a
Reducing the Average Power Consumption of Accelerometers
by Charles Kitchin, Mike Shuster and Bob Briano
AN-378
APPLICATION NOTE
ONE TECHNOLOGY WAY
•
P.O. BOX 9106
•
NORWOOD, MASSACHUSETTS 02062-9106
•
617/329-4700
The use of a simple power cycling circuit provides a dra-
matic reduction in the average current consumption of
the ADXL50 and ADXL05 devices. In low bandwidth
applications such as shipping recorders, a simple, low
cost circuit can provide substantial power reduction. If a
microprocessor is available, only the circuit of Figure 1
is needed; the microprocessor supplies a TTL clock
pulse to gate buffer transistor Q2, which c ycles the supply
voltage on and off. Figures 2 through 4 show typical
wave forms of the accelerometer being operated with a
10% duty cycle: 1 ms on, 9 ms off. This reduces the aver-
age current consumption of the accelerometer from 10
mA to 1 mA, providing a power reduction of 90%.
The lower trace of Figures 2 and 3 is the output voltage
appearing at V PR (Pin 8). The lower trace of Figure 4 is
the buffer output (Pin 9) with the buffer operating at
unity gain. A 0.01 µF capacitor was connected across
the feedback resistor of the buffer to improve its
transient characteristics. The optimum value for this
capacitor will change with buffer gain and the cycling
pulse rate. The µP should sample acceleration during
the interval between the time the 0
g
level has stabilized
(approximately 400 µs using a 0.022 µF demod cap) and
the end of the pulse duration. For the example shown in
Figures 2 through 4, this is between 400 µs and 1 ms
after Q2 receives a logic “low” from the µP.
R3R1
V
OUT
FROM
Q1 OR µP
10kΩQ1
2N3906
0.1µF
BUFFER
+5V
ADXL05
OR
ADXL50
V
PR
V
IN–
V
OUT
C
F
10kΩQ2
2N2222
100kΩ
8 910
1
5
COM
Figure 2. Top Trace: Voltage at Pin 1
Bottom Trace: Output at V
PR
10
0%
200µs
2V
1V
100
90
Figure 3. Top Trace: Voltage at Pin 1
Bottom Trace: Output at V
PR
R3R1
VOUT
FROM
Q1 OR µP
10kΩQ1
2N3906
0.1µF
BUFFER
+5V
ADXL05
OR
ADXL50
VPR VIN– VOUT
CF
10kΩQ2
2N2222
100kΩ
8 910
1
5
COM
Figure 1. Basic Power Cycling Circuit