MEMSIC MXA2500J/K Rev.A
Page 7 of 8
2/17/2006
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.
Power supply
A
OUTX
R
C
A
OUTY
R
C
A
OUTX
Filtered
Output
A
OUTY
Filtered
Output
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. When designing
with MEMSIC ratiometric output accelerometers
(MXR2xxx series), it is highly recommended that an
external, 200 Hz low pass filter be used to eliminate
internally generated periodic noise that is coupled to the
output of the accelerometer.
The low pass filter shown in following figure has a –3dB
breakpoint given by the equation:
RC
f
π
2
1
=
. For the
200 Hz ratiometric output device filter, C=0.1
µF and
R=8k
Ω, ±5%, 1/8W.
A
OUTX
R
C
A
OUTY
R
C
A
OUTX
Filtered
O utput
A
OUTY
Filtered
O utput
Low Pass Filter
POWER SUPPLY NOISE REJECTION
A capacitor is recommended for best rejection of power
supply noise (reference following figure). The capacitor
should be located as close as possible to the device
supply pins VDA. The capacitor lead length should be as
short as possible, and surface mount capacitors are
preferred. For typical applications, capacitors C1 can be
ceramic 0.1 µF.
VDA
0.1uF
C1
Accelerometer
MEMSIC
VDD
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
ground plane should be similar to the MEMSIC
device’s footprint and 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.