MEMSIC MXA2500G/M Rev. E
Page 7 of 8
1/19/2005
EXTERNAL FILTERS
AC Coupling: For applications where only dynamic
accelerations (vibration) are to be measured, it is recommended
to ac couple the accelerometer output as shown in Figure 3.
The advantage of ac coupling is that variations from part to
part of zero g offset and zero g offset versus temperature can
be eliminated. Figure 3 is a HPF (high pass filter) with a –3dB
breakpoint given by the equation:
RC
f
π
2
1
=
. In many
applications it may be desirable to have the HPF –3dB point at
a very low frequency in order to detect very low frequency
accelerations. Sometimes the implementation of this HPF may
result in unreasonably large capacitors, and the designer must
turn to digital implementations of HPFs where very low
frequency –3dB breakpoints can be achieved.
A
OUTX
R
C
A
OUTY
R
C
A
OUTX
Filtered
Output
A
OUTY
Filtered
Output
Figure 3: High Pass Filter
Low Pass Filter: An external low pass filter is useful in low
frequency applications such as tilt or inclination. The low pass
filter limits the noise floor and improves the resolution of the
accelerometer. The low pass filter shown in Figure 4 has a –
3dB breakpoint given by the equation:
RC
f
π
2
1
=
. For the
200 Hz absolute output device filter, C=0.2
µF and R=39kΩ,
±5%, 1/8W.
A
OUTX
R
C
A
OUTY
R
C
A
OUTX
Filtered
Output
A
OUTY
Filtered
Output
Figure 4: Low Pass Filter
POWER SUPPLY NOISE REJECTION
Two capacitors and a resistor are recommended for best
rejection of power supply noise (reference Figure 5 below).
The capacitors should be located as close as possible to the
device supply pins (VDA, VDD). The capacitor lead length
should be as short as possible, and surface mount capacitors are
preferred. For typical applications, capacitors C1 and C2 can
be ceramic 0.1 µF, and the resistor R can be 10 Ω.
R
MEMSIC
Accelerometer
VDA
C1
C2
VDD
V SUPPLY
Figure 5: Power Supply Noise Rejection
PCB LAYOUT AND FABRICATION SUGGESTIONS
1. The Sck pin should be grounded to minimize noise.
2. Liberal use of ceramic bypass capacitors is recommended.
3. Robust low inductance ground wiring should be used.
4. Care should be taken to ensure there is “thermal
symmetry” on the PCB immediately surrounding the
MEMSIC device and that there is no significant heat
source nearby.
5. A metal ground plane should be added directly beneath the
MEMSIC device. The size of the plane should be similar
to the MEMSIC device’s footprint and be as thick as
possible.
6. Vias can be added symmetrically around the ground plane.
Vias increase thermal isolation of the device from the rest
of the PCB.
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