MEMSIC MXA6500E Rev.B
Page 4 of 6
3/22/2005
PIN DESCRIPTIONS
VDD – This is the supply input for the circuits and the
sensor heater in the accelerometer. The DC voltage should
be between 2.7 and 3.6 volts. Refer to the section on PCB
layout and fabrication suggestions for guidance on external
parts and connections recommended.
COM– This is the ground pin for the accelerometer.
TP- This pin should be connected to the ground.
Xout – This pin is the output of the x-axis acceleration
sensor. The user should ensure the load impedance is
sufficiently high as to not source/sink >100
µA. While the
sensitivity of this axis has been programmed at the factory
to be the same as the sensitivity for the y-axis, the
accelerometer
can be programmed for non-equal sensitivities
on the x- and y-axes. Contact the factory for additional
information.
Yout –
This pin is the output of the y-axis acceleration
sensor. The user should ensure the load impedance is
sufficiently high as to not source/sink >100
µA.
While the sensitivity of this axis has been programmed at
the factory to be the same as the sensitivity for the x-axis,
the accelerometer can be programmed for non-equal
sensitivities on the x- and y-axes. Contact the factory for
additional information.
PD – Pin 1 is the power down control pin. Pull this pin HIGH
will put the accelerometer into power down mode. When the part
goes into power down mode, the total current will be smaller than
0.1uA at 3V.
In normal operation mode, this pin should be connected to
Ground.
COMPENSATION FOR THE CHANGE IN
SENSITIVITY OVER TEMPERATURE
All thermal accelerometers display the same sensitivity
change with temperature. The sensitivity change depends
on variations in heat transfer that are governed by the laws
of physics. The sensitivity change is governed by the
following equation (and shown in following figure in
°C):
Si x Ti
3.40 = S
f x Tf
3.40
where Si is the sensitivity at any initial temperature Ti, and
Sf is the sensitivity at any other final temperature Tf with
the temperature values in
°K.
0.0
0.5
1.0
1.5
2.0
2.5
-40
-20
0
20406080
100
Temperature (C)
Thermal Accelerometer Sensitivity
In gaming applications where the game or controller is
typically used in a constant temperature environment,
sensitivity might not need to be compensated in hardware
or software. Any compensation for this effect could be
done instinctively by the game player.
For applications where sensitivity changes of a few percent
are acceptable, the above equation can be approximated
with a linear function. Using a linear approximation, an
external circuit that provides a gain adjustment of –1.1%/
°C
would keep the sensitivity within 10% of its room
temperature value over a 0
°C to +50°C range.
For applications that demand high performance, a low cost
micro-controller can be used to implement the above
equation. A reference design using a Microchip MCU (p/n
16F873/04-SO) and MEMSIC developed firmware is
available by contacting the factory. With this reference
design, the sensitivity variation over the full temperature
range (-40
°C to +85°C) can be kept below 3%. Please visit
the MEMSIC web site at www.memsic.com for reference
design information on circuits and programs including look
up tables for easily incorporating sensitivity compensation.
DISCUSSION OF TILT APPLICATIONS AND
RESOLUTION
Tilt Applications: One of the most popular applications of
the MEMSIC accelerometer product line is in
tilt/inclination measurement. An accelerometer uses the
force of gravity as an input to determine the inclination
angle of an object.
A MEMSIC accelerometer is most sensitive to changes in
position, or tilt, when the accelerometer’s sensitive axis is
perpendicular to the force of gravity, or parallel to the
Earth’s surface. Similarly, when the accelerometer’s axis is
parallel to the force of gravity (perpendicular to the Earth’s
surface), it is least sensitive to changes in tilt.
Following table and figure help illustrate the output
changes in the X- and Y-axes as the unit is tilted from +90
°
to 0
°. Notice that when one axis has a small change in
output per degree of tilt (in mg), the second axis has a large
change in output per degree of tilt. The complementary
nature of these two signals permits low cost accurate tilt