ADXL250 [ADI]
+-5 g to +-50 g, Low Noise, Low Power, Single/Dual Axis iMEMS Accelerometers; + -5克至±50克，低噪声，低功耗，单/双轴加速度计的iMEMS
| 型号： | ADXL250 |
| 厂家： | 
ADI |
| 描述： | +-5 g to +-50 g, Low Noise, Low Power, Single/Dual Axis iMEMS Accelerometers |
| 文件： | 总15页 (文件大小：344K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
؎5 g to ؎50 g, Low Noise, Low Power,
Single/Dual Axis iMEMS Accelerometers
a
ADXL150/ADXL250
FUNCTIONAL BLOCK DIAGRAMS
FEATURES
Complete Acceleration Measurement System
on a Single Monolithic IC
TP
(DO NOT CONNECT)
80 dB Dynamic Range
+V
2
S
+V
S
ADXL150
Pin Programmable ؎50 g or ؎25 g Full Scale
Low Noise: 1 mg/√Hz Typical
Low Power: <2 mA per Axis
Supply Voltages as Low as 4 V
2-Pole Filter On-Chip
5k⍀
0.1F
GAIN
AMP
V
OUT
SENSOR
DEMODULATOR
25k⍀
BUFFER
AMP
CLOCK
9
Ratiometric Operation
COM
SELF-TEST
Complete Mechanical & Electrical Self-Test
Dual & Single Axis Versions Available
Surface Mount Package
OFFSET
NULL
X OFFSET
NULL
TP
(DO NOT CONNECT)
BUFFER
AMP
ADXL250
25k⍀
+V
S
GAIN
AMP
V
V
X
Y
OUT
0.1F
DEMODULATOR
SENSOR
GENERAL DESCRIPTION
5k⍀
+V
2
S
The ADXL150 and ADXL250 are third generation ±50 g sur-
face micromachined accelerometers. These improved replace-
ments for the ADXL50 offer lower noise, wider dynamic range,
reduced power consumption and improved zero g bias drift.
CLOCK
5k⍀
GAIN
AMP
OUT
SENSOR
DEMODULATOR
25k⍀
BUFFER
AMP
The ADXL150 is a single axis product; the ADXL250 is a fully
integrated dual axis accelerometer with signal conditioning on a
single monolithic IC, the first of its kind available on the com-
mercial market. The two sensitive axes of the ADXL250 are
orthogonal (90°) to each other. Both devices have their sensitive
axes in the same plane as the silicon chip.
COM
Y OFFSET
NULL
SELF-TEST
ratiometric to the power supply, eliminating the need for a volt-
age reference when driving ratiometric A/D converters such as
those found in most microprocessors. A power supply bypass
capacitor is the only external component needed for normal
operation.
The ADXL150/ADXL250 offer lower noise and improved
signal-to-noise ratio over the ADXL50. Typical S/N is 80 dB,
allowing resolution of signals as low as 10 mg, yet still providing
a ±50 g full-scale range. Device scale factor can be increased
from 38 mV/g to 76 mV/g by connecting a jumper between
VOUT and the offset null pin. Zero g drift has been reduced to
0.4 g over the industrial temperature range, a 10× improvement
over the ADXL50. Power consumption is a modest 1.8 mA
per axis. The scale factor and zero g output level are both
The ADXL150/ADXL250 are available in a hermetic 14-lead
surface mount cerpac package specified over the 0°C to +70°C
commercial and –40°C to +85°C industrial temperature ranges.
Contact factory for availability of devices specified over automo-
tive and military temperature ranges.
i
MEMS is a registered trademark of Analog Devices, Inc.
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
which 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
World Wide Web Site: http://www.analog.com
© Analog Devices, Inc., 1998
(TA = +25؇C for J Grade, TA = –40؇C to +85؇C for A Grade,
ADXL150/ADXL250–SPECIFICATIONS VS = +5.00 V, Acceleration = Zero g, unless otherwise noted)
ADXL150JQC/AQC
ADXL250JQC/AQC
Parameter
Conditions
Min
Typ Max
Min
Typ Max
Units
SENSOR
Guaranteed Full-Scale Range
Nonlinearity
±40
±50
0.2
±1
±40
±50
0.2
±1
±0.1
±2
g
% of FS
Degrees
Degrees
%
Package Alignment Error1
Sensor-to-Sensor Alignment Error
Transverse Sensitivity2
±2
SENSITIVITY
Sensitivity (Ratiometric)3
Y Channel
X Channel
33.0
33.0
38.0 43.0
38.0 43.0
±0.5
mV/g
mV/g
%
33.0
38.0 43.0
±0.5
Sensitivity Drift Due to Temperature Delta from 25°C to TMIN or TMAX
ZERO g BIAS LEVEL
Output Bias Voltage4
VS/2 – 0.35 VS/2 VS/2 + 0.35 VS/2 – 0.35 VS/2 VS/2 + 0.35 V
Zero g Drift Due to Temperature
Delta from 25°C to TMIN or TMAX
0.2
0.3
g
ZERO-g OFFSET ADJUSTMENT
Voltage Gain
Input Impedance
Delta VOUT/Delta VOS PIN
0.45
20
0.50 0.55
30
0.45
20
0.50 0.55
30
V/V
kΩ
NOISE PERFORMANCE
Noise Density5
Clock Noise
1
5
2.5
1
5
2.5
mg/√Hz
mV p-p
FREQUENCY RESPONSE
–3 dB Bandwidth
Bandwidth Temperature Drift
Sensor Resonant Frequency
900
1000
50
24
900
1000
50
24
Hz
Hz
kHz
T
Q = 5
MIN to TMAX
SELF-TEST
Output Change6
Logic “1” Voltage
Logic “0” Voltage
Input Resistance
ST Pin from Logic “0” to “1”
0.25
VS – 1
0.40 0.60
0.25
VS – 1
0.40 0.60
V
V
V
kΩ
1.0
50
1.0
50
To Common
30
30
OUTPUT AMPLIFIER
Output Voltage Swing
Capacitive Load Drive
I
OUT = ±100 µA
0.25
1000
VS – 0.25 0.25
VS – 0.25
V
pF
1000
POWER SUPPLY (VS)7
Functional Voltage Range
Quiescent Supply Current
4.0
6.0
3.0
4.0
6.0
V
mA
mA
ADXL150
ADXL250 (Total 2 Channels)
1.8
3.5 5.0
TEMPERATURE RANGE
Operating Range J
Specified Performance A
0
–40
+70
+85
0
–40
+70
+85
°C
°C
NOTES
1Alignment error is specified as the angle between the true axis of sensitivity and the edge of the package.
