MXD2020EL - MEMSIC INC - Datasheet.Live

Ultra Low Noise, Low offset Drift ±1 g Dual

Axis Accelerometer with Digital Outputs

MXD2020E/F

FEATURES

Resolution better than 1 mg at 1 Hz

Dual axis accelerometer fabricated on a monolithic CMOS IC

RoHS compliant

On-chip mixed mode signal processing

50,000 g shock survival rating

17 Hz bandwidth

3.00V to 5.25V single supply operation

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

Continuous self-test

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

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

Projectors - Leveling and Keystoning

White Goods – Spin/Vibration Control

Internal

Oscillator

Sck

(optional)

CLK

Heater

Control

X axis

Y axis

Factory Adjust

Offset & Gain

Low Pa ss

Filter

Low Pa ss

Filter

Temperature

Sensor

Voltage

Reference VREF

DOUTX

VDD VDA

Gnd

2-AXIS

SENSOR

DOUTY

TOUT

Continous

Self Test

MXD2020E/F FUNCTIONAL BLOCK DIAGRAM

GENERAL DESCRIPTION

The MXD2020E/F is an ultra low noise and low cost, dual

axis accelerometer built on a standard, submicron CMOS

process. The MXD2020E/F measures acceleration with a

full-scale range of ± 1 g and a sensitivity of 20%/g @5V at

25°C. It can measure both dynamic acceleration (e.g.,

vibration) and static acceleration (e.g., gravity). The

MXD2020E/F 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 up to 50,000 g, leading to

significantly lower failure rates and lower losses due to

handling during assembly.

The MXD2020E/F provides two digital outputs. The

outputs are digital signals with duty cycles (ratio of pulse

width to period) that are proportional to acceleration. The

duty cycles outputs can be directly interfaced to a micro-

processor.

MEMSIC MXD2020E/F Rev H Page 1 of 8 2/26/2007

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

The typical noise floor is 0.2mg / Hz allowing signals

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

MXD2020E/F is available in a LCC surface mount package

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

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

Due to the standard CMOS structure of the MXD2020E/F,

additional circuitry can easily be incorporated into custom

versions for high volume applications. Contact the factory

for more information.

©MEMSIC, Inc.

One Technology Drive Suite 325,Andover MA01810,USA

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

www.memsic.com

MEMSIC MXD2020E/F Rev H Page 2 of 8 2/26/2007

MXD2020E/F SPECIFICATIONS (Measurements @ 25°C, Acceleration = 0g unless otherwise noted, VDD, VDA =

5.0V unless otherwise specified)

Parameter

Conditions

Min MXD2020E/F

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 degrees

Transverse Sensitivity3

±2.0 %

SENSITIVITY

DOUTX and DOUTY

Each Axis

@5.0V supply

19.0

20.0

21.0

% Duty

Cycle/g

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

Δ from 25°C, at +105°C -55 %

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

ZERO g BIAS LEVEL

0 g Offset5

Each Axis

-0.1

0.00

+0.1

0 g Duty Cycle5 48 50 52 % Duty Cycle

0 g Offset over Temperature Δ from 25°C

Δbased on 20%/g

±0.4

±0.008

mg/°C

% / °C

PWM output Frequency For MXD2020EL only

MXD2020E

MXD2020F

380

100

400

103

105

420

NOISE PERFORMANCE

Noise Density, rms

0.2

0.4 mg/ Hz

FREQUENCY RESPONSE

3dB Bandwidth 15 17 19 Hz

TEMPERATURE OUTPUT

Tout Voltage 1.15 1.25 1.35 V

Sensitivity 4.6 5.0 5.4 mV/°K

VOLTAGE REFERENCE OUTPUT

VRef output @3.0V-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 DOUTX, DOUTY under

Failure

@5.0V Supply, output rails to

supply voltage

5.0

Continuous Voltage at DOUTX, DOUTY under

Failure

@3.0V Supply, output rails to

supply voltage

3.0

DOUTX and DOUTY OUTPUTS

Normal Output Range Output High

Output Low

4.8

0.2

Current Source or sink, @ 3.0V-5.25V supply 100 μA

Rise/Fall Time 3.0 to 5.0V Supply 90 100 110 nS

Turn-on Time @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 Current @ 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 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.

