAL OUTPT PIN:
37.5% DCYCLE
50% Y CYCLE
1
7
8
COM
NC
Common
No Connect
12 = 2.5V
12
9
YOUT
XOUT
YFILT
XFILT
VDD
VDD
Y Axis Duty Cycle Ou
X Axis Duty Cycle Ou
Connect Capacitor for
Connect Capitr for X F
+3 V to +55 V, Coect to 14
+3 V to +5Vonneto 13
10
11
12
13
14
1g
TYPICAL OUTPUT AT PIN:
9 = 50% DUTY CYCLE
10 = 37.5% DUTY CYCLE
11 = 2.5V
12 = 2.812V
EARTH'S SURFACE
PACKAGE CHARACTERIST
Package
Device Weight
JA
Figure 1. ADXL202/ADXL210 Nominal Response Due to
Gravity
14-Lead CERPAK
110°C/W
5 Grams
ORDERING GUIDE
g
Temperature
Range
Package
Description
Package
Option
del
Range
ADXL202JQC
ADXL202AQC
ADXL210JQC
ADXL210AQC
±2
±2
±10
±10
0°C to +70°C
–40°C to +85°C
0°C to +70°C
–40°C to +85°C
14-Lead CERPAK
14-Lead CERPAK
14-Lead CERPAK
14-Lead CERPAK
QC-14
QC-14
QC-14
QC-14
CAUTION
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 ADXL202/ADXL210 features proprietary ESD protection circuitry, permanent
damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper
ESD precautions are recommended to avoid performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
REV. B
–3–
ADXL202/ADXL210
TYPICAL CHARACTERISTICS
(@ +25؇C RSET = 125 k⍀, VDD = +5 V, unless otherwise noted)
1.06
4%
3%
1.04
1.02
2%
1%
1.00
0.98
0%
–1%
–2%
–3%
–4%
0.96
0.94
–45
–30
–15
0
15
30
45
60
75
90
–40
2
85
TEMPERATUC
TEMPERATURE – ؇C
Figure 2. Normalized DCM Period (T2) vs. Temperature
Figure 5. Typal X is Sensitivity Drift Due to
Temperate
0.8
0.6
3
g
0.4
C
= 0.01F
FILT
2
1
0
0.2
0
–0.2
g
–0.4
–0.6
–0.8
0
0.4
0.8
1.2
1.4
–40 –30 –20 –10
0
10 20 30 60 70 80 90
TEMPERAT
FREQUENCY – ms
Figure 6. Typical Turn-On Time
Figure 3. Typical Zero g erature
20
18
16
14
12
10
8
0.7
0.6
V
V
= 5 VDC
S
0.5
0.4
0.3
0.2
0.1
0
= 3.5 VDC
S
6
4
2
0
–40
–20
0
20
40
60
80
100
–0.87g –0.64g –0.41g –0.17g 0.06g 0.29g
0.52g 0.75g
TEMPERATURE – ؇C
g/DUTY CYCLE OUTPUT
Figure 7. Typical Zero g Distribution at +25°C
Figure 4. Typical Supply Current vs. Temperature
REV. B
–4–
ADXL202/ADXL210
9
8
7
6
5
4
3
2
1
14
12
10
8
T2 = 1ms
C
= 0.01F
FILT
BW = 500Hz
C
= 0.047F
BW = 100Hz
FILT
g
C
= 0.1F
FILT
BW = 50Hz
6
C
= 0.47F
FILT
BW = 10Hz
4
2
0
11.3
0
11.5 11.7 11.9 12.2 12.4 12.6 12.8 13.1 13.3 13.5 13.7
DUTY CYCLE OUTPUT – % per g
1
4
16
64
NUMBF AVGE SAMPLES
Figure 8. Typical Sensitivity per g at +25°C
Figure 10. pical Ne at Dtal Outputs
20
1
16
12
10
8
14
12
10
8
g
6
6
4
4
2
2
0
0
0.01F
0.047F
0.1F
0.47
500Hz
100Hz
50H
10Hz
C , C
X
Y
DEGREES OF MISALIGNMENT
BANDWIDTH
Figure 9. Typical Noisat XFILT Output
Figure 11. Rotational Die Alignment
REV. B
–5–
ADXL202/ADXL210
DEFINITIONS
APPLICATIONS
POWER SUPPLY DECOUPLING
T1
Length of the “on” portion of the cycle.
