ADXL150AQC Analog Devices Inc, ADXL150AQC Datasheet - Page 12

• Manufacturer Part Number: ADXL150AQC
• Description: IC ACCELEROMETER LP SGL 14CERPAK
• Manufacturer: Analog Devices Inc
• Series: iMEMS®r

Specifications of ADXL150AQC

• Rohs Status: RoHS non-compliant
• Axis: X or Y
• Acceleration Range: ±50g
• Sensitivity: 38mV/g
• Voltage - Supply: 4 V ~ 6 V
• Output Type: Analog
• Bandwidth: 1kHz
• Mounting Type: Surface Mount
• Package / Case: 14-CerPak
• Interface: -

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 A,
providing a power reduction of 90%.
Figure 23 shows the typical power-on settling time of the
ADXL150/ADXL250.
ADXL150/ADXL250
Figure 23. Typical Power-On Settling with Full-Scale
Input. Time Constant of Post Filter Dominates the
Response When a Signal Is Present.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
5.0
4.5
4.0
0
0
0.04
0.08
0.12
0.16
TIME – ms
0.20
0.24
V
V
V
S
OUT
OUT
0.28
0.5V
0.5V
V
– 50 g
+ 50 g
OUT
0.32
= 0 g
0.36
–12–
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.
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.
7
8
Figure 24. Using the Earth’s Gravity to Self-
Calibrate the ADXL150/ADXL250
0 g
(a)
14
1
14
1
(b)
0 g
8
7
14
8
+1 g
(c)
1
7
7
1
–1 g
(d)
REV. 0
14
8