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ADXL50 Datasheet(Arkusz danych) 11 Page - Analog Devices

Numer części ADXL50
Szczegółowy opis  Monolithic Accelerometer With Signal Conditioning
Pobierz  16 Pages
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ADXL50
REV. B
–11–
As an example, if the desired span is
±2.0 V for a = ±50 g input,
then R3/R1 should be chosen such that
R3/R1 = VOUT Span/VPR Span = 2.00/0.95 = 2.105
(1)
where VPR span is the output from the preamplifier and VOUT
span is the buffer amplifier’s output, giving
R3 = 2.105
× R1
(2)
In noncritical applications, a resistor, R2, may simply be con-
nected between VIN– and common to provide an approximate
0 g offset level (see Figure 19). In this simplified configuration
R2 is found using:
R2 = (1.8 V
× R3)/(V
OUT @ 0 g – 1.8 V)
When used with a trim potentiometer, as in Figure 20, resistor
R2 sets the 0 g offset range and also sets the resolution of the
offset trim. A value of 100 k
Ω is typical. Increasing R2 above
this value makes trimming the offset easier, but may not provide
enough trim range to set VOUT equal to +2.5 V for all devices.
To provide an output span of
±2.00 V, with a 0 g output of
+2.5 V, R1 could be set to the standard value of 49.9 k
Ω and
from Equation 2, R3 = 105 k
Ω.
For Figure 20, the circuit transfer function is:
V
OUT =
R3
R1
(1.8V – V
PR )


+
R3
R2
(1.8V – V
X )


+ 1.8V
The summing amplifier configuration allows noninteractive
trimming of offset and span. Since VPR is not always exactly
1.8 V at 0 g, it will contribute to output offset. Therefore, span
must be trimmed first, followed by 0 g offset adjustment.
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
±50 g applications, the resistor R1 from V
PR to VIN– is recom-
mended to have a value greater than 50 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 feed-
back resistor for low-pass filtering does not appear as a capaci-
tive 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 out-
put of the device. Accordingly, a correctly functioning acceler-
ometer will respond by initiating an approximate –1 volt output
change at VPR. If the ADXL50 is experiencing an acceleration
when the self-test is initiated, the VPR output will equal the alge-
braic sum of the two inputs. The output will stay at the self-test
level as long as the ST input remains high and will return to the
0 g level when the ST voltage is removed.
A self-test output that varies more than
±10% from the nominal
–1.0 V change indicates a defective beam or a circuit problem
such as an open or shorted pin or component.
Operating the ADXL50’s buffer amplifier at Gains > 2, to pro-
vide full-scale outputs of less than
±50 g, may cause the self-test
output to overdrive the buffer into saturation. The self-test may
still be used in the case, but the change in the output must then
be monitored at the VPR pin instead of the buffer output.
Note that the value of the self-test delta is not an exact indica-
tion of the sensitivity (mV/g) of the ADXL50 and, therefore,
may not be used to calibrate the device for sensitivity error.
In critical applications, it may be desirable to monitor shifts in
the zero-g bias voltage from its initial value. A shift in the 0 g
bias level may indicate that the 0 g level has shifted which may
warrant an alarm.
POWER SUPPLY DECOUPLING
The ADXL50 power supply should be decoupled with a 0.1
µF
ceramic capacitor from +5 V pin of the ADXL50 to common
using very short component leads. For other decoupling consid-
erations, see EMI/RFI section.
OSCILLATOR DECOUPLING CAPACITOR, C2
An oscillator decoupling capacitor, C2, is used to remove
1 MHz switching transients in the sensor excitation signal, and
is required for proper operation of the ADXL50. A ceramic ca-
pacitor with a minimum value of 0.022
µF is recommended
from the oscillator decoupling capacitor pin to common. Small
amounts of capacitor leakage due to a dc resistance greater than
l M
Ω will not affect operation (i.e., a high quality capacitor is
not needed here). As with the power supply bypass capacitor,
very short component leads are recommended. Although
0.022
µF is a good typical value, it may be increased for reasons
of convenience, but doing this will not improve the noise perfor-
mance of the ADXL50.




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