ADXL210AE [ADI]
Low-Cost 10 g Dual-Axis Accelerometer with Duty Cycle; 低成本10克双轴加速度计与占空比
| 型号： | ADXL210AE |
| 厂家： | 
ADI |
| 描述： | Low-Cost 10 g Dual-Axis Accelerometer with Duty Cycle |
| 文件： | 总12页 (文件大小：177K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Low-Cost ꢀ10 g Dual-Axis
Accelerometer with Duty Cycle
a
ADXL210E
FEATURES
FUNCTIONAL BLOCK DIAGRAM
2-Axis Acceleration Sensor on a Single IC Chip
5 mm ꢃ 5 mm ꢃ 2 mm Ultrasmall Chip Scale Package
2 mg Resolution at 60 Hz
3V TO 5.25V
C
X
V
X
FILT
SELF-TEST
DD
Low Power < 0.6 mA
Direct Interface to Low-Cost Microcontrollers via
Duty Cycle Output
BW Adjustment with a Single Capacitor
3 V to 5.25 V Single-Supply Operation
1000 g Shock Survival
R
32kꢂ
X SENSOR
FILT
X
OUT
C
O
U
N
T
E
R
DEMOD
ANALOG
TO
DUTY
CYCLE
(ADC)
C
DC
OSCILLATOR
ꢁP
ADXL210E
DEMOD
R
32kꢂ
Y
FILT
OUT
APPLICATIONS
Y SENSOR
COM
Y
T2
R
FILT
2-Axis Tilt Sensing with Faster Response than
Electrolytic, Mercury, or Thermal Sensors
Computer Peripherals
C
Y
SET
Information Appliances
Alarms and Motion Detectors
Disk Drives
T2
T1
A(g) = (T1/T2 – 0.5)/4%
0g = 50% DUTY CYCLE
Vehicle Security
T2 = R
/125Mꢂ
SET
GENERAL DESCRIPTION
The typical noise floor is 200 g√Hz, allowing signals below
2 mg (at 60 Hz bandwidth) to be resolved.
The ADXL210E is a low-cost, low-power, complete 2-axis acceler-
ometer with a digital output, all on a single monolithic IC. It is an
improved version of the ADXL210AQC/JQC. The ADXL210E
will measure accelerations with a full-scale range of 10 g. The
ADXL210E can measure both dynamic acceleration (e.g., vibra-
tion) and static acceleration (e.g., gravity).
The bandwidth of the accelerometer is set with capacitors CX and
CY at the XFILT and YFILT pins. An analog output can be recon-
structed by filtering the duty cycle output.
The ADXL210E is available in a 5 mm ϫ 5 mm ϫ 2 mm 8-lead
hermetic LCC package.
The outputs are analog voltage or digital signals whose duty cycles
(ratio of pulsewidth to period) are proportional to acceleration.
The duty cycle outputs can be directly measured by a micro-
processor counter without an A/D converter or glue logic. The
duty cycle period is adjustable from 0.5 ms to 10 ms via a single
resistor (RSET).
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, norforanyinfringementsofpatentsorotherrightsofthirdpartiesthat
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
Fax: 781/326-8703
www.analog.com
© Analog Devices, Inc., 2002
MIN to TMAX, TA = 25ꢄC for J Grade only, VDD = 5 V, RSET = 125 kꢂ, Acceleration = 0 g,
ADXL210E–SPECIFICATIONS (uTnle=ssT otherwise noted.)
