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ADXL250AQC Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer ADXL250AQC
Beschreibung +-5 g to +-50 g/ Low Noise/ Low Power/ Single/Dual Axis iMEMS Accelerometers
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 15 Seiten
ADXL250AQC Datasheet, Funktion
a ؎5 g to ؎50 g, Low Noise, Low Power,
Single/Dual Axis iMEMS® Accelerometers
ADXL150/ADXL250
FEATURES
Complete Acceleration Measurement System
on a Single Monolithic IC
80 dB Dynamic Range
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
Ratiometric Operation
Complete Mechanical & Electrical Self-Test
Dual & Single Axis Versions Available
Surface Mount Package
+VS
0.1F
FUNCTIONAL BLOCK DIAGRAMS
TP
(DO NOT CONNECT)
ADXL150
GAIN
AMP
SENSOR
CLOCK
+VS
2
DEMODULATOR
25k
5k
BUFFER
AMP
9
SELF-TEST
COM
OFFSET
NULL
TP
(DO NOT CONNECT)
X OFFSET
NULL
VOUT
GENERAL DESCRIPTION
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.
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.
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
+VS
0.1F
ADXL250
GAIN
AMP
SENSOR
25k
DEMODULATOR
5k
SENSOR
CLOCK
GAIN
AMP
5k
DEMODULATOR
25k
BUFFER
AMP
VOUTX
+VS
2
BUFFER
AMP
VOUTY
SELF-TEST
COM
Y OFFSET
NULL
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 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.
iMEMS 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
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1998






ADXL250AQC Datasheet, Funktion
ADXL150/ADXL250
20
15
10
5
0
–5
–10 NOISE FROM INTERNAL CLOCK
–15
–20
0 2 4 6 8 10 12 14 16 18 20
TIME – s
Figure 9. Typical Output Noise Voltage with Spikes
Generated by Internal Clock
SELF-TEST
OUTPUT
(0.2V/DIV)
SELF-TEST
INPUT
(2V/DIV)
0 2 4 6 8 10 12 14 16 18 20
TIME – ms
Figure 10. Typical Self-Test Response
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
1.6
1.4
1.2
1.0
0.8
0.6
4.0
4.5 5.0
5.5
SUPPLY VOLTAGE – Volts
6.0
Figure 12. Noise vs. Supply Voltage
30
25
20
15
10
5
0
100
1000
FREQUENCY – kHz
Figure 13. Baseband Error Graph
10000
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.
0.25
10
100
FREQUENCY – Hz
1k
Figure 11. Noise Spectral Density
2k
–6– REV. 0

6 Page









ADXL250AQC pdf, datenblatt
ADXL150/ADXL250
Additional Noise Reduction Techniques
Shielded wire should be used for connecting the accelerometer to
any circuitry that is more than a few inches away—to avoid 60 Hz
pickup from ac line voltage. Ground the cable’s shield at only one
end and connect a separate common lead between the circuits;
this will help to prevent ground loops. Also, if the accelerometer
is inside a metal enclosure, this should be grounded as well.
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.
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.
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.
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 23 shows the typical power-on settling time of the
ADXL150/ADXL250.
VS
5.0
0.5V
4.5
4.0 VOUT – 50g
3.5
3.0 VOUT = 0g
2.5
2.0
1.5 VOUT + 50g
1.0
0.5V
0.5
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.
CALIBRATING THE ADXL150/ADXL250
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.
7
1 14
8
8 14 1
7
0g 0g
(a) (b)
87
14 1
+1g
(c)
1 14
78
–1g
(d)
Figure 24. Using the Earth’s Gravity to Self-
Calibrate the ADXL150/ADXL250
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).
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.
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.
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.
–12–
REV. 0

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