2Transverse sensitivity is measured with an applied acceleration that is 90 degrees from the indicated axis of sensitivity.
3Ratiometric: VOUT = VS/2 + (Sensitivity × VS/5 V × a) where a = applied acceleration in gs, and VS = supply voltage. See Figure 21. Output scale factor can be
doubled by connecting VOUT to the offset null pin.
4Ratiometric, proportional to VS/2. See Figure 21.
5See Figure 11 and Device Bandwidth vs. Resolution section.
6Self-test output varies with supply voltage.
7When using ADXL250, both Pins 13 and 14 must be connected to the supply for the device to function.
Specifications subject to change without notice.
–2–
REV. 0
ADXL150/ADXL250
ABSOLUTE MAXIMUM RATINGS*
Package Characteristics
Acceleration (Any Axis, Unpowered for 0.5 ms) . . . . . . 2000 g
Acceleration (Any Axis, Powered for 0.5 ms) . . . . . . . . . 500 g
+VS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7.0 V
Output Short Circuit Duration
Package
Device Weight
5 Grams
JA
JC
14-Lead Cerpac
110°C/W
30°C/W
(VOUT, VREF Terminals to Common) . . . . . . . . . . . Indefinite
Operating Temperature . . . . . . . . . . . . . . . . . –55°C to +125°C
Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C
ORDERING GUIDE
Temperature Range
Model
*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.
ADXL150JQC
0°C to +70°C
–40°C to +85°C
0°C to +70°C
–40°C to +85°C
ADXL150AQC
ADXL250JQC
ADXL250AQC
PIN CONNECTIONS
Drops onto hard surfaces can cause shocks of greater than 2000 g
and exceed the absolute maximum rating of the device. Care
should be exercised in handling to avoid damage.
1
14
V
NC
NC
S
NC
NC
NC
ADXL150
TOP VIEW
NC
NC
TP (DO NOT CONNECT)
(Not to Scale)
V
OUT
NC
SELF-TEST
ZERO g ADJ
1 ADXL25014
1
ADXL15014
COMMON
7
8
TOP VIEW
TOP VIEW
(Not to Scale)
(Not to Scale)
A
X
A
X
1
14
NC
V
S
90؇
A
Y
V
ZERO g ADJ Y
S
7
8
7
8
V
Y
ADXL250
TOP VIEW
NC
NC
V
OUT
NC
(Not to Scale)
POSITIVE A = POSITIVE V
POSITIVE A = POSITIVE V
OUT
TP (DO NOT CONNECT)
X
OUT
OUT
SELF-TEST
ZERO g ADJ X
NC
COMMON
7
8
Figure 1. ADXL150 and ADXL250 Sensitive Axis
Orientation
NC = NO CONNECT
NOTE: WHEN USING ADXL250, BOTH PINS 13 AND 14 NEED
TO BE CONNECTED TO SUPPLY FOR DEVICE TO FUNCTION
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 ADXL150/ADXL250 feature 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
–3–
REV. 0
ADXL150/ADXL250
GLOSSARY OF TERMS
Acceleration: Change in velocity per unit time.
Zero g Bias Level: The output voltage of the ADXL150/
ADXL250 when there is no acceleration (or gravity) acting
upon the axis of sensitivity. The output offset is the difference
between the actual zero g bias level and (VS/2).
Acceleration Vector: Vector describing the net acceleration
acting upon the ADXL150/ADXL250.
Polarity of the Acceleration Output
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.
The polarity of the ADXL150/ADXL250 output is shown in
Figure 1. When its sensitive axis is oriented to the earth’s gravity
(and held in place), it will experience an acceleration of +1 g.
This corresponds to a change of approximately +38 mV at the
output pin. Note that the polarity will be reversed if the package
is rotated 180°. The figure shows the ADXL250 oriented so that
its “X” axis measures +1 g. If the package is rotated 90° clock-
wise (Pin 14 up, Pin 1 down), the ADXL250’s “Y” axis will now
measure +1 g.
Nonlinearity: The maximum deviation of the ADXL150/
ADXL250 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 (at 50 g).
Resonant Frequency: The natural frequency of vibration of
the ADXL150/ADXL250 sensor’s central plate (or “beam”). At
its resonant frequency of 24 kHz, the ADXL150/ADXL250’s
moving center plate has a slight peak in its frequency response.
A
8
7
8
7
Y
Sensitivity: The output voltage change per g unit of accelera-
tion applied, specified at the VOUT pin in mV/g.
A
A
X
X
Total Alignment Error: Net misalignment of the ADXL150/
ADXL250’s on-chip sensor and the measurement axis of the
application. This error includes errors due to sensor die align-
ment to the package, and any misalignment due to installation
of the sensor package in a circuit board or module.
