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ADXL05 Datasheet(PDF) 13 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) 13 Page - Analog Devices

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ADXL05
–13–
REV. B
The equivalent rms noise of the bandpass filter is equal to
500
µg / Hz × (1.5 F
H )– ( FL /1.5) .
For example, the typical rms noise of the ADXL05 using 1 pole
ac coupling with a bandwidth of 10 Hz and 1 pole low-pass
filter of 100 Hz is:
Noise (rms )
= 500 µg / Hz × 1.5(100) – (10 /1.5)
= 5,987 µg rms or ≈ 5.9 mg rms
Because the ADXL05’s noise is for all practical purposes
Gaussian in amplitude distribution, the highest noise amplitudes
have the smallest (yet nonzero) probability. Peak-to-peak noise
is therefore difficult to measure and can only be estimated due
to its statistical nature. Table II is useful for estimating the
probabilities of exceeding various peak values, given the rms
value.
Table II.
Nominal Peak-to-
% of Time that Noise Will Exceed
Peak Value
Nominal Peak-to-Peak Value
2.0
× rms
32%
4.0
× rms
4.6%
6.0
× rms
0.27%
6.6
× rms
0.1%
8.0
× rms
0.006%
RMS and peak-to-peak noise (for 0.1% uncertainty) for various
bandwidths is estimated in Figure 24. As shown by the figure,
device noise drops dramatically as the operating bandwidth is
reduced. For example, when operated in a 1 kHz bandwidth,
the ADXL05 typically has an rms noise level of 19 mg. With
±5 g applied accelerations, this 19 mg resolution limit is nor-
mally quite satisfactory; but for smaller acceleration levels the
noise is now a much greater percentage of the signal. As shown
by the figure, when the device bandwidth is rolled off to 100 Hz,
the noise level is reduced to approximately 6 mg, and at 10 Hz it
is down to less than 2 mg.
Alternatively, the signal-to-noise ratio may be improved consid-
erably by using a microprocessor to perform multiple measure-
ments and then compute the average signal level. When using
this technique, with 100 measurements, the signal-to-noise ratio
will be increased by a factor of 10 (20 dB).
DEVICE BANDWIDTH VS. MEASUREMENT
RESOLUTION
Although an accelerometer is usually specified according to its
full scale (clipping) g level, the limiting resolution of the device,
i.e., its minimum discernible input level, is extremely important
when measuring low g accelerations.
The limiting resolution is predominantly set by the measure-
ment noise “floor” which includes the ambient background
noise and the noise of the ADXL05 itself. The level of the noise
floor varies directly with the bandwidth of the measurement. As
the measurement bandwidth is reduced, the noise floor drops,
improving the signal-to-noise ratio of the measurement and in-
creasing its resolution.
The bandwidth of the accelerometer can be easily reduced by
adding low-pass or bandpass filtering. Figure 24 shows the typi-
cal noise vs. bandwidth characteristic of the ADXL05.
3dB BANDWIDTH – Hz
100mg
1mg
10mg
10
1k
100
660mg
66mg
6.6mg
Figure 24. Noise Level vs. 3 dB Bandwidth
The output noise of the ADXL05 scales with the square root of
the measurement bandwidth. With a single pole roll-off, the
equivalent rms noise bandwidth is
π divided by 2 or approxi-
mately 1.5 times the 3 dB bandwidth. For example, the typical
rms noise of the ADXL05J using a 100 Hz one pole post filter is:
Noise (rms )
= 500 µg/ Hz × 100(1.5) = 6,124 µgor 6.1 mg rms
For the bandpass filter of Figure 27 where both ac coupling and
low pass filtering are used, the low frequency roll-off, FL, is de-
termined by C4 and R1 and the high frequency roll-off, FH, is
determined by the 1-pole post filter R3, C5.
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