For most applications a single 0.1 µF capacitor, CDC, will ad-
equately decouple the accelerometer from signal and noise on
the power supply. However, in some cases, especially where digital
devices such as microcontrollers share the same power supply, digi-
tal noise on the supply may cause interference on the ADXL202/
ADXL210 output. This is often observed as a slowly undulating
fluctuation of voltage at XFILT and YFILT. If additional decou-
pling is needed, a 100 Ω (or smaller) resistor or ferrite beads,
may be inserted in the ADXL202/ADXL210’s supply line.
T2
Length of the total cycle.
Duty Cycle Ratio of the “on” time (T1) of the cycle to the total
cycle (T2). Defined as T1/T2 for the ADXL202/
ADXL210.
Pulsewidth Time period of the “on” pulse. Defined as T1 for
the ADXL202/ADXL210.
THEORY OF OPERATION
The ADXL202/ADXL210 are complete dual axis acceleration
measurement systems on a single monolithic IC. They contain a
polysilicon surface-micromachined sensor and signal condition-
ing circuitry to implement an open loop acceleration measure-
ment architecture. For each axis, an output circuit converts the
analog signal to a duty cycle modulated (DCM) digital signal
that can be decoded with a counter/timer port on a micropro-
cessor. The ADXL202/ADXL210 are capable of measuring
both positive and negative accelerations to a maximum level of
±2 g or ±10 g. The accelerometer measures static acceleration
forces such as gravity, allowing it to be used as a tilt sensor.
DESIGN PROCEDURE FOR THE DXL202/ADXL210
The design procedure for using thADXL202/ADXL210 with a
duty cycle output involves seleg a ducycle period and a
filter capacitor. A proper deign wte into aount the appli-
cation requirements for bdwidth, nal reution and acqui-
sition time, as discussethe followintions.
VDD
The ADXL202DXL210 havwo power supply (VDD) Pins:
13 and 14. Tse two pins should e connected directly together.
COM
The sensor is a surface micromachined polysilicon structure
built on top of the silicon wafer. Polysilicon springs suspend the
structure over the surface of the wafer and provide a resistance
against acceleration forces. Deflection of the structure is mea-
sured using a differential capacitor that consists of independent
fixed plates and central plates attached to the moving mass. The
fixed plates are driven by 180° out of phase square waves. A
acceleration will deflect the beam and unbalance the diffe
capacitor, resulting in an output square wave whose amp
is proportional to acceleration. Phase sensitive demodulat
techniques are then used to rectify the signal and rmine
direction of the acceleration.
The ADXL2DXL21ave two commons, Pins 4 and 7.
These two pins ould connected directly together and Pin 7
grounded.
VT
This is to left open; make no connections of any kind to
is pin
oupling Capacitor CDC
µF capacitor is recommended from VDD to COM for
er supply decoupling.
ST
The ST pin controls the self-test feature. When this pin is set to
VDD, an electrostatic force is exerted on the beam of the acceler-
ometer. The resulting movement of the beam allows the user to
test if the accelerometer is functional. The typical change in
output will be 10% at the duty cycle outputs (corresponding to
800 mg). This pin may be left open circuit or connected to
common in normal use.
The output of the demodulator drives a duty cle dula
(DCM) stage through a 32 kΩ resistor. At this point a pin i
available on each channel to allow tho set the sign
bandwidth of the device by adding r. This filtering
improves measurement resolutiont aliasing.
After being low-pass filtered, the anaconverted to a
duty cycle modulated sige DC. A single resistor
sets the period for a cT2), hich can be set be-
tween 0.5 ms and 10 2). A 0 g acceleration
produces a nominally e. The acceleration signal
can be determined by he length of the T1 and T2
pulses with a counter/timer or with a polling loop using a low
cost microcontroller.
Duty Cycle Decoding
The ADXL202/ADXL210’s digital output is a duty cycle modu-
lator. Acceleration is proportional to the ratio T1/T2. The
nominal output of the ADXL202 is:
0 g = 50% Duty Cycle
Scale factor is 12.5% Duty Cycle Change per g
The nominal output of the ADXL210 is:
0 g = 50% Duty Cycle
An analog output voltage can be obtained either by buffering the
signal from the XFILT and YFILT pin, or by passing the duty cycle
signal through an RC filter to reconstruct the dc value.
Scale factor is 4% Duty Cycle Change per g
The ADXL202/ADXL210 will operate with supply voltages as
low as 3.0 V or as high as 5.25 V.