14 ADXL150
14 ADXL250
1
1
Transverse Acceleration: Any acceleration applied 90° to the
axis of sensitivity.
Figure 2. Output Polarity
Acceleration Vectors
The ADXL150/ADXL250 is a sensor designed to measure
accelerations that result from an applied force. It responds to
the component of acceleration on its sensitive X axis (ADXL150)
or on both the “X” and “Y” axis (ADXL250).
Transverse Sensitivity Error: The percent of a transverse
acceleration that appears at VOUT
.
Transverse Axis: The axis perpendicular (90°) to the axis of
sensitivity.
REV. 0
–4–
ADXL150/ADXL250
(@+5 V dc, +25؇C with a 38 mV/g Scale Factor unless otherwise noted)
Typical Characteristics
6
5.0
4.0
3.0
2.0
1.0
0
–6
PACKAGE
RESONANCE
–12
–18
0
–1.0
–2.0
–24
–30
–36
–42
BEAM
RESONANCE
–3.0
–4.0
–48
–5.0
4.0
4.5
5.0
5.5
6.0
100
1k
FREQUENCY – Hz
10k
POWER SUPPLY VOLTAGE
Figure 3. Typical Sensitivity Error from Ideal Ratiometric
Response for a Number of Units
Figure 6. Typical Output Response vs. Frequency of
ADXL150/ADXL250 on a PC Board that Has Been
Conformally Coated
30
2.5
2.0
1.5
1.0
20
10
0.5
0
0
g
–0.5
–1.0
–10
–20
–30
–1.5
–2.0
4.0
4.5
5.0
5.5
6.0
–40 –30 –20 –10
0
10 20 30 40 50 60 70 80 90 100
TEMPERATURE – ؇C
SUPPLY VOLTAGE
Figure 4. Offset Error of Zero g Level from Ideal
Figure 7. Typical Zero g Drift for a Number of Units
VS/2 Response as a Percent of Full-Scale for a Number
of Units
2.4
2.2
600g
500g
400g
300g
200g
100g
0g
60g
50g
40g
30g
20g
10g
0g
+105؇C
2
500g INPUT
+25؇C
1.8
–40؇C
OUTPUT RESPONSE
1.6
1.4
1.2
4
4.5
5
5.5
6
TIME – 0.2ms/Div
SUPPLY VOLTAGE – Volts
Figure 5. Typical Supply Current vs. Supply Voltage
Figure 8. Typical 500 g Step Recovery at the Output
–5–
REV. 0
ADXL150/ADXL250
1.6
1.4
1.2
20
15
10
5
g
0
1.0
–5
g
NOISE FROM INTERNAL CLOCK
–10
–15
–20
0.8
0.6
4.0
4.5
5.0
5.5
6.0
0
2
4
6
8
10
12
14
16
18
20
SUPPLY VOLTAGE – Volts
TIME – s
Figure 9. Typical Output Noise Voltage with Spikes
Generated by Internal Clock
Figure 12. Noise vs. Supply Voltage
30
25
20
15
10
5
SELF-TEST
OUTPUT
(0.2V/DIV)
SELF-TEST
INPUT
(2V/DIV)
0
100
1000
10000
0
2
4
6
8
10
12
14
16
18
20
TIME – ms
FREQUENCY – kHz
Figure 10. Typical Self-Test Response
Figure 13. Baseband Error Graph
Figure 13 shows the mV rms error in the output signal if there is
a noise on the power supply pin of 1 mV rms at the internal
clock frequency or its odd harmonics. This is a baseband noise
and can be at any frequency in the 1 kHz passband or at dc.
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
g
0.50
0.25
10
100
1k
2k
FREQUENCY – Hz
Figure 11. Noise Spectral Density
REV. 0
–6–
ADXL150/ADXL250
THEORY OF OPERATION
MEASURING ACCELERATIONS LESS THAN 50 g
The ADXL150 and ADXL250 are fabricated using a propri-
etary surface micromachining process that has been in high
volume production since 1993. The fabrication technique uses
standard integrated circuit manufacturing methods enabling all
the signal processing circuitry to be combined on the same chip
with the sensor.
The ADXL150/ADXL250 require only a power supply bypass
capacitor to measure ±50 g accelerations. For measuring ±50 g
accelerations, the accelerometer may be directly connected to an
ADC (see Figure 25). The device may also be easily modified to
measure lower g signals by increasing its output scale factor.
The scale factor of an accelerometer specifies the voltage change
of the output per g of applied acceleration. This should not be
confused with its resolution. The resolution of the device is the
lowest g level the accelerometer is capable of measuring. Resolu-
tion is principally determined by the device noise and the mea-
surement bandwidth.
The surface micromachined sensor element is made by deposit-
ing polysilicon on a sacrificial oxide layer that is then etched
away leaving the suspended sensor element. Figure 14 is a
simplified view of the sensor structure. The actual sensor has
42 unit cells for sensing acceleration. The differential capacitor
sensor is composed of fixed plates and moving plates attached to
the beam that moves in response to acceleration. Movement of
the beam changes the differential capacitance, which is measured
by the on chip circuitry.
The zero 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.
Pin Programmable Scale Factor Option
The sensor has 12-unit capacitance cells for electrostatically
forcing the beam during a self-test. Self-test is activated by the
user with a logic high on the self-test input pin. During a logic
high, an electrostatic force acts on the beam equivalent to
approximately 20% of full-scale acceleration input, and thus a
proportional voltage change appears on the output pin. When
activated, the self-test feature exercises both the entire mechani-
cal structure and the electrical circuitry.
In its normal state, the ADXL150/ADXL250’s buffer amplifier
provides an output scale factor of 38 mV/g, which is set by an
internal voltage divider. This gives a full-scale range of ±50 g
and a nominal bandwidth of 1 kHz.
