ADXL150JQC Analog Devices Inc, ADXL150JQC Datasheet - Page 10

• Manufacturer Part Number: ADXL150JQC
• Description: IC ACCELEROMETER SGL 14CERPAK
• Manufacturer: Analog Devices Inc
• Series: iMEMS®r

Specifications of ADXL150JQC

• Rohs Status: RoHS non-compliant
• Axis: X or Y
• Acceleration Range: ±50g
• Sensitivity: 38mV/g
• Voltage - Supply: 4 V ~ 6 V
• Output Type: Analog
• Bandwidth: 1kHz
• Mounting Type: Surface Mount
• Package / Case: 14-CerPak
• Interface: -

DEVICE BANDWIDTH VS. MEASUREMENT
RESOLUTION
Although an accelerometer is usually specified according to its
full-scale 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 ADXL150/ADXL250 itself. The level
of the noise floor varies directly with the bandwidth of the mea-
surement. As the measurement bandwidth is reduced, the noise
floor drops, improving the signal-to-noise ratio of the measure-
ment and increasing its resolution.
The bandwidth of the accelerometer can be easily reduced by
adding low-pass or bandpass filtering. Figure 19 shows the
typical noise vs. bandwidth characteristic of the ADXL150/
ADXL250.
The output noise of the ADXL150/ADXL250 scales with the
square root of the measurement bandwidth. With a single pole
roll-off, the equivalent rms noise bandwidth is divided by 2 or
ADXL150/ADXL250
Figure 19. ADXL150/ADXL250 Noise Level vs. 3 dB
Bandwidth (Using a “Brickwall” Filter)
100m g
10m g
1m g
10
+V
0.1 F
S
C1
3dB BANDWIDTH – Hz
14
Figure 20. One-Pole Post Filter Circuit with SF and Zero g Offset Trims
SENSOR
SELF-TEST
100
(DO NOT CONNECT)
ADXL150
9
TP
5
CLOCK
COM
GAIN
AMP
SCALE FACTOR
76mV/ g
100mV/ g
200mV/ g
400mV/ g
DESIRED
OUTPUT
7
DEMODULATOR
1k
OFFSET
0.1 F
660m g
66m g
6.6m g
NULL
RANGE
F.S.
25k
+V
25 g
20 g
10 g
5 g
2
S
8
GAIN
AMP
–10–
5k
EXT
+V
2.0
2.6
5.3
10.5
2
S
BUFFER
AMP
approximately 1.6 times the 3 dB bandwidth. For example, the
typical rms noise of the ADXL150 using a 100 Hz one pole post
filter is:
Because the ADXL150/ADXL250’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 I is useful for esti-
mating the probabilities of exceeding various peak values, given
the rms value.
Nominal Peak-to-
Peak Value
2.0
4.0
6.0
6.6
8.0
RMS and peak-to-peak noise (for 0.1% uncertainty) for various
bandwidths are estimated in Figure 19. 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 ADXL150/ADXL250 typically have an rms noise level of
32 mg. When the device bandwidth is rolled off to 100 Hz, the
noise level is reduced to approximately 10 mg.
Alternatively, the signal-to-noise ratio may be improved consid-
erably by using a microprocessor to perform multiple measure-
ments and then to compute the average signal level.
Low-Pass Filtering
The bandwidth of the accelerometer can easily be reduced by using
post filtering. Figure 20 shows how the buffer amplifier can be
connected to provide 1-pole post filtering, zero g offset trimming,
and output scaling. The table provides practical component values
VALUE
200k
261k
536k
1M
R3
0 g TRIM
200k
10
rms
rms
rms
rms
rms
(OPTIONAL)
RT
Cf ( F)
100Hz
0.0082
0.0056
0.0033
0.0015
75k
R1a
FACTOR
+V
SCALE
S
Noise rms
TRIM
50k
1M
Cf ( F)
R1b
0.027
0.022
0.010
0.0056
R2
30Hz
Cf ( F)
0.082
0.056
0.033
0.015
10Hz
2
3
1mg/ Hz
OP196
Cf
+V
% of Time that Noise Will Exceed
Nominal Peak-to-Peak Value
32%
4.6%
0.27%
0.1%
0.006%
Table I.
7
4
100k
S
R3
0.1 F
6
100 1.6
V
OUT
12.25 mg
REV. 0