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ADXL202JE Datasheet(Arkusz danych) 10 Page - Analog Devices

Numer części ADXL202JE
Szczegółowy opis  Low-Cost -2 g Dual-Axis Accelerometer with Duty Cycle Output
Pobierz  12 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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REV. A
ADXL202E
–10–
With the single pole roll-off characteristic, the typical noise of
the ADXL202E is determined by the following equation:
Noise rms
g
Hz
BW
() =
()××
()
200
1 6
µ /.
At 100 Hz the noise will be:
Noise rms
g
Hz
mg
() =
()×× ()
 =
200
100
1 6
2 53
µ /.
.
Often the peak value of the noise is desired. Peak-to-peak noise
can only be estimated by statistical methods. Table III is useful
for estimating the probabilities of exceeding various peak values,
given the rms value.
Table III. Estimation of Peak-to-Peak Noise
% of Time that Noise
Nominal Peak-to-Peak
Will Exceed Nominal
Value
Peak-to-Peak Value
2.0
× rms
32%
4.0
× rms
4.6%
6.0
× rms
0.27%
8.0
× rms
0.006%
The peak-to-peak noise value will give the best estimate of the
uncertainty in a single measurement.
Table IV gives typical noise output of the ADXL202E for various
CX and CY values.
Table IV. Filter Capacitor Selection, CX and CY
Peak-to-Peak Noise
Estimate 95%
Bandwidth
CX, CY
rms Noise
Probability (rms
4)
10 Hz
0.47
µF
0.8 mg
3.2 mg
50 Hz
0.10
µF
1.8 mg
7.2 mg
100 Hz
0.05
µF
2.5 mg
10.1 mg
200 Hz
0.027
µF 3.6 mg
14.3 mg
500 Hz
0.01
µF
5.7 mg
22.6 mg
CHOOSING T2 AND COUNTER FREQUENCY: DESIGN
TRADE-OFFS
The noise level is one determinant of accelerometer resolution.
The second relates to the measurement resolution of the counter
when decoding the duty cycle output.
The ADXL202E’s duty cycle converter has a resolution of
approximately 14 bits; better resolution than the accelerometer
itself. The actual resolution of the acceleration signal is, how-
ever, limited by the time resolution of the counting devices used
to decode the duty cycle. The faster the counter clock, the higher
the resolution of the duty cycle and the shorter the T2 period
can be for a given resolution. The following table shows some of
the trade-offs. It is important to note that this is the resolution
due to the microprocessors’ counter. It is probable that the
accelerometer’s noise floor may set the lower limit on the resolu-
tion, as discussed in the previous section.
MICROCOMPUTER INTERFACES
The ADXL202E is specifically designed to work with low-cost
microcontrollers. Specific code sets, reference designs, and applica-
tion notes are available from the factory. This section will outline a
general design procedure and discuss the various trade-offs that
need to be considered.
The designer should have some idea of the required performance
of the system in terms of:
Resolution: the smallest signal change that needs to be detected.
Bandwidth: the highest frequency that needs to be detected.
Acquisition Time: the time that will be available to acquire the signal
on each axis.
These requirements will help to determine the accelerometer band-
width, the speed of the microcontroller clock and the length of
the T2 period.
When selecting a microcontroller it is helpful to have a counter
timer port available. The microcontroller should have provisions
for software calibration. While the ADXL202E is a highly accurate
accelerometer, it has a wide tolerance for initial offset. The easi-
est way to null this offset is with a calibration factor saved on the
microcontroller or by a user calibration for zero g. In the case
where the offset is calibrated during manufacture, there are several
options, including external EEPROM and microcontrollers with
“one-time programmable” features.
DESIGN TRADE-OFFS FOR SELECTING FILTER
CHARACTERISTICS: THE NOISE/BW TRADE-OFF
The accelerometer bandwidth selected will determine the measure-
ment resolution (smallest detectable acceleration). Filtering can be
used to lower the noise floor and improve the resolution of the
accelerometer. Resolution is dependent on both the analog filter
bandwidth at XFILT and YFILT and on the speed of the micro-
controller counter.
The analog output of the ADXL202E has a typical bandwidth
of 5 kHz, while the duty cycle modulators’ bandwidth is 500 Hz.
The user must filter the signal at this point to limit aliasing
errors. To minimize DCM errors the analog bandwidth should be
less than 1/10 the DCM frequency. Analog bandwidth may be
increased to up to 1/2 the DCM frequency in many applications.
This will result in greater dynamic error generated at the DCM.
The analog bandwidth may be further decreased to reduce noise
and improve resolution. The ADXL202E noise has the character-
istics of white Gaussian noise that contributes equally at all
frequencies and is described in terms of
µg per root Hz; i.e., the
noise is proportional to the square root of the bandwidth of the
accelerometer. It is recommended that the user limit bandwidth to
the lowest frequency needed by the application, to maximize the
resolution and dynamic range of the accelerometer.




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