ADXL190 查看數據表（PDF） - Analog Devices
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ADXL190 Datasheet PDF : 5 Pages
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ADXL190
APPLICATIONS
All the circuitry needed to drive the sensor and convert the
capacitance change to voltage is incorporated on-chip requiring
no external components except for standard power supply de-
coupling. Both sensitivity and the zero-g value are ratiometric to
the supply voltage, so that ratiometric devices following the
accelerometer (such as an ADC, etc.) will track the accelerom-
eter if the supply voltage changes. The output voltage (VOUT) is
a function of both the acceleration input (a) and the power
supply voltage (VS) as follows:
VOUT = VS/2 – (Sensitivity × VS/5 V × a)
Adjusting the 0 g Bias Level
In some cases the user may have an asymmetrical input or may
want to fine adjust the zero-g output level to obtain maximum
dynamic range. The zero-g level is adjusted by supplying a
voltage to the zero-g adjustment pin (see Figure 2).
+VS
2
ADXL190 5k⍀
ACCELERATION
SIGNAL
25k⍀
GAIN = 3
FILTER
VOUT
C2
0.1F
ZERO g ADJUST
+VS
200k⍀
Figure 2. Optional Zero-g Adjust Circuit Detail
Any voltage difference between the zero-g adjustment pin and
VS/2 is reduced by a factor of 6 by the internal resistor divider.
This is then gained by the factor of 3 in the output stage for a
total gain of 0.5 for the zero-g adjustment. (Note: The ratio of
the resistors in the divider is consistent from part-to-part; how-
ever, the absolute values can have a ± 30% tolerance). The
zero-g adjustment voltage can be set up by a variety of methods
including a potentiometer (as shown in Figure 2), a PWM sig-
nal, or with a simple three-state output.
The simplest way is by adding a resistor between the ZERO g
ADJUST pin and VS or ground. The output will be offset by:
Offset (V) = (7.5 × VS)/(30 + R)
where R is in kΩ and connected to VS.
Offset (V) = (–7.5 × VS)/(30 + R)
where R is in kΩ and connected to ground.
Resistors may also be connected to microcontroller I/O pins as
shown in Figure 3. Using two I/Os that may be set to VS, ground,
or three-state, there are seven possibilities as shown in Table I
(one cannot set one I/O pin to VS and the other to ground).
Using such a system, any ADXL190 may be user trimmed to
output 2.5 V ± 35 mV at zero g.
Table I. Offsets Produced Using the Circuit in Figure 3 for VS
=5V
P1
Three-State
Three-State
0
0
Three-State
1
1
P0
Three-State
0
Three-State
0
1
Three-State
1
Offset Voltage
Produced
0 mV
–71 mV
–134 mV
–191 mV
71 mV
134 mV
191 mV
Offset in g
0
–4
–7.4
–10.6
4
7.4
10.6
Another way to adjust the zero g offset is to supply a voltage to
the ZERO g ADJUST pin. The difference between VS/2 and
the voltage at the ZERO g ADJUST pin is reduced by a factor
of 6 (as a result of the internal 5 kΩ and 25 kΩ voltage divider)
and then multiplied by a factor of 3 in the output stage of the
ADXL190 resulting in a total gain of 0.5. Offset is thus de-
scribed by the following equation:
Offset (V) = (Voltage at the ZERO g ADJUST Pin – VS/2)/2
This voltage may be produced by a variety of methods includ-
ing a PWM signal from a microcontroller. Care must be taken
that the output impedance of this voltage source is less than
5 kΩ and that there is very little ripple (noise). Any noise at the
ZERO g ADJUST pin will cause output errors.
If an asymmetric range of acceleration is required (e.g., +75 g
to –125 g) a resistor may be connected between the ZERO g
ADJUST and ground or VS as described above. For example:
For a range of +75 g to –125 g the offset required is –25 g.
–25 g at 18 mV/g = 450 mV of offset is required.
Rearranging the offset equations above:
R = [(7.5 × VS)/offset] –30 = 53.3 kΩ connected to ground.
For asymmetric operation the g range midpoint may be shifted
up to ± 80 g typically.
P1
MICROCONTROLLER
P0
250k⍀
500k⍀
ZERO g
ADJUST
ADXL190
Figure 3. An Offset Adjustment Scheme
–4–
REV. 0
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