6 Output settled to within ±17mg.

MEMSIC MXD2020E/F Rev H 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 θJA θJC Device Weight

LCC-8 110°C/W 22°C/W < 1 gram

Ordering Guide

Model Package Style Digital

Output

MXD2020EL LCC8 RoHS compliant 100 Hz

MXD2020EF LCC8, Pb-free

RoHS compliant

100Hz

MXD2020FL LCC8 RoHS compliant 400 Hz

MXD2020FF LCC8, Pb-free

RoHS compliant

400Hz

All parts are shipped in tape and reel packaging.

Caution

ESD (electrostatic discharge) sensitive device.

Top View

MEMSIC

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.

Small circle indicates pin one (1).

Pin Description: LCC-8 Package

Pin Name Description I/O

1 TOUT Temperature (Analog Voltage) O

2 DOUTY Y-Axis Acceleration Digital Signal O

3 Gnd Ground I

4 VDA Analog Supply Voltage I

5 DOUTX X-Axis Acceleration Digital Signal O

6 Vref 2.5V Reference Output O

7 Sck Optional External Clock I

8 VDD Digital Supply Voltage I

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.

MEMSIC MXD2020E/F Rev H Page 4 of 8 2/26/2007

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 3.00 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.

DOUTX – This pin is the digital output of the x-axis

acceleration sensor. It is factory programmable to 100 Hz

or 400 Hz. The user should ensure the load impedance is

sufficiently high as to not source/sink >100μA 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.

DOUTY – This pin is the digital output of the y-axis

acceleration sensor. It is factory programmable to 100 Hz

or 400 Hz. The user should ensure the load impedance is

sufficiently high as to not source/sink >100μA 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.

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):

i 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

Temp erature (C)

Sensitivity (normalized)

Figure 1: 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. The 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

MEMSIC MXD2020E/F Rev H Page 5 of 8 2/26/2007

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 to 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

MEMSIC

Figure 2: Accelerometer Position Relative to Gravity

X-Axis Y-Axis

X-Axis

Orientation

To Earth’s

Surface

(deg.)

X Output

(g)

Change

per deg.

of tilt

(mg)

Y 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: Accelerometers can be used in a wide variety

of low g applications such as tilt and orientation. The

device noise floor will vary with the measurement

bandwidth. With the reduction of the bandwidth the noise

floor drops. This will improve the signal to noise ratio of

the measurement and 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%).

DIGITAL INTERFACE

The MXD2020E/F is easily interfaced with low cost

microcontroller. For the digital output accelerometer, one

digital input port is required to read one accelerometer

output. For the analog output accelerometer, many low cost

microcontroller are available today that feature integrated

a/d (analog to digital converters) with resolutions ranging

from 8 to 12 bits.

In many applications the microcontroller provides an

effective approach for the temperature compensation of the

sensitivity and the zero g offset. Specific code set, reference

designs, and applications notes are available from the

factory. The following parameters must be considered in a

digital interface:

Resolution: smallest detectable change in input acceleration

Bandwidth: detectable accelerations in a given period of

time

Acquisition Time: the duration of the measurement of the

acceleration signal

DUTY CYCLE DEFINITION

The MXD2020E/F has two PWM duty cycle outputs (x,y).

The acceleration is proportional to the ratio T1/T2. The

zero g output is set to 50% duty cycle and the sensitivity

scale factor is set to 20% duty cycle change per g. These

nominal values are affected by the initial tolerance of the

device including zero g offset error and sensitivity error.

This device is offered from the factory programmed to

either a 10ms period (100 Hz) or a 2.5ms period (400Hz).

T1 Length of the “on” portion of the cycle.

T2 (Period) Length of the total cycle.

Duty Cycle Ratio of the “0n” time (T1) of the cycle to

the total cycle (T2). Defined as T1/T2.

Pulse width Time period of the “on” pulse. Defined as

T1.

A (g)= (T1/T2 - 0.5)/0.2

At 0g T1=T2

T2= 2.5ms or 10ms (factory programmable)

Figure 3: Typical output Duty Cycle

MEMSIC MXD2020E/F Rev H Page 6 of 8 2/26/2007

CHOOSING T2 AND COUNTER FREQUENCY

DESIGN TRADE-OFFS

The noise level is one determinant of accelerometer

resolution. The second relates to the measurement

resolution of the counter when decoding the duty cycle

output. The actual resolution of the acceleration signal is

limited by the time resolution of the counting devices used

to decode the duty cycle. The faster the counter clock, the

higher the resolution of the duty cycle and the shorter the

T2 period can be for a given resolution. Table 2 shows

some of the trade-offs. It is important to note that this is the

resolution due to the microprocessors’ counter. It is

probable that the accelerometer’s noise floor may set the

lower limit on the resolution.