A factor-of-two increase in sensitivity can be obtained by con-
necting the VOUT pin to the offset null pin, assuming that it is
not needed for offset adjustment. This connection has the effect
of reducing the internal feedback by a factor of two, doubling
the buffer’s gain. This increases the output scale factor to 76 mV/g
and provides a ±25 g full-scale range.
Simultaneously, connecting these two pins also increases the
amount of internal post filtering, reducing the noise floor and
changing the nominal 3 dB bandwidth of the ADXL150/
ADXL250 to 500 Hz. Note that the post filter’s “Q” will also
be reduced by a factor of √2 from 0.58 (Bessel response) to a
much gentler “Q” value of 0.41. The primary effect of this
change in “Q” is only at frequencies within two octaves of the
corner frequency; above this the two filter slopes are essentially
the same. In applications where a flat response up to 500 Hz is
needed, it is better to operate the device at 38 mV/g and use an
external post filter. Note also that connecting VOUT to the offset
pin adds a 30 kΩ load from VOUT to VS/2. When swinging ±2 V
at VOUT, this added load will consume ±60 µA of the ADXL150/
ADXL250’s 100 µA (typical) output current drive.
BEAM
PLATE
CAPACITANCES
UNIT CELL
FIXED
PLATE
ANCHOR
Figure 14. Simplified View of Sensor Under Acceleration
All the circuitry needed to drive the sensor and convert the
capacitance change to voltage is incorporated on the chip requiring
no external components except for standard power supply decou-
pling. Both sensitivity and the zero-g value are ratiometric to
the supply voltage, so that ratiometeric devices following the
accelerometer (such as an ADC, etc.) will track the accelerom-
eter if the supply voltage changes. The output voltage (VOUT) is
a function of both the acceleration input (a) and the power
supply voltage (VS) as follows:
VS
5V
VOUT = VS/2 – (Sensitivity ×
× a)
Both the ADXL150 and ADXL250 have a 2-pole Bessel switched-
capacitor filter. Bessel filters, sometimes called linear phase
filters, have a step response with minimal overshoot and a maxi-
mally flat group delay. The –3 dB frequency of the poles is
preset at the factory to 1 kHz. These filters are also completely
self-contained and buffered, requiring no external components.
–7–
REV. 0
ADXL150/ADXL250
Increasing the
Scale Factor
Figure 15 shows the basic connections for using an external
buffer amplifier to increase the output scale factor.
i
MEM
S
Accelerometer’s Output
For the highest possible accuracy, an external trim is recom-
mended. As shown by Figure 20, this consists of a potentiom-
eter, R1a, in series with a fixed resistor, R1b. Another option is
to select resistor values after measuring the device’s scale factor
(see Figure 17).
The output multiplied by 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 ampli-
fies the signal, but any noise or drift as well. Too much gain can
also cause the buffer to saturate and clip the output waveform.
AC Coupling
If a dc (gravity) response is not required—for example in vibra-
tion measurement applications—ac coupling can be used be-
tween the accelerometer’s output and the external op amp’s
input as shown in Figure 16. The use of ac coupling virtually
eliminates any zero g drift and allows the maximum external
amp gain without clipping.
Note that the “+” input of the external op amp uses the offset
null pin of the ADXL150/ADXL250 as a reference, biasing the
op amp at midsupply, saving two resistors and reducing power
consumption. The offset null pin connects to the VS/2 reference
point inside the accelerometer via 30 kΩ, so it is important not
to load this pin with more than a few microamps.
Resistor R2 and capacitor C3 together form a high pass filter
whose corner frequency is 1/(2 π R2 C3). This filter will reduce
the signal from the accelerometer by 3 dB at the corner fre-
quency, and it will continue to reduce it at a rate of 6 dB/octave
(20 dB per decade) for signals below the corner frequency.
Capacitor C3 should be a nonpolarized, low leakage type.
It is important to use a single-supply or “rail-to-rail” op amp for
the external buffer as it needs to be able to swing close to the
supply and ground.
If ac coupling is used, the self-test feature must be monitored at
the accelerometer’s output rather than at the external amplifier
output (since the self-test output is a dc voltage).
The circuit of Figure 15 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.
TP
(DO NOT CONNECT)
5
+V
2
S
+V
S
14
5k⍀
ADXL150
C1
0.1F
GAIN
AMP
R3
R1
10
SENSOR
DEMODULATOR
25k⍀
BUFFER
AMP
CLOCK
2
3
8
9
7
COM
OFFSET
NULL
+V
2
V
OUT
S
OP196
6
SELF-TEST
7
4
C2
0.1F
C4
0.1F
R3
+V
OUTPUT SCALE FACTOR = 38mV/g ––
S
R1
Figure 15. Using an External Op Amp to Increase Output Scale Factor
TP
(DO NOT CONNECT)
1M⍀
5
+V
2
S
+V
7
S
C4
+V
14
S
ADXL150
5k⍀
0.1F
C1
0.1F
GAIN
AMP
C3
R2
2
10
SENSOR
DEMODULATOR
V
OUT
OP196
6
OUTPUT
BUFFER
AMP
25k⍀
3
CLOCK
9
4
8
7
OFFSET
NULL
COM
+V
S
2
+V
S
SELF-TEST
2
1M ⍀
EXTERNAL AMP GAIN = ––––
R2
C2
0.1F
TYPICAL COMPONENT VALUES FOR AC COUPLED CIRCUIT
C3 VALUE FOR 3dB CORNER FREQ
BUFFER
GAIN
R2
FS RANGE
1Hz
3Hz
10Hz
20Hz
2
4
5
؎25g
1M⍀
0.15F
0.47F
0.68F
0.05F 0.015F 0.0075F
0.15F 0.047F 0.022F
0.22F 0.022F 0.01F
؎12.5g
؎10g
332k⍀
249k⍀
Figure 16. AC Coupled Connection Using an External Op Amp
REV. 0
–8–
ADXL150/ADXL250
R2 (SEE NOTES)
TP
+V OR GND
S
(DO NOT CONNECT)
5
+V
2
S
+V
14
S
ADXL150
5k⍀
C1
0.1F
R3
GAIN
AMP
100k⍀
R1
10
SENSOR
DEMODULATOR
25k⍀
C4
0.1F
+V
7
S
BUFFER
AMP
CLOCK
9
2
8
7
COM
OFFSET
NULL
+V
2
S
OP196
6
V
SELF-TEST
OUT
3
4
C2
0.1F
NOTES:
DESIRED
OUTPUT
SCALE FACTOR
0g “QUICK” CALIBRATION METHOD USING RESISTOR R2 AND A +5V SUPPLY.
