MXA2500EL - Memsic Inc. - Datasheet.Live

MEMSIC MXA2500E Rev G Page 1 of 9 2/26/2007

Ultra Low Noise, Offset Drift

±±

±1 g Dual Axis Accelerometer

with Analog Outputs

MXA2500E

FEATURES

Better than 1 mg resolution

Dual axis accelerometer fabricated on a monolithic CMOS IC

RoHS compliant

On-chip mixed mode signal processing

No moving parts

50,000 g shock survival rating

17 Hz bandwidth expandable to >160 Hz

3V to 5.25V single supply continuous operation

Small (5mm x 5mm x 2mm) surface mount package

Continuous self test

Custom programmable specifications

Independent axis programmability (special order)

APPLICATIONS

Automotive – Vehicle Security/Active Suspension/ABS

Headlight Angle Control/Tilt Sensing

Security – Gas Line/Elevator/Fatigue Sensing

Office Equipment – Computer Peripherals/PDA’s/

Cell Phones

Internal

Oscillator

Sck

(optional)

CLK

Heater

Control

X axis

Y axis

Factory Adjust

Offset & Gain

Low Pass

Filter

Low Pass

Filter

Temperature

Sensor

Voltage

Reference VREF

AOUTX

VDD VDA

Gnd

2-AXIS

SENSOR

AOUTY

TOUT

Continous

Self Test

MXA2500E FUNCTIONAL BLOCK DIAGRAM

Gaming – Joystick/RF Interface/Menu Selection/Tilt Sensing

White Goods – Spin/Vibration Control

GENERAL DESCRIPTION

The MXA2500E is an ultra low noise and low cost, dual

axis accelerometer fabricated on a standard, submicron

CMOS process. It is a complete sensing system with on-

chip mixed mode signal processing. The MXA2500E

measures acceleration with a full-scale range of ±1 g and a

sensitivity of 500mV/g @5V at 25°C. It can measure both

dynamic acceleration (e.g., vibration) and static

acceleration (e.g., gravity). The MXA2500E design is

based on heat convection and requires no solid proof mass.

This eliminates stiction and particle problems associated

with competitive devices and provides shock survival of

50,000 g, leading to significantly lower failure rates and

lower loss due to handling during assembly.

The MXA2500E provides two absolute analog outputs.

The typical noise floor is 0.2 mg/Hz allowing signals

below 1 mg to be resolved at 1 Hz bandwidth. The 3dB

rolloff of the device occurs at 17 Hz but is expandable to

>160 Hz (ref. Application Note AN-00MX-003). The

MXA2500E is available in a LCC surface mount package

(5 mm x 5 mm x 2 mm). It is hermetically sealed and is

operational over a -40°C to +105°C temperature range. It

also contains an on-chip temperature sensor and a bandgap

voltage reference.

Due to the standard CMOS structure of the MXA2500E,

additional circuitry can easily be incorporated into custom

versions for high volume applications. Contact the factory

for more information.

Information furnished by MEMSIC is believed to be accurate and reliable.

However, no responsibility is assumed by MEMSIC for its use, nor for any

infringements of patents or other rights of third parties which may result from

its use. No license is granted by implication or otherwise under any patent or

patent rights of MEMSIC.





MEMSIC, Inc.

One Technology Drive Suite 325,Andover MA01810,USA

Tel: +1 978 738 0900 Fax: +1 978 738 0196

www.memsic.com

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

MXA2500E SPECIFICATIONS (Measurements @ 25°C, Acceleration = 0 g unless otherwise noted; VDD, VDA = 5.0V

unless otherwise specified)

