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ADXL05 Datasheet(PDF) 18 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) 18 Page - Analog Devices

 
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ADXL05
REV. B
–18–
low-pass filtering generally results in smaller capacitance values
and better overall performance. It is also a convenient and more
precise way to set the system bandwidth. Post filtering allows
bandwidth to be controlled accurately by component selection
and avoids the
±40% demodulation tolerance. Note that signal
noise is proportional to the square root of the bandwidth of the
ADXL05 and may be a consideration in component selection—
see section on noise.
Care should be taken to reduce or eliminate any leakage paths
from the demodulator capacitor pins to common or to the +5 V
pin. Even a small imbalance in the leakage paths from these pins
will result in offset shifts in the zero-g bias level. As an example,
an unbalanced parasitic resistance of 30 M
Ω from either de-
modulator pin to ground will result in an offset shift at VPR of
approximately 50 mV. Conformal coating of PC boards with a
high impedance material is recommended to avoid leakage prob-
lems due to aging or moisture.
MINIMIZING EMI/RFI
The architecture of the ADXL05 and its use of synchronous de-
modulation make the device immune to most electromagnetic
(EMI) and radio frequency (RFI) interference. The use of syn-
chronous demodulation allows the circuit to reject all signals ex-
cept those at the frequency of the oscillator driving the sensor
element. However, the ADXL05 does have a sensitivity to RFI
that is within
±5 kHz of the internal oscillator’s nominal fre-
quency of 1 MHz and also to any odd harmonics of this fre-
quency. The internal oscillator frequency will exhibit part to
part variation in the range of 0.5 MHz to 1.4 MHz.
In general the effect is difficult to notice as the interference must
match the internal oscillator within
±5 kHz and must be large
in amplitude. For example: a 1 MHz interference signal of
20 mV p-p applied to the +5 V power supply pin will produce a
200 mV p-p signal at the VPR pin if the internal oscillator and
interference signals are matched exactly or at odd harmonics. If
the same 20 mV interference is applied but 5 kHz above or be-
low the internal oscillator’s frequency, the signal level at VPR will
only be 20 mV p-p in amplitude.
Power supply decoupling, short component leads (especially for
capacitors C1 and C2), physically small (surface mount, etc.)
components and attention to good grounding practices all help
to prevent RFI and EMI problems. Good grounding practices
include having separate analog and digital grounds (as well as
separate power supplies or very good decoupling) on the printed
circuit boards. A single ground line shared by both the digital
and analog circuitry can lead to digital pulses (and clock signals)
interfering with the sensor’s onboard oscillator. In extreme
cases, a low cost radio frequency choke (
≈10 µH) may be
needed in series with the accelerometer’s power supply pin.
This, together with the recommended 0.1
µF power supply by-
pass capacitor, will form an effective RF filter. The use of an RF
choke is preferred over a resistor since any series resistance in
the power supply will “unregulate” the device from the supply,
degrade its power supply rejection and reduce its supply voltage.
output change at VPR. If the ADXL05 is experiencing an
acceleration when the self-test is initiated, the VPR output will
equal the algebraic 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
±15% 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 ADXL05’s buffer amplifier at Gains > 2, to pro-
vide full-scale outputs of less than
±5 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 ADXL05 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 ADXL05 power supply should be decoupled with a 0.1
µF
ceramic capacitor from +5 V pin of the ADXL05 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 ADXL05. A ceramic
capacitor 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
1M
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 ADXL05.
Demodulator Capacitor, C1
The demodulator capacitor is connected across Pins 2 and 3 to
set the bandwidth of the force balance control loop. This capaci-
tor may be used to approximately set the bandwidth of the ac-
celerometer. A capacitor is always required for proper operation.
The frequency response of the ADXL05 exhibits a single pole
roll-off response, see Figure 4.
A nominal value of 0.022
µF is recommended for C1. In gen-
eral, the design bandwidth should be set 40% higher than the
minimum desired system bandwidth due to the
±40% tolerance,
to preserve stability C1 should be kept
> 0.01 µF.
The demodulation capacitor should be a low leakage, low drift
ceramic type with an NPO (best) or X7R (good) dielectric.
In general, it’s best to use the recommended 0.022
µF capacitor
across the demodulator pins and perform any additional low-
pass filtering using the buffer amplifier. The use of the buffer for
OBSOLETE


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