(a) WITH ACCELEROMETER ORIENTED AWAY FROM EARTH’S
GRAVITY (i.e., SIDEWAYS), MEASURE PIN 10 OF THE ADXL150.
(b) CALCULATE THE OFFSET VOLTAGE THAT NEEDS TO BE NULLED:
EXT
AMP
GAIN
R1
VALUE
FS
RANGE
76mV/g
؎25g
؎20g
؎10g
؎5g
2.0
2.6
5.3
10.5
49.9k⍀
38.3k⍀
18.7k⍀
9.53k⍀
V
OS
=(+2.5V – V 10)(R3/R1).
PIN
2.5V (R3)
100mV/g
200mV/g
400mV/g
(c) R2 = ––––––––
V
OS
(d) FOR V
(e) FOR V
10 > +2.5V, R2 CONNECTS TO GND.
10 < +2.5V, R2 CONNECTS TO +V .
S
PIN
PIN
Figure 17. “Quick Zero g Calibration” Connection
Adjusting the Zero g Bias Level
The device scale factor and zero g offset levels can be calibrated
using the earth’s gravity, as explained in the section “calibrating
the ADXL150/ADXL250.”
When a true dc (gravity) response is needed, the output from
the accelerometer must be dc coupled to the external amplifier’s
input. For high gain applications, a zero g offset trim will also be
needed. The external offset trim permits the user to set the zero g
offset voltage to exactly +2.5 volts (allowing the maximum output
swing from the external amplifier without clipping with a +5
supply).
Using the Zero g “Quick-Cal” Method
In Figure 18 (accelerometer alone, no external op amp), a trim
potentiometer connects directly to the accelerometer’s zero g
null pin. The “quick offset calibration” scheme shown in Figure
17 is preferred over using a potentiometer, which could change
its setting over time due to vibration. The “quick offset calibra-
tion” method requires measuring only the output voltage of
the ADXL150/ADXL250 while it is oriented normal to the
earth’s gravity. Then, by using the simple equations shown in
the figures, the correct resistance value for R2 can be calculated.
In Figure 17, an external op amp is used to amplify the signal. A
resistor, R2, is connected to the op amp’s summing junction.
The other side of R2 connects to either ground or +VS depend-
ing on which direction the offset needs to be shifted.
With a dc coupled connection, any difference between the zero g
output and +2.5 V will be amplified along with the signal. To
obtain the exact zero g output desired or to allow the maximum
output voltage swing from the external amplifier, the zero g offset
will need to be externally trimmed using the circuit of Figure 20.
The external amplifier’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 zero g level, the maxi-
mum gain will equal:
2Volts
38
mV/g Times the Max Applied Acceleration in g
TP
(DO NOT CONNECT)
5
+V
2
S
+V
14
S
ADXL150
5k⍀
C1
0.1F
GAIN
AMP
V
OUT
10
DEMODULATOR
25k⍀
SENSOR
BUFFER
AMP
CLOCK
9
8
7
COM
OFFSET
NULL
SELF-TEST
C2
0.1F
R
AT PIN 8
IN
+V
30k⍀ ؎10k⍀
S
200k⍀
Figure 18. Offset Nulling the ADXL150/ADXL250 Using a Trim Potentiometer
–9–
REV. 0
ADXL150/ADXL250
DEVICE BANDWIDTH VS. MEASUREMENT
RESOLUTION
Although an accelerometer is usually specified according to its
full-scale g level, the limiting resolution of the device, i.e., its
minimum discernible input level, is extremely important when
measuring low g accelerations.
approximately 1.6 times the 3 dB bandwidth. For example, the
typical rms noise of the ADXL150 using a 100 Hz one pole post
filter is:
Noise rms =1mg/ Hz × 100 1.6 =12.25mg
(
)
(
)
Because the ADXL150/ADXL250’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. Table I is useful for esti-
mating the probabilities of exceeding various peak values, given
the rms value.
100mg
660mg
66mg
10mg
Table I.
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
6.6 × rms
8.0 × rms
32%
4.6%
0.27%
0.1%
0.006%
6.6mg
1mg
10
100
1k
3dB BANDWIDTH – Hz
Figure 19. ADXL150/ADXL250 Noise Level vs. 3 dB
Bandwidth (Using a “Brickwall” Filter)
RMS and peak-to-peak noise (for 0.1% uncertainty) for various
bandwidths are estimated in Figure 19. 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 ADXL150/ADXL250 typically have an rms noise level of
32 mg. When the device bandwidth is rolled off to 100 Hz, the
noise level is reduced to approximately 10 mg.
