ADXL210 Просмотр технического описания (PDF) - Analog Devices
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ADXL210 Datasheet PDF : 11 Pages
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ADXL202/ADXL210
initial offset. The easiest way to null this offset is with a calibra- Table IV gives typical noise output of the ADXL202/ADXL210
tion factor saved on the microcontroller or by a user calibration
for zero g. In the case where the offset is calibrated during manu-
for various CX and CY values.
facture, there are several options, including external EEPROM
and microcontrollers with “one-time programmable” features.
Table IV. Filter Capacitor Selection, CX and CY
Peak-to-Peak Noise
DESIGN TRADE-OFFS FOR SELECTING FILTER
CHARACTERISTICS: THE NOISE/BW TRADE-OFF
Estimate 95%
Bandwidth CX, CY rms Noise Probability (rms ؋ 4)
The accelerometer bandwidth selected will determine the mea-
surement resolution (smallest detectable acceleration). Filtering 10 Hz
0.47 µF 1.9 mg
7.6 mg
can be used to lower the noise floor and improve the resolution
of the accelerometer. Resolution is dependent on both the ana-
50 Hz
100 Hz
0.10 µF 4.3 mg
0.05 µF 6.1 mg
17.2 mg
24.4 mg
log filter bandwidth at XFILT and YFILT and on the speed of the
microcontroller counter.
The analog output of the ADXL202/ADXL210 has a typical
bandwidth of 5 kHz, much higher than the duty cycle stage is
capable of converting. The user must filter the signal at this
point to limit aliasing errors. To minimize DCM errors the
E 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 dy-
namic error generated at the DCM.
T The analog bandwidth may be further decreased to reduce noise
and improve resolution. The ADXL202/ADXL210 noise has
the characteristics of white Gaussian noise that contributes
E 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 band-
width of the accelerometer. It is recommended that the user limit
bandwidth to the lowest frequency needed by the application, to
L maximize the resolution and dynamic range of the accelerometer.
With the single pole roll-off characteristic, the typical noise of
the ADXL202/ADXL210 is determined by the following equation:
( ) O Noise
rms
=
500
µg
/
Hz
×
BW
×
1.5
At 100 Hz the noise will be:
( ) S Noise
rms
= 500 µg /
Hz
×
100
×
(1.5)
=
6.12
mg
Often the peak value of the noise is desired. Peak-to-peak noise
can only be estimated by statistical methods. Table III is useful
B for estimating the probabilities of exceeding various peak values,
given the rms value.
O Table III. Estimation of Peak-to-Peak Noise
200 Hz
500 Hz
0.027 µF 8.7 mg
0.01 µF 13.7 mg
35.8 mg
54.8 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 ADXL202/ADXL210’s duty cycle converter has a resolu-
tion of approximately 14 bits; better resolution than the acceler-
ometer itself. The actual resolution of the acceleration signal is,
however, 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’s counter. It is probable
that the accelerometer’s noise floor may set the lower limit on
the resolution, as discussed in the previous section.
Table V. Trade-Offs Between Microcontroller Counter Rate,
T2 Period and Resolution of Duty Cycle Modulator
RSET
T2 (ms) (k⍀)
ADXL202/ Counter-
ADXL210 Clock
Sample Rate
Rate
(MHz)
Counts
per T2
Cycle
Counts Resolution
per g (mg)
1.0
124 1000
2.0
1.0
124 1000
1.0
1.0
124 1000
0.5
5.0
625 200
2.0
5.0
625 200
1.0
5.0
625 200
0.5
10.0 1250 100
2.0
10.0 1250 100
1.0
2000 250 4.0
1000 125 8.0
500 62.5 16.0
10000 1250 0.8
5000 625 1.6
2500 312.5 3.2
20000 2500 0.4
10000 1250 0.8
% of Time that Noise
10.0 1250 100
0.5
5000 625 1.6
Nominal Peak-to-Peak Will Exceed Nominal
Value
Peak-to-Peak Value
2.0 × rms
4.0 × rms
6.0 × rms
8.0 × rms
32%
4.6%
0.27%
0.006%
The peak-to-peak noise value will give the best estimate of the
uncertainty in a single measurement.
–8–
REV. B
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