MPXM2202GS datasheet(121/670 Pages) MOTOROLA

121 / 670 page

AN1632

2–85

Motorola Sensor Device Data

www.motorola.com/semiconductors

• Oscillator

The oscillator has been redesigned to center the nominal

frequency within the trimming range and to have better

temperature compensation. As shown in Figure 1, the

oscillator controls three switched capacitor circuit sub–

blocks within the ASIC, thus having direct impact on their

performance. The trimmable oscillator enhances the con-

trol of other performance parameters and enables the part

to meet tighter specification tolerances. Additionally, the

placement of the oscillator on the silicon die has changed,

contributing to a 50% reduction in the noise of the part.

• Power Supply Filter

An internal capacitor has been added between the VDD and

VSS pins to provide some de–coupling of the power supply.

Also, a lowpass filter has been added to the circuitry that

supplies power to the transducer element and that sets the

DC level of the capacitance–to–voltage converter stage.

The filter response suppresses high frequency noise, but

maintains a ratiometric output.

• New Sensing Scheme

The capacitance–to–voltage converter employs innovative

circuit techniques (at the time of this writing, patents are

pending) to improve signal ratiometricity. Amplification is

achieved using an EPROM trimmable gain stage, provid-

ing capability for both coarse and fine tuning. As in the

previous version of the control ASIC, the second gain stage

is cascaded by a switched capacitor four pole Bessel low-

pass filter, with a unity gain response and –3 dB frequency

at 400 Hz.

• Temperature Compensation

The final stage in the ASIC performs temperature com-

pensation of gain. Thus, the temperature coefficient for

sensitivity is set using EPROM trim.

PERFORMANCE ENHANCEMENTS

Motorola’s new MMA1201P accelerometer provides perfor-

mance enhancements in a number of areas, including ratio-

metric output, signal–to–noise ratio, output filter response,

and temperature compensation. For complete details, refer to

the MMA1201P data sheet.

• Ratiometric Output

The offset voltage and the sensitivity of the part are ratio-

metric with supply voltage. Typical error values are less

than 0.5%.

• Signal to Noise Ratio

The noise has been reduced by 50% and is specified at

3.5 mVRMS maximum. Typical values are about

2.0 mVRMS. As a result, the signal to noise ratio of the

part is about 50 dB.

• Lowpass Filter Response

The frequency response of the four pole Bessel lowpass

filter has the –3 dB frequency at 400 Hz. The tolerance has

been narrowed by 60% and is specified at

"40 Hz.

• Temperature Compensation

The sensitivity is very uniform over temperature, with typi-

cal errors of about

"1% over the specified temperature

range. Also, although the spec allows for the equivalent of

5 mV/

°C for the temperature coefficient of offset, typical

values are actually less than 2 mV/

°C, at VDD equal to 5 V.

INTERFACE CONSIDERATIONS

With only four active pin connections, Motorola’s accel-

erometers are very easy to use. There are only a few simple

considerations to be taken into account to ensure reliable

operation and attain the high level of performance that the can

part offer.

• Power Supply

Power is applied to the accelerometer through the VDD pin.

For optimum performance, it is recommended that the part

be powered with a voltage regulator such as the Motorola

MC78L05. An optional 0.1

µF capacitor can be placed on

the VDD pin to complement the accelerometer’s internal

capacitor and provide additional de–coupling of the supply.

The capacitor should be physically located as close as

possible to the accelerometer.

• Ground

Ground is applied through the VSS pin. Whenever pos-

sible it is recommended that a solid ground plane be used

so that the impedance of the ground path is minimized. If

this is not possible, it is strongly recommended that a low

impedance trace (no additional components should be

connected to it) be used to directly connect the VSS pin to

the power supply ground.

• Self–test

The ST pin is an active, high logic level input pin that pro-

vides a way for the user to verify proper operation of the

part. It is pulled down internally. Therefore, for normal

operation, the user could apply a logic level “0” or leave it

unconnected. Applying a logic level “1” to the ST pin will

apply the equivalent of a 25 g acceleration to the trans-

ducer, and the user should see a change in the output

equivalent to 25 times the part’s rated sensitivity.

• Output

The accelerometer’s output is measured at the VOUT pin.

As shown in Figure 1, the ASIC’s oscillator controls the

switched capacitor lowpass filter, with a nominal operating

frequency of 65 kHz. As a result, a clock noise component

of about 2 mVpeak may be present at 65 kHz. Therefore, it

is recommended that the user place a simple RC lowpass

filter on the VOUT pin to reduce the clock noise present in

the output signal. Recommended values are a 1 k

Ω resistor

and a 0.01

µF capacitor. These values produce a filter with

a –3 dB frequency at about 16 kHz, which will not interfere

with the response of the internal Bessel filter, yet will pro-

vide sufficient attenuation (approximately –12 dB) of the

clock noise.

Placing a filter on the output is especially recommended for

applications where the signal will be fed into a stand–alone

A/D converter, and in cases where the signal will be ampli-

fied to a level where the amplified clock noise may begin to

contribute significantly to the noise floor of the system.

However, if using an MCU or microprocessor in the system,

the user may choose to use a software algorithm to digitally

filter the signal, instead of using the analog RC filter. This

option would have to be evaluated based on the system

performance requirements.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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