A
ADXL210JE
ADXL210AE
Typ
Parameter
Conditions
Min
Typ
Max
Min
Max
Unit
SENSOR INPUT
Measurement Range1
Nonlinearity
Each Axis
8
10
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, 3
Alignment Error
Cross-Axis Sensitivity2, 4
SENSITIVITY
Duty Cycle per g2
Duty Cycle per g2
Each Axis
T1/T2, VDD = 5 V
T1/T2, VDD = 3 V
VDD = 5 V
VDD = 3 V
Delta from 25ЊC
3.3
3.2
85
4.0
3.8
100
55
4.9
4.4
125
65
3.2
3.0
80
4.0
3.8
100
55
5
%/g
%/g
mV/g
mV/g
%
4.6
130
70
2
Sensitivity XFILT, YFILT
2
Sensitivity XFILT, YFILT
45
40
Temperature Drift2, 5
0.5
0.5
ZERO g BIAS LEVEL
0 g Duty Cycle2
Each Axis
T1/T2, VDD = 5 V
T1/T2, VDD = 3 V
VDD = 5 V
44
40
2.3
1.35
50
50
2.5
1.5
1.0
2.0
56
60
2.7
1.65
4.0
42
38
2.3
1.3
50
50
2.5
1.5
1.0
2.0
58
62
2.7
1.7
4.0
%
%
V
V
%/V
mg/ЊC
0 g Duty Cycle2
2
0 g Voltage XFILT, YFILT
2
0 g Voltage XFILT, YFILT
VDD = 3 V
0 g Duty Cycle vs. Supply2
0 g Offset vs. Temperature2, 5 Delta from 25ЊC
NOISE PERFORMANCE
Noise Density2
@ 25ЊC
200
200
1000
µg√Hz rms
FREQUENCY RESPONSE
3 dB Bandwidth
Sensor Resonant Frequency
At Pins XFILT, YFILT
6
10
6
10
kHz
kHz
FILTER
RFILT Tolerance
Minimum Capacitance
32 kΩ Nominal
15
3
15
3
%
pF
At Pins XFILT, YFILT
1000
1000
0.7
SELF-TEST
Duty Cycle Change
Self-Test “0” to “1”
%
DUTY CYCLE OUTPUT STAGE
FSET
Output High Voltage
Output Low Voltage
T2 Drift vs. Temperature
Rise/Fall Time
RSET = 125 kΩ
I = 25 µA
I = 25 µA
0.7
VS – 200 mV
1.3
1.3
kHz
V
mV
ppm/ЊC
ns
VS – 200 mV
200
200
50
200
50
200
POWER SUPPLY
Operating Voltage Range
Quiescent Supply Current
Turn-On Time
3
5.25
1.0
3.0
5.25
1.0
V
mA
ms
0.6
0.6
CFILT in µF
160 ϫ CFILT + 0.3
160 ϫ CFILT + 0.3
TEMPERATURE RANGE
Specified Performance AE
Operating Range
–40
–40
+85
+85
ЊC
ЊC
0
70
NOTES
1Guaranteed by measurement of initial offset and sensitivity.
2See Typical Performance Characteristics.
3Alignment error is specified as the angle between the true and indicated axis of sensitivity (see TPC 15).
4Cross-axis sensitivity is the algebraic sum of the alignment and the inherent sensitivity errors.
5Defined as the output change from ambient to maximum temperature or ambient to minimum temperature.
Specifications subject to change without notice.
–2–
REV. 0
ADXL210E
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 +6.0 V
Output Short Circuit Duration, (Any Pin to Common)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Indefinite
Operating Temperature . . . . . . . . . . . . . . . . –55ЊC to +125ЊC
Storage Temperature . . . . . . . . . . . . . . . . . . –65ЊC to +150ЊC
V
DD
8
1
2
3
7
6
5
ST
X
Y
FILT
T2
FILT
OUT
X
COM
4
Y
OUT
BOTTOM VIEW
*Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent damage to the device. This is a stress rating only; 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.
PIN FUNCTION DESCRIPTIONS
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
No.
