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ADXL50JH Datasheet(Arkusz danych) 10 Page - Analog Devices

Numer części ADXL50JH
Szczegółowy opis  Monolithic Accelerometer With Signal Conditioning
Pobierz  16 Pages
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Producent  AD [Analog Devices]
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ADXL50
–10–
REV. B
calibration are used and there is a desire to eliminate trim po-
tentiometers, the design should leave room at either supply rail
to account for signal swing and or variations in initial zero g bias.
For example, in the circuit in Figure 19, the initial zero g bias of
±250 mV will be reflected to the output by the gain of the R3/R1
network, resulting in an output offset of
±526 mV worst case.
The offset, combined with a full-scale signal of 50 g, (+2.0 V)
will cause the output buffer amplifier to saturate at the supply
rail.
The full
±2.25 V output swing of the buffer amplifier can be
utilized if the user is able to trim the zero-g bias to exactly
2.5 V. In applications where the full-scale range will be
±25 g or
less, a bias trim such as that shown in Figure 20 will almost al-
ways be required.
VARYING THE OUTPUT SENSITIVITY AND 0 g LEVEL
USING THE INTERNAL BUFFER AMPLIFIER
The uncommitted buffer amplifier may be used to change the
output sensitivity to provide useful full-scale ranges of
±50 g
and below. Table II provides recommended resistor values for
several standard ranges down to
±10 g. As the full-scale range is
decreased, buffer amplifier gain is increased, and the noise con-
tribution as a percentage of full scale will also increase. For all
ranges, the signal-to-noise ratio can be improved by reducing
the circuit bandwidth, either by increasing the demodulator ca-
pacitor, C1, or by adding a post filter using the buffer amplifier.
Table II. Recommended Resistor Values for Setting the
Circuit of Figure 20 to Several Common Full-Scale Ranges
Buffer
SF in
FS (g)
Gain
mV/g
R1
R3
R2
±50.0
2.11
40
49.9 k
105 k
100 k
±40.0
2.63
50
39.2 k
103 k
100 k
±30.8
3.42
65
40.2 k
137 k
100 k
±26.7
3.95
75
28.7 k
113 k
100 k
±20.0
5.26
100
26.1 k
137 k
100 k
±10.0
10.53
200
23.7 k
249 k
100 k
Note that the value of resistor R1 should be selected to limit the
output current flowing into VPR to less than 25
µA (to provide a
safety margin). For a “J” grade device, this current is equal to:
I
PR =
(2.05 V – The peak full -scale output voltage at V
PR )– 1.8V
R1 in ohms
For a
±50 g full-scale range, R1 needs to be 49.9 kΩ or larger in
value; but at the lower full-scale g ranges, if the VPR swing is
much less, then it is possible to use much lower resistance val-
ues. For this table, the circuit of Figure 20 is used, as a 0 g off-
set trim will be required for most applications. In all cases, it is
assumed that the zero-g bias level is 2.5 V with an output span
of
±2 V.
Note that for full scales below
±20 g the self-test is unlikely to
operate correctly because the VPR pull-down current is not guar-
anteed to be large enough to drive R1 to the required –1.0 V
swing. In these cases, the self-test command will cause VOUT to
saturate at the rail, and it will be necessary to monitor the self-
test at VPR. Self-test can remain operational at VPR for all g
ranges listed by keeping R1 > 49.9 k
Ω, with the subsequent
tradeoff that the required values for R3 will become very large.
The user always has the option of adding external gain and fil-
tering stages after the ADXL50 to make lower full-scale ranges.
Measuring Full-Scale Accelerations Less than
5 g
Applications, such as motion detection, and tilt sensing, have
signal amplitudes in the 1 g to 2 g range. Although designed for
higher full-scale ranges, the ADXL50 may be adapted for use in
BUFFER
AMP
ADXL50
PRE-AMP
0.022
µF
C1
C1
0.022
µF
C2
COM
0.1
µF
+5V
V
OUT
V
IN–
V
PR
VREF
+3.4V
50k
R1
R2
V
X
+1.8V
R3
0g
LEVEL
TRIM
1.8V
2
3
4
1
5
6
9
8
10
Figure 20. ADXL50 Circuit Using the Buffer Amplifier to
Set the Output Scaling and 0 g Offset Level
low g applications; the two main design considerations are noise
and 0 g offset drift (BH, KH grades recommended).
At its full 1 kHz bandwidth, the ADXL50 will typically exhibit
1 g p-p of noise. With
±50 g accelerations this is generally not a
problem, but at a
±2 g full-scale level the signal-to-noise ratio
will be very poor. However, reducing the bandwidth to 100 Hz
or less considerably improves the S/N ratio. Figure 25 shows the
relationship between ADXL50 bandwidth and noise.
The ADXL50 exhibits offset drifts that are typically 0.02 g per
°C but which may be as large as 0.1 g per °C. With the buffer
amplifier configured for a 2 g full scale, the ADXL50 will typi-
cally drift 1/2 of its full-scale range with a 50
°C increase in
temperature.
There are several cures for offset drift. If a dc response is not
required, for example in motion sensing or vibration measurement
applications, consider ac coupling the acceleration signal to re-
move the effects of offset drift. See the section on ac coupling.
Periodically recalibrating the accelerometer’s 0 g level is another
option. Autozero or long term averaging can be used to remove
long term drift using a microprocessor or the autozero circuit of
Figure 29. Be sure to keep the buffer amplifier’s full-scale out-
put range much larger than the measurement range to allow for
the 0 g level drift.
CALCULATING COMPONENT VALUES FOR SCALE
FACTOR AND 0 g SIGNAL LEVEL
The ADXL50 buffer’s scale factor is set by –R3/R1 (since the
amplifier is in the inverter mode).




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