ADXL05_15 Datasheet, PDF (11/20 Pages) Analog Devices – Single Chip Accelerometer with Signal Conditioning

ADXL05
USING THE INTERNAL BUFFER AMPLIFIER TO VARY
THE ACCELEROMETER’S OUTPUT SCALE FACTOR
AND 0 g BIAS LEVEL
The ADXL05 accelerometer has an onboard buffer amplifier
that allows the user to change the output scale factor and 0 g
bias level.
C2
4
0.022µF
0.022µF
C1
2
3
C1
ADXL05
PRE-AMP
1
1.8V
BUFFER
AMP
9
+5V
0.1µF
V_OUT
The output scale factor of an accelerometer is simply how many
volts output are provided 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 mea-
suring. Resolution is principally determined by the device noise
and the measurement bandwidth.
5
COM
6
8
10
+3.4V ^VPR
REF
V_{IN– R3}
R1a R1b
OUTPUT SCALE FACTOR =
V_PR OUTPUT: 200mV/g
R3
(R1a + R1b)
x V_PR OUTPUT
The 0 g bias level is simply the dc output level of the accelerom-
OBSOLETE eter when it is not in motion or being acted upon by the Earth’s
gravity.
Setting the Accelerometer’s Scale Factor
Figure 20 shows the basic connections for using the onboard
buffer amplifier to increase the output scale factor. The nominal
output level in volts from V_PR (the preamplifier output) is equal
to the g forces applied to the sensor (along its sensitive axis)
times 200 mV/g. The use of the buffer is always recommended,
even if the preset scale factor is adequate, as the buffer helps
prevent any following circuitry from loading down the V_PR
output.
C2
4
0.022µF
0.022µF
C1
2
ADXL05
PRE-AMP
1
1.8V
BUFFER
AMP
+5V
C3
0.1µF
Figure 21. External Scale Factor Trimming
Setting the Accelerometer’s 0 g Bias Level, AC Coupled
Response
If a dc (gravity) response is not required—for example in motion
sensing or vibration measurement applications—ac coupling can
be used between the preamplifier output and the buffer input as
shown in Figure 22. The use of ac coupling between V_PR and
the buffer input virtually eliminates any 0 g drift and allows the
maximum buffer gain without clipping.
Resistor R1 and capacitor C4 together form a high pass filter
whose corner frequency is 1/(2 π R1 C4). This means that this
simple filter will reduce the signal from V_PR by 3 dB at the
corner frequency, and it will continue to reduce it at a rate of
6 dB/octave (20 dB per decade) for signals below the corner
frequency.
Note that capacitor C4 should be a nonpolarized, low leakage
3
C1
9
V_OUT
type. If a polarized capacitor is used, tantalum types are pre-
5
ferred, rather than electrolytic. With polarized capacitors, V_PR
COM
should be measured on each device and the positive terminal of
6
+3.4V
REF
8
V_{PR R1}
10
V_{IN– R3}
the capacitor connected toward either V_PR or V_IN—whichever is
more positive
OUTPUT SCALE FACTOR =
V_PR OUTPUT: 200mV/g
R3
R1
x
V_PR
OUTPUT
Figure 20. Basic Buffer Connections
The 0 g offset level of the ADXL05 accelerometer is preset at
+1.8 V. This can easily be changed to a more convenient level,
such as +2.5 V which, being at the middle of the supply voltage,
provides the greatest output voltage swing.
In Figure 20, the output scale factor at Pin 9 (V_OUT) is the out-
put at V_PR times 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 amplifies the sig-
nal but any noise or drift as well. Too much gain can also cause
the buffer to saturate and clip the output wave form.
The circuit of Figure 20 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. For the highest possible accuracy, an external trim is rec-
ommended. As shown by Figure 21, this consists of a potenti-
ometer, R1a, in series with a fixed resistor, R1b.
When using the ac coupled circuit of Figure 22, only a single re-
sistor, R2, is required to swing the buffer output to +2.5 V.
Since the “+” input of the buffer is referenced at +1.8 V, its
summing junction, Pin 10, is also held constant at +1.8 V.
Therefore, to swing the buffer’s output to the desired +2.5 V
0 g bias level, its output must move up +0.7 V (2.5 V – 1.8 V =
0.7 V). Therefore, the current needed to flow through R3 to
cause this change, IR3, is equal to:
IR 3
=
0.7 Volts
R3 in Ohms
REV. B
–11–