The limiting resolution is predominantly set by the measure-
ment noise “floor,” which includes the ambient background
noise and the noise of the ADXL150/ADXL250 itself. The level
of the noise floor varies directly with the bandwidth of the mea-
surement. As the measurement bandwidth is reduced, the noise
floor drops, improving the signal-to-noise ratio of the measure-
ment and increasing its resolution.
Alternatively, the signal-to-noise ratio may be improved consid-
erably by using a microprocessor to perform multiple measure-
ments and then to compute the average signal level.
The bandwidth of the accelerometer can be easily reduced by
adding low-pass or bandpass filtering. Figure 19 shows the
typical noise vs. bandwidth characteristic of the ADXL150/
ADXL250.
Low-Pass Filtering
The bandwidth of the accelerometer can easily be reduced by using
post filtering. Figure 20 shows how the buffer amplifier can be
connected to provide 1-pole post filtering, zero g offset trimming,
and output scaling. The table provides practical component values
The output noise of the ADXL150/ADXL250 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
+V
S
R2
RT
200k⍀
0g TRIM
1M⍀
TP
(DO NOT CONNECT)
5
Cf
+V
2
S
+V
14
S
ADXL150
5k⍀
R1a
75k⍀ 50k⍀
R1b
C1
0.1F
R3
100k⍀
GAIN
AMP
10
SENSOR
DEMODULATOR
25k⍀
SCALE
FACTOR
TRIM
+V
7
S
BUFFER
AMP
0.1F
CLOCK
COM
(OPTIONAL)
2
3
8
9
7
+V
2
OFFSET
NULL
S
6
OP196
V
OUT
SELF-TEST
4
0.1F
DESIRED
OUTPUT
SCALE FACTOR
EXT
AMP
GAIN
R3
VALUE
F.S.
RANGE
Cf (F) Cf (F) Cf (F)
100Hz 30Hz 10Hz
76mV/g
؎25g
؎20g
؎10g
؎5g
2.0
2.6
5.3
10.5
200k⍀ 0.0082 0.027 0.082
261k⍀ 0.0056 0.022 0.056
536k⍀ 0.0033 0.010 0.033
100mV/g
200mV/g
400mV/g
1M⍀
0.0015 0.0056 0.015
Figure 20. One-Pole Post Filter Circuit with SF and Zero g Offset Trims
–10–
REV. 0
ADXL150/ADXL250
Figure 22 shows how both the zero g offset and output sensitiv-
ity of the ADXL150/ADXL250 vary with changes in supply
voltage. If they are to be used with nonratiometric devices, such
as an ADC with a built-in 5 V reference, then both components
should be referenced to the same source, in this case the ADC
reference. Alternatively, the circuit can be powered from an
external +5 volt reference.
for various full-scale g levels and approximate circuit band-
widths. For bandwidths other than those listed, use the
formula:
1
Cf =
2π R3 Desired 3dB Bandwidth in Hz
(
)
or simply scale the value of capacitor Cf accordingly; i.e., for an
application with a 50 Hz bandwidth, the value of Cf 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, a 2- or 3-pole post filter is recommended. These provide
a much steeper roll-off of noise above the pole frequency. Fig-
ure 21 shows a circuit that provides 2-pole post filtering. Com-
ponent values for the 2-pole filter were selected to operate the
first op amp at unity gain. Capacitors C3 and C4 were chosen
to provide 3 dB bandwidths of 10 Hz, 30 Hz, 100 Hz and
300 Hz.
2.65
2.60
2.55
2.50
2.45
2.40
40.25
39.50
38.75
38.00
37.25
36.50
35.75
g
The second 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.
2.35
5.25 5.20 5.15 5.10 5.05 5.00 4.95 4.90 4.85 4.80 4.75
POWER SUPPLY VOLTAGE
APPLICATION HINTS
ADXL250 Power Supply Pins
When wiring the ADXL250, be sure to connect BOTH power
supply terminals, Pins 14 and 13.
Figure 22. Typical Ratiometric Operation
Since any voltage variation is transferred to the accelerometer’s
output, it is important to reduce any power supply noise. Simply
following good engineering practice of bypassing the power supply
right at Pin 14 of the ADXL150/ADXL250 with a 0.1 µF ca-
pacitor should be sufficient.
Ratiometric Operation
Ratiometric operation means that the circuit uses the power
supply as its voltage reference. If the supply voltage varies, the
accelerometer and the other circuit components (such as an
ADC, etc.) track each other and compensate for the change.
TP
R3
82.5k⍀
(DO NOT CONNECT)
5
+V
2
S
C4
2
+V
8
S
+V
14
S
5k⍀
ADXL150
0.1F
C1
0.1F
R2
42.2k⍀
R1
82.5k⍀
GAIN
AMP
10
DEMODULATOR
25k⍀
SENSOR
1/2
OP296
1
C3
BUFFER
AMP
TYPICAL FILTER VALUES
3
CLOCK
COM
2-POLE
FILTER
BW
C3
C4
8
9
7
300Hz 0.027F 0.0033F
100Hz 0.082F 0.01F
OFFSET
NULL
+V
S
2
+V
S
2
SELF-TEST
30Hz
10Hz
0.27F 0.033F
0.82F 0.1F
C2
0.1F
SCALING
AMPLIFIER
5
6
DESIRED
OUTPUT
SCALE FACTOR
EXT
1/2
R5
VALUE
F.S.
RANGE
OUTPUT
R4
100k⍀
7
AMP
OP296
GAIN
4
76mV/g
±25g
±20g
±10g
±5g
2.0
2.6
5.3
10.5
200k⍀
261k⍀
536k⍀
1M⍀
+V
S
R6
1M⍀
100mV/g
200mV/g
400mV/g
R5
200k⍀
0g TRIM
Figure 21. Two-Pole Post Filter Circuit
–11–
REV. 0
ADXL150/ADXL250
Additional Noise Reduction Techniques
CALIBRATING THE ADXL150/ADXL250
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.
