AD ADXL210JE

a
FEATURES
2-Axis Acceleration Sensor on a Single IC Chip
5 mm 5 mm 2 mm Ultrasmall Chip Scale Package
2 mg Resolution at 60 Hz
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
APPLICATIONS
2-Axis Tilt Sensing with Faster Response than
Electrolytic, Mercury, or Thermal Sensors
Computer Peripherals
Information Appliances
Alarms and Motion Detectors
Disk Drives
Vehicle Security
GENERAL DESCRIPTION
The ADXL210E is a low-cost, low-power, complete 2-axis accelerometer 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., vibration) and static acceleration (e.g., gravity).
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 microprocessor 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).
Low-Cost 10 g Dual-Axis
Accelerometer with Duty Cycle
ADXL210E
FUNCTIONAL BLOCK DIAGRAM
3V TO 5.25V
CX
VDD
XFILT
RFILT
32k
X SENSOR
XOUT
DEMOD
CDC
OSCILLATOR
ANALOG
TO
DUTY
CYCLE
(ADC)
ADXL210E
DEMOD
Y SENSOR
COM
SELF-TEST
RFILT
32k
YOUT
YFILT
C
O
U
N P
T
E
R
T2
CY
RSET
T2
T1
A(g) = (T1/T2 – 0.5)/4%
0g = 50% DUTY CYCLE
T2 = RSET/125M
The typical noise floor is 200 ␮g√Hz, allowing signals below
2 mg (at 60 Hz bandwidth) to be resolved.
The bandwidth of the accelerometer is set with capacitors CX and
CY at the XFILT and YFILT pins. An analog output can be reconstructed by filtering the duty cycle output.
The ADXL210E is available in a 5 mm ⫻ 5 mm ⫻ 2 mm 8-lead
hermetic LCC package.
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, nor for any infringements of patents or other rights of third parties that
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
www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 2002
(TA = TMIN to TMAX, TA = 25C for J Grade only, VDD = 5 V, RSET = 125 k, Acceleration = 0 g,
ADXL210E–SPECIFICATIONS unless otherwise noted.)
Parameter
Conditions
SENSOR INPUT
Measurement Range1
Nonlinearity
Alignment Error2, 3
Alignment Error
Cross-Axis Sensitivity2, 4
Each Axis
±8
± 10
0.2
±1
0.01
±2
3.3
3.2
85
45
4.0
3.8
100
55
± 0.5
44
40
2.3
1.35
Delta from 25⬚C
50
50
2.5
1.5
1.0
2.0
@ 25⬚C
At Pins XFILT, YFILT
X Sensor to Y Sensor
Each Axis
T1/T2, VDD = 5 V
T1/T2, VDD = 3 V
VDD = 5 V
VDD = 3 V
Delta from 25⬚C
ZERO g BIAS LEVEL
0 g Duty Cycle2
0 g Duty Cycle2
0 g Voltage XFILT, YFILT2
0 g Voltage XFILT, YFILT2
0 g Duty Cycle vs. Supply2
0 g Offset vs. Temperature2, 5
Each Axis
T1/T2, VDD = 5 V
T1/T2, VDD = 3 V
VDD = 5 V
VDD = 3 V
FREQUENCY RESPONSE
3 dB Bandwidth
Sensor Resonant Frequency
FILTER
RFILT Tolerance
Minimum Capacitance
32 kΩ Nominal
At Pins XFILT, YFILT
SELF-TEST
Duty Cycle Change
Self-Test “0” to “1”
DUTY CYCLE OUTPUT STAGE
RSET = 125 kΩ
FSET
Output High Voltage
I = 25 µA
Output Low Voltage
I = 25 µA
T2 Drift vs. Temperature
Rise/Fall Time
POWER SUPPLY
Operating Voltage Range
Quiescent Supply Current
Turn-On Time
ADXL210JE
Typ
Max
Best Fit Straight Line
SENSITIVITY
Duty Cycle per g2
Duty Cycle per g2
Sensitivity XFILT, YFILT2
Sensitivity XFILT, YFILT2
Temperature Drift2, 5
NOISE PERFORMANCE
Noise Density2
Min
ADXL210AE
Typ
Max
± 10
0.2
±1
0.01
±2
4.9
4.4
125
65
3.2
3.0
80
40
4.0
3.8
100
55
± 0.5
5
4.6
130
70
%/g
%/g
mV/g
mV/g
%
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/⬚C
200
200
1000
µg√Hz rms
6
10
6
10
kHz
kHz
± 15
%
pF
3
%
± 15
g
% of FS
Degrees
Degrees
%
1000
3
0.7
VS – 200 mV
1.3
0.7
VS – 200 mV
1.3
200
200
50
200
CFILT in µF
50
200
5.25
0.6
1.0
160 ⫻ CFILT + 0.3
0
Unit
±8
1000
3
TEMPERATURE RANGE
Specified Performance AE
Operating Range
Min
70
3.0
5.25
0.6
1.0
160 ⫻ CFILT + 0.3
–40
–40
+85
+85
kHz
V
mV
ppm/⬚C
ns
V
mA
ms
⬚C
⬚C
NOTES
1
Guaranteed by measurement of initial offset and sensitivity.