These nominal values are affected by the initial tolerance of the
device including zero g offset error and sensitivity error.
T2
T2 does not have to be measured for every measurement cycle.
It need only be updated to account for changes due to tempera-
ture, (a relatively slow process). Since the T2 time period is
shared by both X and Y channels, it is necessary only to mea-
sure it on one channel of the ADXL202/ADXL210. Decoding
algorithms for various microcontrollers have been developed.
Consult the appropriate Application Note.
T1
A(g) = (T1/T2 – 0.5)/12.5%
0g = 50% DUTY CYCLE
T2(s) = R
(⍀)/125M⍀
SET
Figure 12. Typical Output Duty Cycle
REV. B
–6–
ADXL202/ADXL210
+3.0V TO +5.25V
C
X
SELF TEST
X
V
FILT
V
DD
DD
ADXL202/
ADXL210
X SENSOR
R
FILT
32k⍀
X OUT
Y OUT
T2
DEMOD
DEMOD
C
DC
C
O
U
N
T
E
R
DUTY
CYCLE
MODULATOR
(DCM)
T1
P
OSCILLATOR
A(g) = (T1/T2 – 0.5)/12.5%
0g = 50% DUTY CYCLE
R
FILT
32k⍀
T2 = R
/125M⍀
SET
Y SENSOR
Y
T2
FILT
COM
R
SET
C
Y
Figure 13. Block Diagram
Setting the Bandwidth Using CX and CY
Table II. Reor Values to Set T2
The ADXL202/ADXL210 have provisions for bandlimiting the
XFILT and YFILT pins. Capacitors must be added at these pins to
implement low-pass filtering for antialiasing and noise reduc-
tion. The equation for the 3 dB bandwidth is:
2
RSET
1 ms
2
ms
10 ms
125 kΩ
250 kΩ
625 kΩ
1.25 MΩ
1
F–3 dB
=
2 π (32 kΩ) × C(x, y)
(
)
Nott the RSET should always be included, even if only an
analooutput is desired. Use an RSET value between 500 kΩ
and 2 MΩ when taking the output from XFILT or YFILT. The
RSET resistor should be place close to the T2 Pin to minimize
parasitic capacitance at this node.
5 µF
C(X,Y )
F–3 dB
=
or, more simply,
The tolerance of the internal resistor (RFILT), cn vary
as ±25% of its nominal value of 32 kΩ; so e bandwidth w
vary accordingly. A minimum capacitancf 100or C(X, Y)
is required in all cases.
Selecting the Right Accelerometer
For most tilt sensing applications the ADXL202 is the most
appropriate accelerometer. Its higher sensitivity (12.5%/g allows
the user to use a lower speed counter for PWM decoding while
maintaining high resolution. The ADXL210 should be used in
applications where accelerations of greater than ±2 g are expected.
Table I. Filter Capacition, CCY
itor
Bandwidth
MICROCOMPUTER INTERFACES
10 Hz
50 Hz
100 Hz
200 Hz
500 Hz
5 kHz
47 µF
0.10 µF
0.05 µF
0.027 µF
0.01 µF
0.001 µF
The ADXL202/ADXL210 were specifically designed to work
with low cost microcontrollers. Specific code sets, reference
designs, and application notes are available from the factory.
This section will outline a general design procedure and discuss
the various trade-offs that need to be considered.
The designer should have some idea of the required perfor-
mance of the system in terms of:
Setting the DCM Period with RSET
The period of the DCM output is set for both channels by a
single resistor from RSET to ground. The equation for the period
is:
Resolution: the smallest signal change that needs to be detected.
Bandwidth: the highest frequency that needs to be detected.
Acquisition Time: the time that will be available to acquire the
signal on each axis.
R
SET (Ω)
T2 =
125 MΩ
These requirements will help to determine the accelerometer
bandwidth, the speed of the microcontroller clock and the
length of the T2 period.
A 125 kΩ resistor will set the duty cycle repetition rate to ap-
proximately 1 kHz, or 1 ms. The device is designed to operate at
duty cycle periods between 0.5 ms and 10 ms.
When selecting a microcontroller it is helpful to have a counter
timer port available. The microcontroller should have provisions
for software calibration. While the ADXL202/ADXL210 are
highly accurate accelerometers, they have a wide tolerance for
REV. B
–7–
ADXL202/ADXL210
initial offset. The easiest way to null this offset is with a calibra-
tion factor saved on the microcontroller or by a user calibration
for zero g. In the case where the offset is calibrated during manu-
facture, there are several options, including external EEPROM
and microcontrollers with “one-time programmable” features.