Mnemonic
Description
1
2
3
4
5
6
7
8
ST
T2
Self-Test
Connect RSET to Set T2 Period
Common
Y-Channel Duty Cycle Output
X-Channel Duty Cycle Output
Y-Channel Filter Pin
X-Channel Filter Pin
3 V to 5.25 V
PACKAGE CHARACTERISTICS
Package
COM
YOUT
XOUT
YFILT
XFILT
VDD
Weight
JA
JC
Device
8-Lead LCC
120°C/W
TBD°C/W
<1.0 grams
ORDERING GUIDE
Temperature
No. of
Axes
Specified
Voltage
Package
Description
Package
Option
Model
Range
ADXL210JE
ADXL210AE*
2
2
3 V to 5 V
3 V to 5 V
0 to 70ЊC
–40ЊC to +85ЊC
8-Lead LCC
8-Lead LCC
E-8
E-8
*Available Soon
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 ADXL210E 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. 0
–3–
*
ADXL210E–Typical Performance Characteristics
VDD = 3 V
VDD = 5 V
35
30
25
20
15
10
5
35
30
25
20
15
10
5
0
0
1.42 1.44 1.46 1.48 1.50 1.52 1.54 1.56 1.58 1.60
VOLTS
2.38 2.40 2.43 2.45 2.48 2.50 2.53 2.55 2.58 2.60 2.63
VOLTS
TPC 1. X-Axis Zero g Bias Distribution at XFILT, VDD = 3 V
TPC 4. X-Axis Zero g Bias Distribution at XFILT, VDD = 5 V
35
30
25
20
15
10
5
35
30
25
20
15
10
5
0
0
1.40 1.42 1.44 1.46 1.48 1.50 1.52 1.54 1.56 1.58 1.60
2.38 2.40 2.43 2.45 2.48 2.50 2.53 2.55 2.58 2.60 2.63
VOLTS
VOLTS
TPC 2. Y-Axis Zero g Bias Distribution at YFILT, VDD = 3 V
TPC 5. Y-Axis Zero g Bias Distribution at YFILT, VDD = 5 V
35
30
25
20
15
10
5
70
60
50
40
30
20
10
0
0
52.5 53.3 54.2 55.0 55.8 56.7 57.5 58.3 59.2 60.0
97.5
100.0
103.0
105.0
108.0
110.0
113.0
mV/g
mV/g
TPC 3. X-Axis Sensitivity Distribution at XFILT, VDD = 3 V
TPC 6. X-Axis Sensitivity Distribution at XFILT, VDD = 5 V
*Data taken from 14,500 parts over 3 lots minimum.
–4–
REV. 0
ADXL210E
VDD = 3 V
VDD = 3 V
70
60
50
40
30
20
10
0
35
30
25
20
15
10
5
0
95.0
97.5
100.0 103.0 105.0 108.0 110.0 113.0
52.5 53.3 54.2 55.0 55.8 56.7 57.5 58.3 59.2 60.0
mV/g
mV/g
TPC 7. Y-Axis Sensitivity Distribution at YFILT, VDD = 3 V
TPC 10. Y-Axis Sensitivity Distribution at YFILT, VDD = 5 V
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
3.5
3.6
3.7
3.8
3.9
4.0
4.1
3.9
4.0
4.1
4.2
4.3
4.4
PERCENT DUTY CYCLE PER g
PERCENT DUTY CYCLE PER g
TPC 8. X-Axis Sensitivity Distribution at XOUT, VDD = 3 V
TPC 11. X-Axis Sensitivity Distribution at XOUT, VDD = 5 V
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
3.4
3.5
3.6
3.7
3.8
3.9
4.0
4.1
3.8
3.9
4.0
4.1
4.2
4.3
4.4
PERCENT DUTY CYCLE PER g
PERCENT DUTY CYCLE PER g
TPC 9. Y-Axis Sensitivity Distribution at YOUT, VDD = 3 V
TPC 12. Y-Axis Sensitivity Distribution at YOUT, VDD = 5 V
REV. 0
–5–
ADXL210E
35
30
25
20
15
10
5
25
20
15
10
5
0
0
230 250 270 290 310 330 350 370 390 410
150
170
190
210
230
250
270
290
310
NOISE DENSITY – ꢁg Hz rms
NOISE DENSITY – ꢁg Hz rms
TPC 13. Noise Density Distribution, VDD = 3 V
TPC 16. Noise Density Distribution, VDD = 5 V
40
35
30
25
20
15
10
5
0.7
0.6
V
V
= 5 VDC
S
0.5
0.4
0.3
0.2
0.1
0
= 3.5 VDC
S
0
–40
–20
0
20
40
60
80
100
–3
–2
–1
0
1
2
3
PERCENT
TEMPERATURE – ꢄC
TPC 14. Typical Supply Current vs. Temperature
TPC 17. Cross-Axis Sensitivity Distribution
20
18
16
14
12
