MXR7900DF datasheet(4/6 Pages) MEMSIC

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MEMSIC MXR7900A/C/D Rev.A

Page 4 of 6

4/09/2009

THEORY OF OPERATION

The MEMSIC device is a complete dual-axis acceleration

measurement system fabricated on a monolithic CMOS IC

process. The device operation is based on heat transfer by

natural convection and operates like other accelerometers

having a proof mass except it is a gas in the MEMSIC

sensor.

A single heat source, centered in the silicon chip is

suspended across a cavity. Equally spaced

aluminum/polysilicon thermopiles (groups of

thermocouples) are located equidistantly on all four sides of

the heat source (dual axis). Under zero acceleration, a

temperature gradient is symmetrical about the heat source,

so that the temperature is the same at all four thermopiles,

causing them to output the same voltage.

Acceleration in any direction will disturb the temperature

profile, due to free convection heat transfer, causing it to be

asymmetrical. The temperature, and hence voltage output

of the four thermopiles will then be different. The

differential voltage at the thermopile outputs is directly

proportional to the acceleration. There are two identical

acceleration signal paths on the MXR7900A/C/D, one to

measure acceleration in the x-axis and one to measure

acceleration in the Y-axis. For more details visit the

MEMSIC website at www.memsic.com for a

picture/graphic description of the free convection heat

transfer principle.

MXR7900A/C/D PIN DESCRIPTIONS

VDD – This is the supply input for the circuits and the sensor

heater in the accelerometer. The DC voltage should be

between 4.5 and 5.25 volts. Refer to the section on PCB

layout and fabrication suggestions for guidance on external

parts and connections recommended.

GND– 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 typical.

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 typical.

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.

CLK – This is an optional serial clock input. The standard

product is delivered with an internal clock (1000 kHz.

However, an external clock between 400 kHz and 1.6 MHz

can be used as an option, if the sensor is programmed from

the factory to run in external clock mode.

This pin is grounded internally with a 50 Kohm resistor if

an external clock is used it should be able to drive this load.

But if external clock is not used, in order to minimize noise

and ESD this pin should be grounded externally as well.

STIN– Self –Test Input

This pin controls the self-test function of the sensor

Bringing STIN high will cause a negative deflection around

-1g signal to the Xout and Yout from the 0g value.

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 m

g), 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 sensing to be

achieved with the MEMSIC device (reference application

note AN-00MX-007).

X-Axis

Y-Axis

X-Axis

Orientation

To Earth’s

Surface

(deg.)

X Output

(

Change

per deg.

of tilt

Y Output

(

Change

per deg.

of tilt

-1.000

0.15

0.000

17.45

-0.996

1.37

0.087

17.37

-0.985

2.88

0.174

17.16

-0.940

5.86

0.342

16.35

-0.866

8.59

0.500

15.04

-0.707

12.23

0.707

12.23

-0.500

15.04

0.866

8.59

-0.342

16.35

0.940

5.86

-0.174

17.16

0.985

2.88

-0.087

17.37

0.996

1.37

0.000

17.45

1.000

0.15

Changes in Tilt for X- and Y-Axes

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