Parameter

Conditions

Min

MXA2500E

Typ

Max

Units

SENSOR INPUT

Measurement Range1

Each Axis

±1.0

Nonlinearity Best fit straight line 0.5 1.0 % of FS

Alignment Error2 ±1.0 degree

Transverse Sensitivity3 ±2.0 %

SENSITIVITY

Sensitivity, Analog Outputs at pins

AOUTX and AOUTY6

Each Axis

475

500

525

mV/g

Change over Temperature (uncompensated)4 ∆ from 25°C, at –40°C +120 %

∆ from 25°C, at +105°C -55 %

Change over Temperature (compensated) 4 ∆ from 25°C, –40°C to +105°C <3.0 %

ZERO g BIAS LEVEL

0 g Offset6

Each Axis

-0.1

0.00

+0.1

0 g Voltage6 1.20 1.25 1.30 V

0 g Offset over Temperature ∆ from 25°C

∆ from 25°C, based on 500mV/g

±0.4

±0.2

mg/°C

mV/°C

NOISE PERFORMANCE

Noise Density, rms

Without frequency compensation

0.2

0.4

mg/Hz

FREQUENCY RESPONSE

3dB Bandwidth - uncompensated 17 Hz

3dB Bandwidth - compensated5 >160 Hz

TEMPERATURE OUTPUT

Tout Voltage 1.15 1.25 1.35 V

Sensitivity 4.6 5.0 5.4 mV/°C

VOLTAGE REFERENCE OUTPUT

VRef output @3V-5.25V supply 2.4 2.5 2.65 V

Change over Temperature 0.1 mV/°C

Current Drive Capability Source 100 µA

SELF TEST

Continuous Voltage at AOUTX, AOUTY under

Failure

@5.0V Supply, output rails to

supply voltage

5.0

Continuous Voltage at AOUTX, AOUTY under

Failure

@3.0V Supply, output rails to

supply voltage

3.0

AOUTX and AOUTY OUTPUTS

Normal Output Range @5.0V Supply

@3.0V Supply

0.1

4.9

2.9

Current Source or sink, @ 3.0V-5.25V supply

100 µA

Turn-On Time7 @5.0V Supply

@3.0V Supply

160

300

POWER SUPPLY

Operating Voltage Range 3.0 5.25 V

Supply Current @ 5.0V 2.7 3.3 4.1 mA

Supply Current6 @ 3.0V 3.2 4.0 4.8 mA

TEMPERATURE RANGE

Operating Range -40 +105 °C

NOTES

1 Guaranteed by measurement of initial offset and sensitivity.

2 Alignment error is specified as the angle between the true and indicated

axis of sensitivity.

3 Transverse sensitivity is the algebraic sum of the alignment and the

inherent sensitivity errors.

4 The sensitivity change over temperature for thermal accelerometers is based

on variations in heat transfer that are governed by the laws of physics and it is

highly consistent from device to device. Please refer to the section in this data

sheet titled “Compensation for the Change of Sensitivity over Temperature”

for more information.

5 External circuitry is required to extend the 3dB bandwidth. (ref.

Application Note: AN-00MX-003).

6 The device operates over a 3.0V to 5.25V supply range. Please note that

sensitivity and zero g bias level will be slightly different at 3.0V operation.

For devices to be operated at 3.0V in production, they can be trimmed at the

factory specifically for this lower supply voltage operation, in which case the

sensitivity and zero g bias level specifications on this page will be met.

Please contact the factory for specially trimmed devices for low supply

voltage operation.

7 Output settled to within ±17mg.

MEMSIC MXA2500E Rev G Page 3 of 8 2/26/2007

ABSOLUTE MAXIMUM RATINGS*

Supply Voltage (VDD, VDA) ………………...-0.5 to +7.0V

Storage Temperature ……….…………-65°C to +150°C

Acceleration ……………………………………..50,000 g

*Stresses above those listed under Absolute Maximum Ratings may cause permanent

damage to the device. This is a stress rating only; the functional operation of the device

at these or any other conditions above those indicated in the operational sections of this

specification is not implied. Exposure to absolute maximum rating conditions for

extended periods may affect device reliability.

Package Characteristics

Package θ

θθ

θJA θ

θθ

θJC Device Weight

LCC-8 110°C/W 22°C/W < 1 gram

Pin Description: LCC-8 Package

Pin Name Description I/O

1 TOUT Temperature (Analog Voltage) O

2 AOUTY Y-Axis Acceleration Signal O

3 Gnd Ground I

4 VDA Analog Supply Voltage I

5 AOUTX X-Axis Acceleration Signal O

6 Vref 2.5V Reference Output O

7 Sck Optional External Clock I

8 VDD Digital Supply Voltage I

Ordering Guide

Model Package Style

MXA2500EL LCC8 RoHS compliant

MXA2500EF LCC8, Pb-free RoHS compliant

*LCC parts are shipped in tape and reel packaging.