10
8
V
DD
3
2
1
0
C
= 0.01ꢁF
X
FILT
OUT
6
4
2
0
0
0.4
0.8
1.2
1.4
TIME – ms
DEGREES OF MISALIGNMENT
TPC 15. Rotational Die Alignment
TPC 18. Typical Turn-On Time
–6–
REV. 0
ADXL210E
20
18
16
14
12
10
8
20
18
16
14
12
10
8
6
6
4
4
2
2
0
0
mg/ꢄC
mg/ꢄC
TPC 19. X-Axis Zero g Drift Due to Temperature
TPC 22. Y-Axis Zero g Drift Due to Temperature
Distribution, –40°C to +85°C
Distribution, –40°C to +85°C
60
50
40
30
20
10
60
50
40
30
20
10
0
0
–0.0292 –0.0245 –0.0198 –0.0152 –0.0105 –0.0058 –0.0012
–0.0156 –0.0123 –0.0090 –0.0056 –0.0023 0.0010 0.0043 0.0077
PERCENT/ꢄC
PERCENT/ꢄC
TPC 20. X-Axis Sensitivity Drift at XFILT Due to
TPC 23. Y-Axis Sensitivity Drift at XFILT Due to
Temperature Distribution, –40°C to +85°C
Temperature Distribution, –40°C to +85°C
2.60
2.58
2.56
2.54
2.52
2.50
2.48
2.46
2.44
2.57
2.55
2.53
2.51
2.49
2.47
2.45
2.43
2.41
–40 –30 –20 –10
0
10 20 30 40 50 60 70 80 90
–40 –30 –20 –10
0
10 20 30 40 50 60 70 80 90
TEMPERATURE – ꢄC
TEMPERATURE – ꢄC
TPC 21. Typical X-Axis Zero g Output vs. Tempera-
ture for 16 Parts
TPC 24. Typical Y-Axis Zero g Output vs. Tempera-
ture for 16 Parts
REV. 0
–7–
ADXL210E
1.06
1.04
1.02
1.00
0.98
0.96
0.94
–45
–30
–15
0
15
30
45
60
75
90
TEMPERATURE – ꢄC
TPC 25. Normalized DCM Period (T2) vs. Temperature
DEFINITIONS
nominally 50% duty cycle. The acceleration signal can be deter-
T1
T2
Length of the “on” portion of the cycle.
Length of the total cycle.
mined by measuring the length of the T1 and T2 pulses with
a counter/timer or with a polling loop using a low cost micro-
controller.
Duty Cycle Ratio of the “on” time (T1) of the cycle to the total
cycle (T2). Defined as T1/T2 for the ADXL210E/
ADXL210.
Pulsewidth Time period of the “on” pulse. Defined as T1 for
the ADXL210E/ADXL210.
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.
The ADXL210E will operate with supply voltages as low as 3.0 V
or as high as 5.25 V.
THEORY OF OPERATION
T2
The ADXL210E is a complete, dual-axis acceleration measure-
ment system on a single monolithic IC. It contains a polysilicon
surface-micromachined sensor and signal conditioning circuitry
to implement an open loop acceleration measurement architec-
ture. 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 microprocessor. The ADXL210E
is capable of measuring both positive and negative accelerations
to 10 g. The accelerometer can measure static acceleration
forces such as gravity, allowing it to be used as a tilt sensor.
T1
A(g) = (T1/T2 – 0.5)/4%
0g = 50% DUTY CYCLE
T2(s) = R
(ꢂ)/125Mꢂ
SET
Figure 1. Typical Output Duty Cycle
APPLICATIONS
POWER SUPPLY DECOUPLING
For most applications a single 0.1 µF capacitor, CDC, will
adequately 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, digital noise on the supply may cause interference on
the ADXL210E output. This may be observed as a slowly
undulating fluctuation of voltage at XFILT and YFILT. If additional
decoupling is needed, a 100 Ω (or smaller) resistor or ferrite
beads, may be inserted in the supply line of the ADXL210E.