2
See Typical Performance Characteristics.
3
Alignment error is specified as the angle between the true and indicated axis of sensitivity (see TPC 15).
4
Cross-axis sensitivity is the algebraic sum of the alignment and the inherent sensitivity errors.
5
Defined 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 . . . . . . . . . . . . . . . . –55⬚C to +125⬚C
Storage Temperature . . . . . . . . . . . . . . . . . . –65⬚C to +150⬚C
VDD
8
PACKAGE CHARACTERISTICS
Package
Weight
␪JA
␪JC
Device
8-Lead LCC
120°C/W
TBD°C/W
<1.0 grams
7
1
ST
YFILT
6
2
T2
XOUT
5
3
COM
4
YOUT
BOTTOM VIEW
*Stresses above those listed under Absolute Maximum Ratings may cause permanent 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.
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.
XFILT
PIN FUNCTION DESCRIPTIONS
Pin
No.
Mnemonic
Description
1
2
3
4
5
6
7
8
ST
T2
COM
YOUT
XOUT
YFILT
XFILT
VDD
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
ORDERING GUIDE
Model
No. of
Axes
Specified
Voltage
Temperature
Range
Package
Description
Package
Option
ADXL210JE
ADXL210AE*
2
2
3 V to 5 V
3 V to 5 V
0 to 70⬚C
–40⬚C to +85⬚C
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.
REV. 0
–3–
WARNING!
ESD SENSITIVE DEVICE
ADXL210E–Typical Performance Characteristics*
VDD = 5 V
35
35
30
30
25
25
PERCENT OF PARTS
PERCENT OF PARTS
VDD = 3 V
20
15
10
1.44
1.46
1.48
1.50 1.52
VOLTS
1.54
1.56
1.58
1.60
35
35
30
30
25
25
20
15
10
5
0
20
15
10
5
0
1.40 1.42 1.44 1.46 1.48 1.50 1.52 1.54 1.56 1.58 1.60
VOLTS
TPC 2. Y-Axis Zero g Bias Distribution at YFILT, VDD = 3 V
2.38 2.40 2.43 2.45 2.48 2.50 2.53 2.55 2.58 2.60 2.63
VOLTS
TPC 5. Y-Axis Zero g Bias Distribution at YFILT, VDD = 5 V
70
30
60
25
50
PERCENT OF PARTS
35
20
15
10
5
0
2.38 2.40 2.43 2.45 2.48 2.50 2.53 2.55 2.58 2.60 2.63
VOLTS
TPC 4. X-Axis Zero g Bias Distribution at XFILT, VDD = 5 V
PERCENT OF PARTS
PERCENT OF PARTS
10
0
1.42
TPC 1. X-Axis Zero g Bias Distribution at XFILT, VDD = 3 V
PERCENT OF PARTS
15
5
5
0
20
40
30
20
10
52.5
53.3
54.2
55.0
55.8 56.7
mV/g
57.5
58.3
59.2
0
60.0
TPC 3. X-Axis Sensitivity Distribution at XFILT, VDD = 3 V
97.5
100.0
103.0
105.0
mV/g
108.0
110.0
113.0
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
35
70
30
60
25
50
PERCENT OF PARTS
PERCENT OF PARTS
VDD = 3 V
20
15
10
54.2
53.3
55.0
55.8 56.7
mV/g
57.5
58.3
59.2
60.0
70
60
60
50
50
PERCENT OF PARTS
PERCENT OF PARTS
70
40
30
20
0
97.5
100.0
103.0 105.0
mV/g
108.0
110.0
113.0
40
30
20
10
3.5
3.6
3.7
3.8
3.9
4.0
PERCENT DUTY CYCLE PER g
0
4.1
TPC 8. X-Axis Sensitivity Distribution at XOUT, VDD = 3 V
3.9
4.0
4.1
4.3
4.2
PERCENT DUTY CYCLE PER g
4.4
TPC 11. X-Axis Sensitivity Distribution at XOUT, VDD = 5 V
70
60
60
50
50
PERCENT OF PARTS
70
40
30
20
10
0
95.0
TPC 10. Y-Axis Sensitivity Distribution at YFILT, VDD = 5 V
10
PERCENT OF PARTS
20
0
52.5
TPC 7. Y-Axis Sensitivity Distribution at YFILT, VDD = 3 V
40
30
20
10
3.4
3.5
3.7
3.8
3.9
3.6
PERCENT DUTY CYCLE PER g
4.0
0
4.1
TPC 9. Y-Axis Sensitivity Distribution at YOUT, VDD = 3 V
REV. 0
30
10
5
0
40
3.8
3.9
4.0
4.3
4.2
4.1
PERCENT DUTY CYCLE PER g
4.4
TPC 12. Y-Axis Sensitivity Distribution at YOUT, VDD = 5 V
–5–
ADXL210E
35
25
30
PERCENT OF PARTS
PERCENT OF PARTS
20
15
10
25
20
15
10
5
5
0
0
250
230
270
290 310 330 350 370