Table IV gives typical noise output of the ADXL202/ADXL210
for various CX and CY values.
Table IV. Filter Capacitor Selection, CX and CY
Peak-to-Peak Noise
Estimate 95%
rms Noise Probability (rms
؋
4) DESIGN TRADE-OFFS FOR SELECTING FILTER
CHARACTERISTICS: THE NOISE/BW TRADE-OFF
The accelerometer bandwidth selected will determine the mea-
surement resolution (smallest detectable acceleration). Filtering
can be used to lower the noise floor and improve the resolution
of the accelerometer. Resolution is dependent on both the ana-
log filter bandwidth at XFILT and YFILT and on the speed of the
microcontroller counter.
Bandwidth CX, CY
10 Hz
50 Hz
100 Hz
200 Hz
500 Hz
0.47 µF
0.10 µF
0.05 µF
0.027 µF 8.7 mg
0.01 µF 13.7 mg
1.9 mg
4.3 mg
6.1 mg
7.6 mg
17.2 mg
24.4 mg
35.8 mg
54.8 mg
The analog output of the ADXL202/ADXL210 has a typical
bandwidth of 5 kHz, much higher than the duty cycle stage is
capable of converting. The user must filter the signal at this
point to limit aliasing errors. To minimize DCM errors the
analog bandwidth should be less than 1/10 the DCM frequency.
Analog bandwidth may be increased to up to 1/2 the DCM
frequency in many applications. This will result in greater dy-
namic error generated at the DCM.
CHOOSING T2 AND COUNTFRQUENCY: DESIGN
TRADE-OFFS
The noise level is one dminant of elemeter resolution.
The second relates to he asurement olution of the
counter when decoding the y cycle output.
The ADXL20ADXL210’s dutycle converter has a resolu-
tion of apprximatel14 bits; better resolution than the acceler-
ometer itsThe tual resolution of the acceleration signal is,
however, limiby the tie resolution of the counting devices
used to decode dutcycle. The faster the counter clock, the
igher the resolutiof the duty cycle and the shorter the T2
pod can be for a given resolution. The following table shows
somf the tre-offs. It is important to note that this is the
esolutdto the microprocessors’s counter. It is probable
the acelerometer’s noise floor may set the lower limit on
esolution, as discussed in the previous section.
The analog bandwidth may be further decreased to reduce noise
and improve resolution. The ADXL202/ADXL210 noise has
the characteristics of white Gaussian noise that contributes
equally at all frequencies and is described in terms of µg per root
Hz; i.e., the noise is proportional to the square root of the band-
width of the accelerometer. It is recommended that the user limi
bandwidth to the lowest frequency needed by the applicati
maximize the resolution and dynamic range of the accelerom
With the single pole roll-off characteristic, the typical nois
the ADXL202/ADXL210 is determined by the follequati
ble V. Trade-Offs Between Microcontroller Counter Rate,
T2 Period and Resolution of Duty Cycle Modulator
Noise rms = 500 µg/ Hz
×
B×
(
)
ADXL202/ Counter-
ADXL210 Clock
Counts
per T2 Counts Resolution
At 100 Hz the noise will be:
RSET Sample
Rate
T2 (ms) (k⍀) Rate
(MHz)
Cycle
per g
(mg)
Noise rms = 500 µg/ Hz
= 6.12 mg
(
)
1.0
124 1000
124 1000
124 1000
625 200
625 200
625 200
1250 100
1250 100
1250 100
2.0
1.0
0.5
2.0
1.0
0.5
2.0
1.0
0.5
2000
1000
500
10000
5000
2500
20000
10000
5000
250
125
4.0
8.0
16.0
0.8
1.6
3.2
0.4
0.8
1.6
1.0
1.0
Often the peak value of tis desik-to-peak noise
can only be estimated ethoTable III is useful
for estimating the proeding various peak values,
given the rms value.
62.5
1250
625
312.5
2500
1250
625
5.0
5.0
5.0
10.0
10.0
10.0
Table III. Estimatof Peak-to-Peak Noise
% of Time that Noise
Nominal Peak-to-Peak Will Exceed Nominal
Value
Peak-to-Peak Value
2.0 × rms
4.0 × rms
6.0 × rms
8.0 × rms
32%
4.6%
0.27%
0.006%
The peak-to-peak noise value will give the best estimate of the
uncertainty in a single measurement.