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. An
acceleration will deflect the beam and unbalance the differential
capacitor, resulting in an output square wave whose amplitude
is proportional to acceleration. Phase sensitive demodulation
techniques are then used to rectify the signal and determine the
direction of the acceleration.
FERRITE BEAD
100ꢂ
V
V
X
OUT
DD
DD
C
DC
ADXL210E
The output of the demodulator drives a duty cycle modulator
(DCM) stage through a 32 kΩ resistor. At this point a pin is
available on each channel to allow the user to set the signal band-
width of the device by adding a capacitor. This filtering improves
measurement resolution and helps prevent aliasing.
COM
ST
Y
X
Y
OUT
FILT
FILT
X
FILT
T2
After being low-pass filtered, the analog signal is converted to a
duty cycle modulated signal by the DCM stage. A single resistor
sets the period for a complete cycle (T2), which can be set between
0.5 ms and 10 ms (see TPC 12). A 0 g acceleration produces a
R
SET
Y
FILT
Figure 2.
–8–
REV. 0
ADXL210E
DESIGN PROCEDURE FOR THE ADXL210E
Setting the Bandwidth Using CX and CY
The ADXL210E has provisions for bandlimiting the XFILT and
FILT pins. Capacitors must be added at these pins to implement
low-pass filtering for antialiasing and noise reduction. The equa-
tion for the 3 dB bandwidth is:
The design procedure for using the ADXL210E with a duty cycle
output involves selecting a duty cycle period and a filter capacitor.
A proper design will take into account the application requirements
for bandwidth, signal resolution and acquisition time, as discussed
in the following sections.
Y
1
F–3 dB
=
Decoupling Capacitor CDC
A 0.1 µF capacitor is recommended from VDD to COM for power
supply decoupling.
2 π (32 kΩ)× C(x, y)
(
)
or, more simply,
ST
5µF
C(X,Y )
F–3dB
=
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 accelerometer.
The resulting movement of the beam allows the user to test if the
accelerometer is functional. The typical change in output will be 3%
at the duty cycle outputs (corresponding to 800 mg). This pin
may be left open circuit or connected to common in normal use.
The tolerance of the internal resistor (RFILT), can vary typically as
much as 15% of its nominal value of 32 kΩ; so the bandwidth
will vary accordingly. A minimum capacitance of 1000 pF for
C(X, Y) is required in all cases.
Duty Cycle Decoding
Table I. Filter Capacitor Selection, CX and CY
Capacitor
The ADXL210E’s digital output is a duty cycle modulator.
Acceleration is proportional to the ratio T1/T2. The nominal
output of the ADXL210E is:
Bandwidth
Value
0 g = 50% Duty Cycle
10 Hz
50 Hz
100 Hz
200 Hz
500 Hz
5 kHz
0.47 µF
0.10 µF
0.05 µF
0.027 µF
0.01 µF
0.001 µF
Scale factor is 4% Duty Cycle Change per g
These nominal values are affected by the initial tolerance of the
device including zero g offset error and sensitivity error.
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 measure it on
one channel of the ADXL210E. Decoding algorithms for various
microcontrollers have been developed. Consult the appropriate
Application Note.
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:
R
SET (Ω)
125 MΩ
T2 =
3V TO 5.25V
C
X
A 125 kΩ resistor will set the duty cycle repetition rate to approxi-
mately 1 kHz, or 1 ms. The device is designed to operate at duty
cycle periods between 0.5 ms and 10 ms.
V
X
FILT
SELF-TEST
X
DD
R
X SENSOR
FILT
32kꢂ
OUT
C
O
U
N
T
E
R
DEMOD
ANALOG
TO
DUTY
CYCLE
(ADC)
Table II. Resistor Values to Set T2
C
DC
OSCILLATOR
ꢁP
ADXL210E
T2
RSET
DEMOD
R
1 ms
2 ms
5 ms
10 ms
125 kΩ
250 kΩ
625 kΩ
1.25 MΩ
Y
FILT
32kꢂ
OUT
Y SENSOR
COM
Y
T2
R
FILT
C
Y
SET
Note that the RSET should always be included, even if only an
analog output 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 placed close to the T2 Pin to minimize parasitic
capacitance at this node.