NOISE DENSITY – g Hz rms
390
410
TPC 13. Noise Density Distribution, VDD = 3 V
190
210
230
250
270
NOISE DENSITY – g Hz rms
290
310
TPC 16. Noise Density Distribution, VDD = 5 V
40
0.7
0.6
35
VS = 5 VDC
30
0.5
PERCENT OF PARTS
SUPPLY CURRENT – mA
170
150
VS = 3.5 VDC
0.4
0.3
0.2
25
20
15
10
0.1
5
0
–40
0
–20
20
40
60
80
0
100
–2
–3
–1
TEMPERATURE – C
TPC 14. Typical Supply Current vs. Temperature
0
PERCENT
1
2
3
TPC 17. Cross-Axis Sensitivity Distribution
20
18
VDD
3
14
CFILT = 0.01F
XOUT
2
VOLTS
12
10
8
1
6
4
0
0
1.375
1.125
0.875
0.625
0.375
0.125
–0.125
–0.375
–0.625
–0.875
0
–1.125
2
–1.375
PERCENT OF PARTS
16
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
20
18
18
16
16
14
14
8
mg/C
60
60
50
50
PERCENT OF PARTS
PERCENT OF PARTS
3.06
2.15
2.60
1.70
TPC 22. Y-Axis Zero g Drift Due to Temperature
Distribution, –40 °C to +85 °C
40
30
20
40
30
20
10
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/C
PERCENT/C
TPC 20. X-Axis Sensitivity Drift at XFILT Due to
Temperature Distribution, –40 °C to +85 °C
TPC 23. Y-Axis Sensitivity Drift at XFILT Due to
Temperature Distribution, –40 °C to +85 °C
2.57
2.60
2.55
2.58
2.53
2.56
2.51
2.54
VOLTS
VOLTS
0.79
mg/C
TPC 19. X-Axis Zero g Drift Due to Temperature
Distribution, –40 °C to +85 °C
2.49
2.52
2.47
2.50
2.45
2.48
2.43
2.46
2.41
–40 –30 –20 –10
0
10 20 30 40 50
TEMPERATURE – C
60
70
80
2.44
–40 –30 –20 –10
90
TPC 21. Typical X-Axis Zero g Output vs. Temperature for 16 Parts
REV. 0
1.24
0
0.34
0
–0.12
2
–1.02
4
2
–0.57
4
–1.93
6
–1.47
6
10
–2.83
8
–2.38
10
12
–3.29
12
–3.74
PERCENT OF PARTS
20
–5.34
–4.83
–4.32
–3.81
–3.30
–2.79
–2.28
–1.77
–1.26
–0.76
–0.25
0.26
0.77
1.28
1.79
2.30
2.81
3.32
3.83
4.34
PERCENT OF PARTS
ADXL210E
0
10 20 30 40 50
TEMPERATURE – C
60
70
80
90
TPC 24. Typical Y-Axis Zero g Output vs. Temperature for 16 Parts
–7–
ADXL210E
PERIOD NORMALIZED TO 1 @ 25C
1.06
1.04
1.02
1.00
0.98
0.96
0.94
–45
–30
–15
0
15
30
45
TEMPERATURE – C
60
90
75
TPC 25. Normalized DCM Period (T2) vs. Temperature
nominally 50% duty cycle. The acceleration signal can be determined by measuring the length of the T1 and T2 pulses with
a counter/timer or with a polling loop using a low cost microcontroller.
DEFINITIONS
T1
Length of the “on” portion of the cycle.
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 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 measurement system on a single monolithic IC. It contains a polysilicon
surface-micromachined sensor and signal conditioning circuitry
to implement an open loop acceleration measurement 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 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) = RSET()/125M
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 measured 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
VDD
100
VDD
CDC
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 bandwidth of the device by adding a capacitor. This filtering improves
measurement resolution and helps prevent aliasing.
XOUT
ADXL210E
COM
YOUT
ST
XFILT
XFILT
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
RSET
YFILT
YFILT
Figure 2.
–8–
REV. 0
ADXL210E
DESIGN PROCEDURE FOR THE ADXL210E
Setting the Bandwidth Using C X and CY
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.