REV. B
–8–
ADXL202/ADXL210
STRATEGIES FOR USING THE DUTY CYCLE OUTPUT
WITH MICROCONTROLLERS
A DUAL AXIS TILT SENSOR: CONVERTING
ACCELERATION TO TILT
Application notes outlining various strategies for using the duty
cycle output with low cost microcontrollers are available from
the factory.
When the accelerometer is oriented so both its X and Y axes are
parallel to the earth’s surface it can be used as a two axis tilt
sensor with a roll and a pitch axis. Once the output signal from
the accelerometer has been converted to an acceleration that
varies between –1 g and +1 g, the output tilt in degrees is calcu-
lated as follows:
USING THE ADXL202/ADXL210 AS A DUAL AXIS TILT
SENSOR
One of the most popular applications of the ADXL202/ADXL210
is tilt measurement. An accelerometer uses the force of gravity
as an input vector to determine orientation of an object in space.
Pitch = ASIN (Ax/1 g)
Roll = ASIN (Ay/1 g)
Be sure to account for overranges. It is possible for the acceler-
ometers to output a signal greatehan ±1 g due to vibration,
shock or other accelerations.
An accelerometer is most sensitive to tilt when its sensitive axis
is perpendicular to the force of gravity, i.e., parallel to the
earth’s surface. At this orientation its sensitivity to changes in
tilt is highest. When the accelerometer is oriented on axis to
gravity, i.e., near its +1 g or –1 g reading, the change in output
acceleration per degree of tilt is negligible. When the accelerom-
eter is perpendicular to gravity, its output will change nearly
17.5 mg per degree of tilt, but at 45° degrees it is changing only
at 12.2 mg per degree and resolution declines. The following
table illustrates the changes in the X and Y axes as the device is
tilted ±90° through gravity.
MEASURING 360؇ OF TIL
It is possible to measua full ° of oritation through grav-
ity by using two acrometers otperpendicular to one
another (see Fige 1When one nsor is reading a maxi-
mum change in output degree, the other is at its minimum.
+90؇
Y
360؇ OF TILT
1g
Y
X
0؇
1g
X
–90؇
X OUTPUT
⌬ PER
DEGREE OF
Y OUTPU
Figure 15. Using a Two-Axis Accelerometer to Measure
360° of Tilt
X AXIS
ORIENTATION
DE
TO HORIZON (؇) X OUTPUT (g) TILT (mg)
Y UTPUT (g) TILT (mg
–90
–75
–60
–45
–30
–15
0
–1.000
–0.966
–0.866
–0.707
–0.500
–0.259
0.000
–0.2
4.4
0
0.259
0.500
0.707
6
6
.000
0.966
0.866
0.707
0.500
0.259
0.000
7.5
6.9
15.2
12.4
8.9
17
16.9
5.2
.4
8.9
4.7
–
–
–
–
–
–
–
0.2
15
–4.4
–8.6
–12.2
–15.0
–16.8
–17.5
30
45
60
75
0.
4.7
90
1.000
0.2
Figure 14. How the X and Y Axes Respond to Changes in
Tilt
REV. B
–9–
ADXL202/ADXL210
USING THE ANALOG OUTPUT
Power Cycling When Using the Digital Output
The ADXL202/ADXL210 was specifically designed for use with
its digital outputs, but has provisions to provide analog outputs
as well.
An alternative is to run the microcontroller at a higher clock
rate and put it into shutdown between readings, allowing the
use of the digital output. In this approach the ADXL202/
ADXL210 should be set at its fastest sample rate (T2 = 0.5 ms),
with a 500 Hz filter at XFILT and YFILT. The concept is to ac-
quire a reading as quickly as possible and then shut down the
ADXL202/ADXL210 and the microcontroller until the next
sample is needed.
Duty Cycle Filtering
An analog output can be reconstructed by filtering the duty
cycle output. This technique requires only passive components.
The duty cycle period (T2) should be set to 1 ms. An RC filter
with a 3 dB point at least a factor of 10 less than the duty cycle
frequency is connected to the duty cycle output. The filter resis-
tor should be no less than 100 kΩ to prevent loading of the
output stage. The analog output signal will be ratiometric to the
supply voltage. The advantage of this method is an output scale
factor of approximately double the analog output. Its disadvan-
tage is that the frequency response will be lower than when
using the XFILT, YFILT output.