T2 (ms)

MEMSIC

Sample

Rate

Counter-

Clock

Rate

(MHz)

Counts

Per T2

Cycle

Counts

per g

Reso-

lution

(mg)

10.0 100 1.0 10000 2000 0.5

10.0 100 0.5 5000 1000 1.0

2.5 400 1.0 2500 500 2.0

2.5 400 0.5 1250 250 4.0

Table 2: Trade-Offs Between Microcontroller Counter Rate and

T2 Period.

COMPENSATION FOR ZERO G OFFSET CHANGE

OVER TEMPERATURE

The compensation of offset is performed with the following

equation: Aoc = A + ( a + b * T + c * T * T)

where Aoc is the offset compensated acceleration, A is the

uncompensated acceleration, T is temperature and a, b, c

are constants characteristic to each accelerometer.

Computer programs are used to determine these constants.

The constants can be read from and written to the MCU

EEPROM via the RS-232. The constants a,b,c are normally

stored in the MCU EEPROM. To determine the values of

the constants, each accelerometer is taken to three different

temperatures, preferably evenly spread across the desired

temperature span. The zero g bias (A0, A1 and A2) and the

temperatures (T0, T1 and T2) are recorded at each

temperature. The data collected (A0, T0, A1, T1, A2, T2) is

used in a quadratic interpolation (or LaGrange polynomial)

to determine a, b and c as follows:

r0 = A0 / ( (T0-T1)*(T0-T2) )

r1 = A1 / ( (T1-T0)*(T1-T2) )

r2 = A2 / ( (T2-T0)*(T2-T1) )

a = r0 * T1 * T2 + r1 * T0 * T2 + r2 * T0 * T1

b = - r0 * (T1+T2) – r1 * (T0+T2) – r2 *(T0+T1)

c = r0 + r1 + r2

In many cases a computer is used to control the

temperature, communicate with the MCU, and to calculate

the constants. After calculating the constants, the computer

downloads the constants to EEPROM.

For a more detail discussion of temperature compensation

reference MEMSIC application note #AN-00MX-002

Microcontroller

MEMSIC

Accel

I/O

A/D

Figure 4: Zero g Offset Temperature Compensation Circuit

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 digital filter can be used to equally compensate

all sensors. The compensating filter does not require

adjustment for individual accelerometers. The function of

the compensating filter is to apply gain in proportion with

the acceleration changes. The faster the acceleration

changes occur, the higher the gain that the filter applies.

For analog output accelerometers, the compensating filter

can be implemented with a circuit involving two op-amps

and some resistors and capacitors. For digital output

accelerometers, a digital filter is necessary.

In applications where high frequency accelerations need to

be measured, a DSP (digital signal processor) may be

necessary to implement the digital filter. DSP IC’s and

development tools are readily available from major IC

manufacturers.

However, if the bandwidth requirement is relatively low

(i.e. 100Hz), it is possible to implement a digital frequency

compensating filter with an 8 bit microcontroller. The

microcontroller will likely have to be capable of operating

at relatively high clock frequencies (20MHz).

CONVERTING THE DIGITAL OUTPUT TO AN

ANALOG OUTPUT

The PWM output can be easily converted into an analog

output by integration. A simple RC filter can do the

conversion. Note that that the impedance of the circuit

following the integrator must be much higher than the

impedance of the RC filter. Reference figure 5 for an

example.

1uF

DOUT AOUT

10K

MEMSIC

Accel.

Figure 5: Converting the digital output to an analog voltage

MEMSIC MXD2020E/F Rev H Page 7 of 8 2/26/2007

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 6.

Filtered TOUT

8.2K

0.1uFMEMSIC

Accel.

TOUT

Figure 6: Temperature Output Noise Reduction

POWER SUPPLY NOISE REJECTION

One capacitor is recommended for best rejection of power

supply noise (reference Figure 7 below). The capacitor

should be located as close as possible to the device supply

pin (VDD). The capacitor lead length should be as short as

possible, and surface mount capacitor is preferred. For

typical applications, the capacitor can be ceramic 0.1 μF.

VDA VDD

MEMSIC

Accelerometer

Power Supply

0.1uF

Figure 7: 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 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.

MEMSIC MXD2020E/F Rev H Page 8 of 8 2/26/2007

PACKAGE DRAWING

Fig 8: Hermetically Sealed Packa ge Outline