MXD2020GL datasheet(5/7 Pages) ETC

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MEMSIC MXD2020GL/ML/NL/HL

Page 5 of 7

2002.08.29.1

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 ) *

)

(

Bandwidth

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 MXD2020GL/HL/ML/NL is easily interfaced with low

cost microcontrollers. For the digital output accelerometer,

one digital input port is required to read one accelerometer

output. For the analog output accelerometer, many low cost

microcontrollers 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 MXD2020GL/HL/ML/NL 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).

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)/20%

0g = 50% Duty Cycle

T2= 2.5ms or 10ms (factory programmable)

Figure 3: Typical output Duty C ycle

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)

2.5

400

2.0

5000

1000

1.0

2.5

400

1.0

2500

500

2.0

2.5

400

0.5

1250

250

4.0

10.0

100

2.0

20000

4000

0.25

10.0

100

1.0

10000

2000

0.5

10.0

100

0.5

5000

1000

1.0

Table 2: Trade-Offs Between Microcontroller Counter Rate and

T2 Period.

USING THE ACCELEROMETER IN VERY LOW

POWER APPLICATIONS (BATTERY OPERATION)

In applications with power limitations, power cycling can

be used to extend the battery operating life. One important

consideration when power cycling is that the accelerometer

turn on time limits the frequency bandwidth of the

accelerations to be measured. For example, operating at

3.0V the turn on time is 40mS. To double the operating

time, a particular application may cycle power ON for

40mS, then OFF for 40mS, resulting in a measurement

period of 80mS, or a frequency of 12.5Hz. With a

frequency of measurements of 12.5Hz, accelerations

changes as high as 6.25Hz can be detected. Power cycling

can be used effectively in many inclinometry applications,

where inclination changes can be slow and infrequent.

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