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ADXL05 Datasheet(PDF) 17 Page - Analog Devices

Part No. ADXL05
Description  Single Chip Accelerometer with Signal Conditioning
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADXL05 Datasheet(HTML) 17 Page - Analog Devices

 
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ADXL05
–17–
REV. B
10
0%
100
90
200µs
2V
500mV
Figure 33. Top Trace: Voltage at Pin 1; Bottom Trace:
Buffer Output With R1 = R3 = 100 k
Ω, C
F = 0.01 µF
COMPONENT SELECTION
LOAD DRIVE CAPABILITIES OF THE VPR AND BUFFER
OUTPUTS
The VPR and the buffer amplifier outputs are both capable of
driving a load to voltage levels approaching that of the supply
rail. However, both outputs are limited in how much current
they can supply, affecting component selection.
VPR Output
The VPR pin has the ability to source current up to 500
µA but
only has a sinking capability of 30
µA which limits its ability to
drive loads. It is recommended that the buffer amplifier be used
in most applications, to avoid loading down VPR. In standard
±5 g applications, the resistor R1 from V
PR to VIN– is recom-
mended to have a value greater than 20 k
Ω to reduce loading
effects.
Capacitive loading of the VPR pin should be minimized. A load
capacitance between the VPR pin and common will introduce an
offset of approximately 1 mV for every 10 pF of load. The VPR
pin may be used to directly drive an A/D input or other source
as long as these sensitivities are taken into account. It is always
preferable to drive A/D converters or other sources using the
buffer amplifier (or an external op amp) instead of the VPR pin.
Buffer Amplifier Output
The buffer output can drive a load to within 0.25 V of either
power supply rail and is capable of driving 1000 pF capacitive
loads. Note that a capacitance connected across the buffer
feedback resistor for low-pass filtering does not appear as a
capacitive load to the buffer. The buffer amplifier is limited to
sourcing or sinking a maximum of 100
µA. Component values
for the resistor network should be selected to ensure that the
buffer amplifier can drive the filter under worst case transient
conditions.
Self-Test Function
The digital self-test input is compatible with both CMOS and
TTL signals. A Logic “l” applied to the self-test (ST) input will
cause an electrostatic force to be applied to the sensor which
will cause it to deflect to the approximate negative full-scale
output of the device. Accordingly, a correctly functioning accel-
erometer will respond by initiating an approximate –1 volt
so that the duty cycle is correct when the pulse is re-inverted
again by transistor, Q2, which cycles the accelerometer’s supply
voltage on and off.
ADXL05
VPR
VIN–
VOUT
58
10
R3
9
R1
CF
VOUT
FROM µP
OR
FIGURE 1b
Q1
10k
2N3906
0.1µF
BUFFER
+5V
1
2N2222
100k
10k
Q2
Figure 31. Basic Power Cycling Circuit
Figures 32 and 33 show typical waveforms of the accelerometer
being operated with a 10% duty cycle: 1 ms on, 9 ms off. This
reduces the average current consumption of the accelerometer
from 8 mA to 800
µA, providing a power reduction of 90%. The
µP should sample acceleration during the interval between the
time the 0 g level has stabilized (
≈ 400 µs using a 0.022 µF
demod cap) and the end of the pulse duration. The measure-
ment bandwidth of a power-cycled circuit will be set by the
clock pulse rate and duty cycle. In this example, 1 sample can
be taken every 10 ms which is 100 samples per second or 100
Hz. As defined by the “Nyquist criteria,” the best case measure-
ment bandwidth is FS/2 or half the clock frequency. Therefore
50 Hz signals can be processed if adequate filtering is provided.
10
0%
200µs
2V
1V
100
90
Figure 32. Top Trace: Voltage at Pin 1;
Bottom Trace: Output at VPR
Higher measurement bandwidths can be achieved by reducing
the size of the demodulation capacitor below 0.022
µF and in-
creasing the pulse frequency. A 0.01
µF capacitor was con-
nected across the feedback resistor of the ADXL05 buffer to
improve its transient characteristics. The optimum value for this
capacitor will change with buffer gain and the cycling pulse rate.
For more details, refer to application note AN-378.
OBSOLETE


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