MXA2500EL Datasheet MXA2500EL | P4-P6

MEMSIC MXA2500E Rev G Page 4 of 8 2/26/2007

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 accelerometer, one to

measure acceleration in the x-axis and one to measure

acceleration in the y-axis. Please visit the MEMSIC

website at www.memsic.com for a picture/graphic

description of the free convection heat transfer principle.

PIN DESCRIPTIONS

– This is the supply input for the digital circuits and

the sensor heater in the accelerometer. The DC voltage

should be between 3.0 volts and 5.25 volts. Refer to the

section on PCB layout and fabrication suggestions for

guidance on external parts and connections recommended.

– This is the power supply input for the analog

amplifiers in the accelerometer. 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.

OUTX

– 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 >100µA. 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 on this feature.

OUTY

– 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 >100µA. 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 on this feature.

OUT

– This pin is the buffered output of the temperature

sensor. The analog voltage at T

OUT

is an indication of the

die temperature. This voltage is useful as a differential

measurement of temperature from ambient and not as an

absolute measurement of temperature. After correlating

the voltage at TOUT to 25°C ambient, the change in this

voltage due to changes in the ambient temperature can be

used to compensate for the change over temperature of the

accelerometer offset and sensitivity. Please refer to the

section on Compensation for the Change in Sensitivity

Over Temperature for more information.

Sck – The standard product is delivered with an internal

clock option (800kHz). This pin should be grounded

when operating with the internal clock. An external

clock option can be special ordered from the factory

allowing the user to input a clock signal between 400kHz

and 1.6MHz.

ref

– This pin is the output of a reference voltage. It is set

at 2.50V typical and has 100µA of drive capability.

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. Manufacturing variations do not influence the

sensitivity change, so there are no unit-to-unit differences

in sensitivity change. The sensitivity change is governed

by the following equation (and shown in Figure 1 in °C):

x T

2.90

= S

x T

2.90

where S

is the sensitivity at any initial temperature T

, and

is the sensitivity at any other final temperature T

with

the temperature values in °K.

0.0

0.5

1.0

1.5

2.0

2.5

-40 -20 0 20 40 60 80 100

Temperature (C)

Sensitivity (normalized)

Figure 1: Thermal Accelerometer Sensitivity

MEMSIC MXA2500E Rev G Page 5 of 8 2/26/2007

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 –

0.9%/°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 +105°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.

Table 1 and Figure 2 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 sensing to

be achieved with the MEMSIC device (reference

application note AN-00MX-007).

Top View

+90

gravity

Figure 2: Accelerometer Position Relative to Gravity

X-Axis Y-Axis

X-Axis

Orientation

To Earth’s

Surface

(deg.)

Output

(g)

Change

per deg.

of tilt

(mg)

Output

(g)

Change

per deg.

of tilt

(mg)

90 1.000 0.15 0.000 17.45

85 0.996 1.37 0.087 17.37

80 0.985 2.88 0.174 17.16

70 0.940 5.86 0.342 16.35

60 0.866 8.59 0.500 15.04

45 0.707 12.23 0.707 12.23

30 0.500 15.04 0.866 8.59

20 0.342 16.35 0.940 5.86

10 0.174 17.16 0.985 2.88

5 0.087 17.37 0.996 1.37

0 0.000 17.45 1.000 0.15

Table 1: Changes in Tilt for X- and Y-Axes

Resolution: The accelerometer resolution is limited by

noise. The output noise will vary with the measurement

bandwidth. With the reduction of the bandwidth, by

applying an external low pass filter, the output noise drops.

Reduction of bandwidth will improve the signal to noise

ratio and the resolution. The output noise scales directly

with the square root of the measurement bandwidth. The

maximum amplitude of the noise, its peak- to- peak value,

approximately defines the worst case resolution of the

measurement. With a simple RC low pass filter, the rms

noise is calculated as follows:

Noise (mg rms) = Noise(mg/ Hz ) *

)6.1*)(( HzBandwidth

The peak-to-peak noise is approximately equal to 6.6 times

the rms value (for an average uncertainty of 0.1%).

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

MEMSIC MXA2500E Rev G Page 6 of 8 2/26/2007

equation:

= . 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.

OUT X

OUT Y

OUT X

Filtered

Output

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:

= . For the 200 Hz ratiometric output device

filter, C=0.1µF and R=8kΩ, ±5%, 1/8W.

OUT X

OUTY

OUTX

Filtered

Output

OUT Y

Filtered

Output

Figure 4: Low Pass Filter

COMPENSATION FOR EXTENDING THE

FREQUENCY RESPONSE

The response of the thermal accelerometer is a function of

the internal gas physical properties, the natural convection

mechanism and the sensor electronics. Since the gas

properties of MEMSIC's mass produced accelerometer are

uniform, a simple circuit can be used to equally

compensate all sensors. For most applications, the

compensating circuit does not require adjustment for

individual units.

A simple compensating network comprising two

operational amplifiers and a few resistors and capacitors

provides increasing gain with increasing frequency (see

Figure 5). The circuit shown is for an absolute output

accelerometer operating at 5 V supply. It provides a DC

gain of X2, so the offset at the output is 2.5V and the

sensitivity is doubled. The 14.3 KΩ and the 5.9KΩ

resistors along with the non-polarized 0.82µF capacitors

tune the gain of the network to compensate for the output

attenuation at the higher frequencies. The resistors and the

capacitors provide noise reduction and stability.

Figure 5: Frequency Response Extension Circuit

The accelerometer response (bottom trace), the network

response (top trace) and the compensated response (middle

trace) are shown in Figure 6. The amplitude remains above

–3db beyond 100 Hz, and there is useable signal well

after this frequency.

-60

-45

-30

-15

1 0 10 0 10 0 0

F req u e ncy - H z

Amplitude - dB

8.06K

5.9K

160K

1.5uF

0.01uF

8.06K

14.3K

0.01uF

0.047uF

5.9K

1.5uF

0.047uF

14.3K

0.0022uF

Aout X or Y

Freq. Comp. Output

P1-P3 P4-P6 P7-P8

MXA2500EL

Mfr. #:

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Accelerometers Ultra Low Noise, Offset Drift 1 g Dual Axis Accelerometer with Analog Outputs

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