In either of the above approaches, the ADXL202/ADXL210
can be turned on and off directly using a digital port pin on the
microcontroller to power the accelerometer without additional
components. The port should be ud to switch the common
pin of the accelerometer so the pt pin is pulling down.”
CALIBRATING THE ADL202/L210
The initial value of the ot and scale ctor the ADXL202/
ADXL210 will requicration for acations such as tilt
measurement. The ADXL2/ADXL210 architecture has been
designed so thahese calibrats take place in the software of
the microconoller used to decothe duty cycle signal. Cali-
bration fars can stored in EEPROM or determined at
turn-on anavin dynamic memory.
XFILT, YFILT Output
The second method is to use the analog output present at the
XFILT and YFILT pin. Unfortunately, these pins have a 32 kΩ
output impedance and are not designed to drive a load directly.
An op amp follower may be required to buffer this pin. The
advantage of this method is that the full 5 kHz bandwidth of the
accelerometer is available to the user. A capacitor still must be
added at this point for filtering. The duty cycle converter should
be kept running by using RSET <10 MΩ. Note that the acceler-
ometer offset and sensitivity are ratiometric to the supply volt-
age. The offset and sensitivity are nominally:
For low g appltionshe force of gravity is the most stable,
accurate and convnt acceleration reference available. A
ding of the 0 g point can be determined by orientating the
devparallel the earth’s surface and then reading the output.
0 g Offset = VDD/2
ADXL202 Sensitivity = (60 mV × VS)/g 300 mV/g at +5 V
ADXL210 Sensitivity = (20 mV × VS)/g 100 mV/g at +5
2.5 V at +5 V
more rate calibration method is to make a measurements
1 g and –1 g. The sensitivity can be determined by the two
urements.
calibrate, the accelerometer’s measurement axis is pointed
irectly at the earth. The 1 g reading is saved and the sensor is
turned 180° to measure –1 g. Using the two readings, the sensi-
tivity is:
USING THE ADXL202/ADXL210 IN VERY LOOWER
APPLICATIONS
An application note outlining low power strgiet
ADXL202/ADXL210 is available. Some key points are p
sented here. It is possible to reduce thXL202/ADXL0’s
average current from 0.6 mA to le0 µA by using the
following techniques:
Let A = Accelerometer output with axis oriented to +1 g
Let B = Accelerometer output with axis oriented to –1 g then:
Sensitivity = [A – B]/2 g
For example, if the +1 g reading (A) is 55% duty cycle and the
–1 g reading (B) is 32% duty cycle, then:
1. Power Cycle the accelerometer.
2. Run the accelerometewer VDown to 3 V).
Sensitivity = [55% – 32%]/2 g = 11.5%/g
Power Cycling with a
These equations apply whether the output is analog, or duty
cycle.
Depending on the valapacitor, the ADXL202/
ADXL210 is capable od giving a good reading in
1.6 ms. Most microcontrd A/Ds can acquire a reading
in another 25 µs. Thus it is possible to turn on the ADXL202/
ADXL210 and take a reading in <2 ms. If we assume that a
20 Hz sample rate is sufficient, the total current required to
take 20 samples is 2 ms × 20 samples/s × 0.6 mA = 24 µA aver-
age current. Running the part at 3 V will reduce the supply
current from 0.6 mA to 0.4 mA, bringing the average current
down to 16 µA.
Application notes outlining algorithms for calculating accelera-
tion from duty cycle and automated calibration routines are
available from the factory.
The A/D should read the analog output of the ADXL202/
ADXL210 at the XFILT and YFILT pins. A buffer amplifier is
recommended, and may be required in any case to amplify the
analog output to give enough resolution with an 8-bit to 10-bit
converter.
REV. B
–10–
ADXL202/ADXL210
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
14-Lead CERPAK
(QC-14)
0.390 (9.906)
MAX
14
1
8
0.291 (7.391)
0.285 (7.239)
0.419 (10.643)
0.394 (10.008)
7
PIN 1
0.300 (7.62)
0.195 (4.953)
0.345 (8.763
0.290 (7.3
0.115 (2.921)
0.020 (0.508)
0.004 (0.102)
0.215 (5.461)
0.119 (3.023)
8°
0.050
(1.27)
BSC
0.020 (0.508)
0.013 (0.330)
0.050 (1.270)
0.016 (0.406)
0.012.318)
0(0.229)
SEATING
PLANE
REV. B
–11–
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