If a calibrated shaker is not available, both the zero g level and
scale factor of the ADXL150/ADXL250 may be easily set to fair
accuracy by using a self-calibration technique based on the 1 g
acceleration of the earth’s gravity. Figure 24 shows how gravity
and package orientation affect the ADXL150/ADXL250’s
output. With its axis of sensitivity in the vertical plane, the
ADXL150/ADXL250 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 zero g
bias level) should be indicated. The use of an external buffer
amplifier may invert the polarity of the signal.
Mounting Fixture Resonances
A common source of error in acceleration sensing is resonance
of the mounting fixture. For example, the circuit board that the
ADXL150/ADXL250 mounts to may have resonant frequencies
in the same range as the signals of interest. This could cause the
signals measured to be larger than they really are. A common
solution to this problem is to damp these resonances by mount-
ing the ADXL150/ADXL250 near a mounting post or by add-
ing extra screws to hold the board more securely in place.
1
14
8
7
7
8
8
7
14
1
1
14
When testing the accelerometer in your end application, it is
recommended that you test the application at a variety of fre-
quencies to ensure that no major resonance problems exist.
8
7
14
1
0g
0g
+1g
–1g
(c)
(d)
(a)
(b)
REDUCING POWER CONSUMPTION
The use of a simple power cycling circuit provides a dramatic
reduction in the accelerometer’s average current consumption.
In low bandwidth applications such as shipping recorders, a
simple, low cost circuit can provide substantial power reduction.
If a microprocessor is available, it can supply a TTL clock pulse
to toggle the accelerometer’s power on and off.
Figure 24. Using the Earth’s Gravity to Self-
Calibrate the ADXL150/ADXL250
A 10% duty cycle, 1 ms on, 9 ms off, reduces the average cur-
rent consumption of the accelerometer from 1.8 mA to 180 µA,
providing a power reduction of 90%.
Figure 24 shows how to self-calibrate the ADXL150/ADXL250.
Place the accelerometer on its side with its axis of sensitivity
oriented as shown in “a.” (For the ADXL250 this would be the
“X” axis—its “Y” axis is calibrated in the same manner, but the
part is rotated 90° clockwise.) The zero g offset potentiometer
RT is then roughly adjusted for midscale: +2.5 V at the external
amp output (see Figure 20).
Figure 23 shows the typical power-on settling time of the
ADXL150/ADXL250.
V
S
5.0
4.5
0.5V
Next, the package axis should be oriented as in “c” (pointing
down) and the output reading noted. The package axis should
then be rotated 180° to position “d” and the scale factor poten-
tiometer, R1b, adjusted so that the output voltage indicates a
change of 2 gs in acceleration. For example, if the circuit scale
factor at the external buffer’s output is 100 mV per g, the scale
factor trim should be adjusted so that an output change of
200 mV is indicated.
4.0
3.5
3.0
2.5
V
– 50g
OUT
V
= 0g
OUT
2.0
1.5
V
+ 50g
OUT
1.0
0.5
0.5V
Self-Test Function
A Logic “1” applied to the self-test (ST) input will cause an
electrostatic force to be applied to the sensor that will cause it to
deflect. If the accelerometer is experiencing an acceleration
when the self-test is initiated, the output will equal the algebraic
sum of the two inputs. The output will stay at the self-test level
as long as the ST input remains high, and will return to the
actual acceleration level when the ST voltage is removed.
0
0
0.04 0.08 0.12 0.16 0.20 0.24 0.28 0.32 0.36
TIME – ms
Figure 23. Typical Power-On Settling with Full-Scale
Input. Time Constant of Post Filter Dominates the
Response When a Signal Is Present.
Using an external amplifier to increase output scale factor may
cause the self-test output to overdrive the buffer into saturation.
The self-test may still be used in this case, but the change in the
output must then be monitored at the accelerometer’s output
instead of the external amplifier’s output.
Note that the value of the self-test delta is not an exact indica-
tion of the sensitivity (mV/g) and therefore may not be used to
calibrate the device for sensitivity error.
REV. 0
–12–
ADXL150/ADXL250
MINIMIZING EMI/RFI
In selecting an appropriate ADC to use with our accelerometer
we need to find a device that has a resolution better than the
measurement resolution but, for economy’s sake, not a great
deal better.
The architecture of the ADXL150/ADXL250, and its use of
synchronous demodulation, makes the device immune to most
electromagnetic (EMI) and radio frequency (RFI) interference.
The use of synchronous demodulation allows the circuit to
reject all signals except those at the frequency of the oscillator
driving the sensor element. However, the ADXL150/ADXL250
have a sensitivity to noise on the supply lines that is near its
internal clock frequency (approximately 100 kHz) or its odd
harmonics and can exhibit baseband errors at the output. These
error signals are the beat frequency signals between the clock
and the supply noise.
For most applications, an 8- or 10-bit converter is appropriate.
The decision to use a 10-bit converter alone, or to use a gain
stage together with an 8-bit converter, depends on which is more
important: component cost or parts count and ease of assembly.
Table II shows some of the tradeoffs involved.
Table II.
Such noise can be generated by digital switching elsewhere in
the system and must be attenuated by proper bypassing. By
inserting a small value resistor between the accelerometer and
its power supply, an RC filter is created. This consists of the
resistor and the accelerometer’s normal 0.1 µF bypass capacitor.
For example if R = 20 Ω and C = 0.1 µF, a filter with a pole at
80 kHz is created, which is adequate to attenuate noise on the
supply from most digital circuits, with proper ground and sup-
ply layout.