T2
T1
A(g) = (T1/T2 – 0.5)/4%
0g = 50% DUTY CYCLE
T2 = R
/125Mꢂ
SET
Selecting the Right Accelerometer
Figure 3. Block Diagram
For most tilt sensing applications the ADXL202E 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 ADXL210E should be used in
applications where accelerations of greater than 2 g are expected.
REV. 0
–9–
ADXL210E
MICROCOMPUTER INTERFACES
With the single pole roll-off characteristic, the typical noise of
the ADXL210E is determined by the following equation:
The ADXL210E is specifically designed to work with low-cost
microcontrollers. Specific code sets, reference designs, and applica-
tion 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.
Noise rms = 200 µg/ Hz × BW ×1.6
(
)
(
)
(
)
At 100 Hz the noise will be:
The designer should have some idea of the required performance
of the system in terms of:
Noise rms = 200 µg/ Hz
×
100 × 1.6 = 2.53 mg
(
)
(
)
(
)
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
Often the peak value of the noise is desired. Peak-to-peak noise
can only be estimated by statistical methods. Table III is useful
for estimating the probabilities of exceeding various peak values,
given the rms value.
on each axis.
These requirements will help to determine the accelerometer band-
width, the speed of the microcontroller clock and the length of
the T2 period.
Table III. Estimation of Peak-to-Peak Noise
% of Time that Noise
When selecting a microcontroller it is helpful to have a counter
timer port available. The microcontroller should have provisions
for software calibration. While the ADXL210E is a highly accurate
accelerometer, it has a wide tolerance for initial offset. The
easiest way to null this offset is with a calibration factor saved on
the microcontroller or by a user calibration for zero g. In the
case where the offset is calibrated during manufacture, there are
several options, including external EEPROM and microcontrol-
lers with “one-time programmable” features.
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.
Table IV gives typical noise output of the ADXL210E for various
DESIGN TRADE-OFFS FOR SELECTING FILTER
CHARACTERISTICS: THE NOISE/BW TRADE-OFF
The accelerometer bandwidth selected will determine the measure-
ment 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 analog filter
bandwidth at XFILT and YFILT and on the speed of the micro-
controller counter.
C
X and CY values.
Table IV. Filter Capacitor Selection, CX and CY
Peak-to-Peak Noise
Estimate 95%
rms Noise Probability (rms ꢃ 4)
Bandwidth CX, CY
The analog output of the ADXL210E has a typical bandwidth
of 5 kHz, while the duty cycle modulators’ bandwidth is 500 Hz.
The user must filter the signal at this point to limit aliasing
errors. To minimize DCM errors the analog bandwidth should be
less than one-tenth the DCM frequency. Analog bandwidth
may be increased to up to half the DCM frequency in many
applications. This will result in greater dynamic error generated
at the DCM.
10 Hz
50 Hz
100 Hz
200 Hz
500 Hz
0.47 µF
0.10 µF
0.05 µF
0.8 mg
1.8 mg
2.5 mg
3.2 mg
7.2 mg
10.1 mg
14.3 mg
22.6 mg
0.027 µF 3.6 mg
0.01 µF 5.7 mg
CHOOSING T2 AND COUNTER FREQUENCY: DESIGN
TRADE-OFFS
The noise level is one determinant of accelerometer resolution.
The second relates to the measurement resolution of the counter
when decoding the duty cycle output.
The analog bandwidth may be further decreased to reduce noise
and improve resolution. The ADXL210E noise has the character-
istics 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 bandwidth of the
accelerometer. It is recommended that the user limit bandwidth to
the lowest frequency needed by the application to maximize the
resolution and dynamic range of the accelerometer.
The ADXL210E’s duty cycle converter has a resolution of
approximately 14 bits; better resolution than the accelerometer
itself. The actual resolution of the acceleration signal is, how-
ever, limited by the time resolution of the counting devices used
to decode the duty cycle. The faster the counter clock, the higher
the resolution of the duty cycle and the shorter the T2 period
can be for a given resolution. The following table shows some of
the trade-offs. It is important to note that this is the resolution
due to the microprocessors’ counter. It is probable that the
accelerometer’s noise floor may set the lower limit on the resolu-
tion, as discussed in the previous section.
