MXC6202XJ データシートの表示（PDF） - Unspecified

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MXC6202XJ

Ultra Low Cost, ±2.0 g Dual Axis Accelerometer With I2C Interface

Unspecified 

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(Hz) *1.6)

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

times the rms value (for an average uncertainty of

0.1%).

HARDWARE DESIGN CONSIDERATIONS

1. One capacitor is recommended for best rejection

of power supply noise (reference figure 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 a surface mount capacitor is

preferred. For typical applications, the capacitor

can be ceramic 0.1 µF.

5. Vias can be added symmetrically around the

ground plane. These vias will increase the

thermal isolation of the device from the rest of the

PCB and improve performance.

SOFTWARE DESIGN CONSIDERATION

We recommended that the following software limiter

filter be used in the acceleration signal process

routine in order to further reduce the noise on the I2C

bus:

Int iAccReal[0x02];//Real-time acceleration data array

Int iAccFilter;//Filtered acceleration data

SoftwareLimiterFilter()

{

if (abs(iAccReal[0x00]-iAccReal[0x01])<0x80) then

iAccFilter=iAccReal[0x00];

iAccReal[0x01]= iAccReal[0x00];

}

I2C INTERFACE DESCRIPTION

A slave mode I2C (or Inter IC bus) circuit has been

implemented into the MEMSIC thermal accelerometer

as a standard interface for customer applications.

The A/D converter and MCU functionality have been

added to the MEMSIC sensor, thereby increasing

ease-of-use, and lowering power consumption,

footprint and total solution cost.

The I2C is an industry standard bi-directional two-wire

interface bus. A master I2C device can operate

READ/WRITE controls to an unlimited number of

devices on the bus by proper addressing. The

MEMSIC accelerometer operates only in a slave

mode, i.e. only responding to calls by a master device

I2C BUS CHARACTERISTICS

Power supply noise rejection

2. Robust low inductance ground wiring should be

used.

3. 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. Based on the

experiment, with a 85° C heating source at 15mm

away of MEMSIC device, the offset change will be

within 0.7g.

4. A metal ground plane should be added directly

beneath the MEMSIC device. The size of the

plane should be similar to the MEMSIC device’s

footprint and be as thick as possible.

I2C Bus

The two wires in I2C bus are called SDA (serial data

line) and SCL (serial clock line). In order for a data

transfer to start, the bus has to be free, which is

defined by both wires in a HIGH output state. Due to

the open-drain/pull-up resistor structure and wired

Boolean “AND” operation, any device on the bus can

pull lines low and overwrite a HIGH signal. The data

MEMSIC MXC6202xJ/K Rev.A

Page 8 of 11

3/20/2006

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