8-Bit Converter and 10-Bit (or 12-Bit)
Op Amp Preamp
Converter
Advantages:
Low Cost Converter
No Zero g Trim Required
Higher Cost Converter
Disadvantages:
Needs Op Amp
Needs Zero g Trim
Adding amplification between the accelerometer and the ADC
will reduce the circuit’s full-scale input range but will greatly
reduce the resolution requirements (and therefore the cost) of
the ADC. For example, using an op amp with a gain of 5.3
following the accelerometer will increase the input drive to the
ADC from 38 mV/g to 200 mV/g. Since the signal has been
gained up, but the maximum full-scale (clipping) level is still the
same, the dynamic range of the measurement has also been
reduced by 5.3.
Power supply decoupling, short component leads, 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.
INTERFACING THE ADXL150/ADXL250 SERIES
iMEM
S
ACCELEROMETERS WITH POPULAR ANALOG-TO-
DIGITAL CONVERTERS.
Basic Issues
Table III. Typical System Resolution Using Some Popular
ADCs Being Driven with and without an Op Amp Preamp
The ADXL150/ADXL250 Series accelerometers were designed
to drive popular analog-to-digital converters (ADCs) directly.
In applications where both a ±50 g full-scale measurement range
and a 1 kHz bandwidth are needed, the VOUT terminal of the
accelerometer is simply connected to the VIN terminal of the
ADC as shown in Figure 25a. The accelerometer provides its
(nominal) factory preset scale factor of +2.5 V ±38 mV/g which
drives the ADC input with +2.5 V ±1.9 V when measuring a
50 g full-scale signal (38 mV/g × 50 g = 1.9 V).
Converter
mV/Bit
SF
Preamp in
FS
System
Converter
Type
Range Resolution
2n
(5 V/2n)
Gain
None
2
mV/g in g’s
in g’s (p-p)
0.51
8 Bit
256 19.5 mV
256 19.5 mV
256 19.5 mV
256 19.5 mV
1,024 4.9 mV
1,024 4.9 mV
1,024 4.9 mV
1,024 4.9 mV
4,096 1.2 mV
4,096 1.2 mV
4,096 1.2 mV
4,096 1.2 mV
38
±50
±25
±20
±10
±50
±25
±20
±10
±50
±25
±20
±10
76
0.26
2.63
5.26
None
2
100
200
38
0.20
As stated earlier, the use of post filtering will dramatically
improve the accelerometer’s low g resolution. Figure 25b shows
a simple post filter connected between the accelerometer and
the ADC. This connection, although easy to implement, will
require fairly large values of Cf, and the accelerometer’s signal
will be loaded down (causing a scale factor error) unless the
ADC’s input impedance is much greater than the value of Rf.
ADC input impedance’s range from less than 1.5 kΩ up to
greater than 15 kΩ with 5 kΩ values being typical. Figure 25c is
the preferred connection for implementing low-pass filtering
with the added advantage of providing an increase in scale
factor, if desired.
0.10
10 Bit
12 Bit
0.13
76
0.06
2.63
5.26
None
2
100
200
38
0.05
0.02
0.03
76
0.02
2.63
5.26
100
200
0.01
0.006
Calculating ADC Requirements
The resolution of commercial ADCs is specified in bits. In an
ADC, the available resolution equals 2n, where n is the number
of bits. For example, an 8-bit converter provides a resolution of
28 which equals 256. So the full-scale input range of the converter
divided by 256 will equal the smallest signal it can resolve.
Table III is a chart showing the required ADC resolution vs. the
scale factor of the accelerometer with or without a gain ampli-
fier. Note that the system resolution specified in the table refers
–13–
REV. 0
ADXL150/ADXL250
+V
+V
S
to that provided by the converter and preamp (if used). It is
necessary to use sufficient post filtering with the accelerometer
to reduce its noise floor to allow full use of the converter’s reso-
lution (see post filtering section).
S
XL
ADC
V
OUT
The use of a gain stage following the accelerometer will nor-
mally require the user to adjust the zero g offset level (either by
trimming or by resistor selection—see previous sections).
For many applications, a modern “economy priced” 10-bit
converter, such as the AD7810 allows you to have high resolu-
tion without using a preamp or adding much to the overall
circuit cost. In addition to simplicity and cost, it also meets two
other necessary requirements: it operates from a single +5 V
supply and is very low power.
a. Direct Connection, No Signal Amplification or
Post Filtering
+V
+V
S
S
R
ADC
F
XL
V
OUT
INPUT
RESISTANCE
Cf
b. Single-Pole Post Filtering, No Signal Amplification
+V
+V
S
S
Cf
0g
OFFSET
ADJUST
R
F
R1
ADC
V
OUT
XL
V
OS
NULL PIN
c. Single-Pole Post Filtering and Signal Amplification
Figure 25. Interfacing the ADXL150/ADXL250 Series
Accelerometers to an ADC
REV. 0
–14–
ADXL150/ADXL250
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
14-Lead Cerpac
(QC-14)
0.390 (9.906)
MAX
14
1
8
7
0.291 (7.391)
0.285 (7.239)
0.419 (10.643)
0.394 (10.008)
PIN 1
0.300 (7.62)
0.195 (4.953)
0.115 (2.921)
0.345 (8.763)
0.290 (7.366)
0.020 (0.508)
0.004 (0.102)
0.215 (5.461)
0.119 (3.023)
8؇
0؇
0.050
(1.27)
BSC
0.020 (0.508)
0.013 (0.330)
0.050 (1.270)
0.016 (0.406)
0.0125 (0.318)
0.009 (0.229)
SEATING
PLANE
–15–
REV. 0
相关型号：
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