–10–
REV. 0
ADXL210E
Table V. Trade-Offs Between Microcontroller Counter Rate,
T2 Period, and Resolution of Duty Cycle Modulator
Power Cycling with an External A/D
Depending on the value of the XFILT capacitor, the ADXL210E
is capable of turning on and giving a good reading in 1.6 ms.
Most microcontroller-based A/Ds can acquire a reading in
another 25 µs. Thus it is possible to turn on the ADXL210E
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:
Counter-
ADXL210E Clock
Counts
RSET Sample
Rate
per T2 Counts Resolution
T2 (ms) (kꢂ) Rate
(MHz)
Cycle
per g
(mg)
1.0
1.0
1.0
5.0
5.0
5.0
10.0
10.0
10.0
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
80
40
20
400
200
100
800
400
200
12.50
25.00
50.00
2.50
5.00
10.00
1.25
2 ms ϫ 20 Samples/s ϫ 0.6 mA = 24 µA
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.
The A/D should read the analog output of the ADXL210E 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.
2.50
5.00
Power Cycling When Using the Digital Output
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 ADXL210E 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 acquire a reading as quickly as
possible and then shut down the ADXL210E and the microcon-
troller until the next sample is needed.
USING THE ANALOG OUTPUT
The ADXL210E was specifically designed for use with its digital
outputs, but has provisions to provide analog outputs as well.
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 fre-
quency is connected to the duty cycle output. The filter resistor
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 disadvantage is that
In either of the above approaches, the ADXL210E can be turned
on and off directly using a digital port pin on the microcontroller to
power the accelerometer without additional components.
CALIBRATING THE ADXL210E
The initial value of the offset and scale factor for the ADXL210E will
require calibration for applications such as tilt measurement. The
ADXL210E architecture has been designed so that these calibra-
tions take place in the software of the microcontroller used to decode
the duty cycle signal. Calibration factors can be stored in EEPROM
or determined at turn-on and saved in dynamic memory.
the frequency response will be lower than when using the XFILT
,
YFILT output.
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 voltage.
The offset and sensitivity are nominally:
For low g applications, the force of gravity is the most stable,
accurate and convenient acceleration reference available. A reading
of the 0 g point can be determined by orientating the device
parallel to the earth’s surface and then reading the output.
A more accurate calibration method is to make measurements at
+1 g and –1 g. The sensitivity can be determined by the two
measurements.
To calibrate, the accelerometer’s measurement axis is pointed
directly 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:
0 g Offset = VDD/2
ADXL210E Sensitivity = (20 mV ϫ VS)/g
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
USING THE ADXL210E IN VERY LOW POWER
APPLICATIONS
An application note outlining low power strategies for the
ADXL210E is available. Some key points are presented here.
It is possible to reduce the ADXL210E’s average current from
0.6 mA to less than 20 µA by using the following techniques:
For example, if the +1 g reading (A) is 55% duty cycle and the
–1 g reading (B) is 47% duty cycle, then:
Sensitivity = [55% – 47%]/2 g = 4%/g
These equations apply whether the output is analog or duty cycle.
1. Power cycle the accelerometer.
Application notes outlining algorithms for calculating accelera-
tion from duty cycle and automated calibration routines are
available from the factory.
2. Run the accelerometer at a lower voltage (down to 3 V).
REV. 0
–11–
ADXL210E
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
8-Terminal Ceramic Leadless Chip Carrier
(E-8)
0.050 (1.27)
7
0.070 (1.78)
0.197 (5.00)
SQ
ꢅ
0.015 (0.38)
1
0.075
0.099
(2.50)
(1.91)
0.177
(4.50)
SQ
0.050 (1.27)
TOP VIEW
0.050 (1.27)
0.025
0.099
(2.50)
(0.64)
3
5
R0.008
(0.20)
ꢅ
0.015 (0.38)
R0.028 (0.70)
0.008
(0.20)
BOTTOM VIEW
CONTROLLING DIMENSIONS ARE IN MILLIMETERS
–12–
REV. 0
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