Caution

ESD (electrostatic discharge) sensitive device.

Top View

M E M S IC

X +g

Y +g

Note: The MEMSIC logo’s arrow indicates the +X sensing

direction of the device. The +Y sensing direction is rotated 90°

away from the +X direction following the right-hand rule.

Small circle indicates pin one (1).

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

VDD – 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.

VDA – 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.

AOUTX – 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.

AOUTY – 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.

TOUT – This pin is the buffered output of the temperature

sensor. The analog voltage at TOUT 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.

Vref – 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):

Si x Ti

2.90 = Sf x Tf

2.90

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 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

+900

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: RC

=. 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.

AOUT X

AOUT Y

AOUT X

Filtered

Output

AOUTY

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.

AOUT X

AOUTY

AOUTX

Filtered

Output

AOUT 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 1 0 0 1 0 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

MEMSIC MXA2500E Rev G Page 7 of 8 2/26/2007

Figure 6: Amplitude Frequency Response

COMPENSATION FOR ZERO G OFFSET CHANGE

OVER TEMPERATURE

In applications where a stable zero g offset is required, and

where the AC coupling external filter described earlier can

not be used, analog or digital temperature compensation

can be applied. The compensation requires individual

calibration because the magnitude of the zero g offset

change over temperature is different for each unit. To

compensate the change, a calibrated temperature

dependent signal equal in magnitude but with opposite

polarity is added to the accelerometer output. The circuit

in Figure 7 shows a circuit example applying an analog

linear compensation technique. In this circuit the

accelerometer temperature sensor output is added to or

subtracted from the accelerometer output. The calibration

sequence is: start at room temperature with the 100K pot

set so that its wiper is at Vref. Next, soak the accelerometer

at the expected extreme temperature and observe the

direction of the change. Then set the switch to the non-

inverting input if the change is negative or vice versa.

Finally, adjust the 100K pot while monitoring the circuit

output, until the zero g offset change is removed.

+5V

100K

Aoutx or y

zero g drift

compensated

SW SPDT

100K

Aoutx or y

100K

10K

100K

10K

Tout

Vref

10K

Figure 7: Zero g Offset Temperature Compensation Circuit

Various digital compensation techniques can be applied

using a similar concept. Digital techniques can provide

better compensation because they can compensate for non-

linear zero g offset vs. temperature. A micro-controller or

micro-processor would perform the compensation. The

acceleration signal and the temperature signal would be

digitized using an analog to digital converter. Like in the

analog compensation, the first step is to test and

characterize the zero g change. The purpose of the

characterization is to create a look up table or to estimate a

mathematical representation of the change. For example,

the change could be characterized by an equation of the

form:

Change = a * Temperature 2 + b * Temperature + c

where a,b,c are unique constants for each accelerometer.

In normal operation the processor calculates the output:

Compensated Output = Acceleration – Change.

For a more detail discussion of temperature compensation

reference MEMSIC application note #AN-00MX-002.

TEMPERATURE OUTPUT NOISE REDUCTION

It is recommended that a simple RC low pass filter is used

when measuring the temperature output. Temperature

output is typically a very slow changing signal, so a very

low frequency filter eliminates erroneous readings that may

result from the presence of higher frequency noise. A

simple filter is shown in Figure 8.

Filtered TOUT

8.2K

0.1uFMEMSIC

Accel.

TOUT

Figure 8: Temperature Output Noise Reduction

POWER SUPPLY NOISE REJECTION

Two capacitors and a resistor are recommended for best

rejection of power supply noise (reference Figure 9 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.

MEMSIC

Accelerometer

VDA

C1 C2

VDD

V SUPPLY

Figure 9: Power Supply Noise Rejection

PCB LAYOUT AND FABRICATION SUGGESTIONS

1. The Sck pin should be grounded to minimize noise.

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

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 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.

PACKAGE DRAWING

Fig 10: Hermetically Sealed Package Outline