The ADXL210E has provisions for bandlimiting the XFILT and
YFILT pins. Capacitors must be added at these pins to implement
low-pass filtering for antialiasing and noise reduction. The equation for the 3 dB bandwidth is:
F –3 dB =
Decoupling Capacitor C DC
A 0.1 µF capacitor is recommended from VDD to COM for power
supply decoupling.
F –3 dB =
5 µF
C(X ,Y )
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
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:
0 g = 50% Duty Cycle
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 temperature (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.
)
or, more simply,
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 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.
(
1
2 π (32 kΩ) × C(x, y)
Bandwidth
Capacitor
Value
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
Setting the DCM Period with R SET
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:
T2 =
RSET (Ω)
125 MΩ
3V TO 5.25V
CX
VDD
XFILT
RFILT
32k
X SENSOR
XOUT
DEMOD
CDC
OSCILLATOR
DEMOD
Y SENSOR
COM
ANALOG
TO
DUTY
CYCLE
(ADC)
ADXL210E
A 125 kΩ resistor will set the duty cycle repetition rate to approximately 1 kHz, or 1 ms. The device is designed to operate at duty
cycle periods between 0.5 ms and 10 ms.
SELF-TEST
RFILT
32k
YOUT
YFILT
C
O
U
N P
T
E
R
Table II. Resistor Values to Set T2
T2
CY
RSET
T2
T1
A(g) = (T1/T2 – 0.5)/4%
0g = 50% DUTY CYCLE
T2 = RSET/125M
Figure 3. Block Diagram
T2
RSET
1 ms
2 ms
5 ms
10 ms
125 kΩ
250 kΩ
625 kΩ
1.25 MΩ
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.
Selecting the Right Accelerometer
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
The ADXL210E is 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.
With the single pole roll-off characteristic, the typical noise of
the ADXL210E is determined by the following equation:
(
) (
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
on each axis.
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.
These requirements will help to determine the accelerometer bandwidth, the speed of the microcontroller clock and the length of
the T2 period.
Table III. Estimation of Peak-to-Peak 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 microcontrollers with “one-time programmable” features.
Nominal Peak-to-Peak
Value
% of Time that Noise
Will Exceed Nominal
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
CX and CY values.
DESIGN TRADE-OFFS FOR SELECTING FILTER
CHARACTERISTICS: THE NOISE/BW TRADE-OFF
The accelerometer bandwidth selected will determine the measurement 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 microcontroller counter.
Table IV. Filter Capacitor Selection, CX and CY
Bandwidth
CX, CY
rms Noise
Peak-to-Peak Noise
Estimate 95%
Probability (rms 4)
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
0.027 µF
0.01 µF
0.8 mg
1.8 mg
2.5 mg
3.6 mg
5.7 mg
3.2 mg
7.2 mg
10.1 mg
14.3 mg
22.6 mg
The analog bandwidth may be further decreased to reduce noise
and improve resolution. The ADXL210E 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 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 noise level is one determinant of accelerometer resolution.
The second relates to the measurement resolution of the counter
when decoding the duty cycle output.
CHOOSING T2 AND COUNTER FREQUENCY: DESIGN
TRADE-OFFS
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, however, 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 resolution, 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
CounterADXL210E Clock
Counts
RSET Sample
Rate
per T2 Counts Resolution
T2 (ms) (k) 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
124
124
625
625
625
1250
1250
1250
1000
1000
1000
200
200
200
100
100
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
2.50
5.00
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 frequency 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
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
X FILT 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 accelerometer offset and sensitivity are ratiometric to the supply voltage.
The offset and sensitivity are nominally:
0 g Offset = VDD/2
ADXL210E Sensitivity = (20 mV ⫻ VS)/g
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:
2 ms ⫻ 20 Samples/s ⫻ 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.
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 microcontroller until the next sample is needed.
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 calibrations 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.
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 sensitivity is:
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:
1. Power cycle the accelerometer.
2. Run the accelerometer at a lower voltage (down to 3 V).
REV. 0
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.
Application notes outlining algorithms for calculating acceleration from duty cycle and automated calibration routines are
available from the factory.
–11–
ADXL210E
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
0.197 (5.00)
SQ
0.177
(4.50)
SQ
0.070 (1.78)
0.050 (1.27)
0.015
7
1
5
3
0.050 (1.27)
C02778–0–2/02(0)
8-Terminal Ceramic Leadless Chip Carrier
(E-8)
(0.38)
0.075
(1.91)
0.099
(2.50)
0.025
(0.64)
0.099
(2.50)
TOP VIEW
0.050 (1.27)
R0.008
0.015 (0.38)
(0.20)
0.008
BOTTOM VIEW
(0.20)
CONTROLLING DIMENSIONS ARE IN MILLIMETERS
PRINTED IN U.S.A.
R0.028 (0.70)
–12–
REV. 0