ADXL105JQC Analog Devices Inc, ADXL105JQC Datasheet - Page 7

• Manufacturer Part Number: ADXL105JQC
• Description: IC ACCELEROMETER SGL 14CERPAK
• Manufacturer: Analog Devices Inc
• Series: iMEMS®r

Specifications of ADXL105JQC

• Rohs Status: RoHS non-compliant
• Axis: X, Z
• Acceleration Range: ±7g
• Sensitivity: 250mV/g
• Voltage - Supply: 2.7 V ~ 5.25 V
• Output Type: Analog
• Bandwidth: 12kHz
• Mounting Type: Surface Mount
• Package / Case: 14-CerPak
• Interface: -

Output Scaling
The acceleration output (A
250 mV/g. This scale factor may not be appropriate for all appli-
cations. The UCA may be used to increase the scale factor. The
simplest implementation would be as shown in Figure 14a.
Since the 0 g offset of the ADXL105 is 2.5 V
gain of greater than 4 could result in having the UCA output at
0 V or 5 V at 0 g. The solution is to add R3 and VR1, as shown
in Figure 14b, turning the UCA into a summing amplifier. VR1
is adjusted such that the UCA output is V
Device Bandwidth vs. Resolution
In general the bandwidth selected will determine the noise floor
and hence, the measurement resolution (smallest detectable
acceleration) of the ADXL105. Since the noise of the ADXL105
has the characteristic of white Gaussian noise that contributes
equally at all frequencies, the noise amplitude may be reduced
by simply reducing the bandwidth. So the typical noise of the
ADXL105 is:
Where
K
K
*For other corner frequencies, consult an active filter handbook.
REV. A
1.6 for a single-pole filter
1.4 for a 2-pole filter
Noise (rms) = (225 g/ Hz)
IN
IN
IN
Figure 15. UCA Used as Active Filters*
IN
C
0.39 F
20k
d. 2-Pole Bessel High-Pass Filter
b. 2-Pole Bessel Low-Pass Filter
R2
R2
R3
V
V
MID
c. 1-Pole High-Pass Filter
a. 1-Pole Low-Pass Filter
MID
20k
0.39 F
V
MID
V
R1
V
MID
MID
C
OUT
0.18 F
59k
R1
44.2k
0.22 F
) of the ADXL105 is nominally
OUT
OUT
Bandwidth
DD
f
GAIN = – R1
–3dB
/2 at 0 g.
f
f
GAIN = – R1
f
R3
–3dB
–3dB
–3dB
=
~ ~
625 mV, using a
OUT
OUT
2 CR1
2.5 R1
= 30Hz
=
= 10Hz
R2
1
2 CR2
R2
1
K)
–7–
So given a bandwidth of 1000 Hz, the typical rms noise floor of
an ADLX105 will be:
Noise = (225 g/ Hz)
and
Noise = (225 g/ Hz)
Often the peak value of the noise is desired. Peak-to-peak noise
can only be estimated by statistical means. Table I may be used
for estimating the probabilities of exceeding various peak values
given the rms value. The peak-to-peak noise value will give the
best estimate of the uncertainty in a single measurement.
Nominal Peak-to-
Peak Value
2
3
4
5
6
7
8
The UCA may be configured to act as an active filter with gain
and 0 g offset control as shown in Figure 16.
EMC and Electrical Noise
The design of the ADXL105 is such that EMI or magnetic
fields do not normally affect it. Since the ADXL105 is ratiomet-
ric, conducted electrical noise on V
This is particularly true for noise at the ADXL105’s internal
clock frequency (200 kHz) and its odd harmonics. So maintain-
ing a clean supply voltage is key in preserving the low noise and
high resolution properties of the ADXL105.
One way to ensure that V
to add an R-C low-pass filter near the V
Figure 17. Using the component values shown in Figure 17,
noise at 200 kHz is attenuated by approximately –23 dB. As-
suming the ADXL105 consumes 2 mA, there will be a 100 mV
drop across R1. This can be neglected simply by using the
ADXL105’s V
shown in Figure 17.
Figure 16. UCA Configured as an Active Low-Pass Filter
with Gain and Offset
rms
rms
rms
rms
rms
rms
rms
= 9 mg rms for a single-pole filter
= 8.4 mg rms for 2-pole filter
IN
Table I. Estimation of Peak-to-Peak Noise
47k
DD
as the A-to-D converter’s reference voltage as
47k
10k
V
DD
( 1000
( 1000
DD
47k
contains no high frequency noise is
0.1 F
0.1 F
% of Time that Noise Will
Exceed Peak-to-Peak Value
32%
13%
4.6%
1.2%
0.27%
0.047%
0.0063%
1.6)
1.4)
DD
100k
does affect the output.
DD
pin as shown in
GAIN = 2
f
ADXL105
–3dB
OUT
= 30Hz