MPXM2102AS datasheet(528/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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POWER SUPPLY

Since the sensor output is ratiometric with the supply volt-

age, any variation in supply voltage will also proportionally

appear at the output of the sensor. The integrated pressure

sensor is designed, characterized and trimmed to be powered

with a 5 V +/– 5% power supply which can supply the maxi-

mum 10 mA current requirement of the sensor. Powering the

integrated sensor at another voltage than specified is not rec-

ommended because the offset, temperature coefficient of off-

set (TCO) and temperature coefficient of span (TCS) trim will

be invalidated and will affect the sensor accuracy.

From a noise point of view, adequate de–coupling is impor-

tant. A 0.33

µF to 1.0 µF ceramic capacitor in parallel with a

0.01

µF ceramic capacitor works well for this purpose. Also,

with respect to noise, it is preferable to use a linear regulator

such as an MC78L05 rather than a relatively more noisy

switching power supply 5 volt output. An additional consider-

ation is that the power to the sensor and the A/D voltage refer-

ence should be tied to the same supply. Doing this takes

advantage of the sensor output ratiometricity. Since the A/D

resolution is also ratiometric to its reference voltage, varia-

tions in supply voltage will be canceled by the system.

LAYOUT OPTIMIZATION

In mixed analog and digital systems, layout is a critical part

of the total design. Often, getting a system to work properly

depends as much on layout as on the circuit design. The fol-

lowing discussion covers some general layout principles, digi-

tal section layout and analog section layout.

General Principles:

There are several general layout principles that are impor-

tant in mixed systems. They can be described as five rules:

Rule 1: Minimize Loop Areas. This is a general principle

that applies to both analog and digital circuits. Loops are

antennas. At noise sensitive inputs, the area enclosed by an

incoming signal path and its return is proportional to the

amount of noise picked up by the input. At digital output ports,

the amount of noise that is radiated is also proportional to

loop area.

Rule 2: Cancel fields by running equal currents that

flow in opposite directions as close as possible to each

other. If two equal currents flow in opposite directions, the

resulting electromagnetic fields will cancel as the two currents

are brought infinitely close together. In printed circuit board

layout, this situation can be approximated by running signals

and their returns along the same path but on different layers.

Field cancellation is not perfect due to the finite physical sepa-

ration, but is sufficient to warrant serious attention in critical

paths. Looked at from a different perspective, this is another

way of looking at Rule # 1, i.e., minimize loop areas.

Rule 3: On traces that carry high speed signals avoid 90

degree angles, including “T” connections. If you think of

high speed signals in terms of wavefronts moving down a

trace, the reason for avoiding 90 degree angles is simple. To

a high speed wavefront, a 90 degree angle is a discontinuity

that produces unwanted reflections. From a practical point of

view, 90 degree turns on a single trace are easy to avoid by

using two 45 degree angles or a curve. Where two traces

come together to form a “T” connection, adding some material

to cut across the right angles accomplishes the same thing.

Rule 4: Connect signal circuit grounds to power

grounds at only one point. The reason for this constraint is

that transient voltage drops along the power grounds can be

substantial, due to high values of di/dt flowing through finite in-

ductance. If signal processing circuit returns are connected to

power ground at multiple points, then these transients will

show up as return voltage differences at different points in the

signal processing circuitry. Since signal processing circuitry

seldom has the noise immunity to handle power ground tran-

sients, it is generally necessary to tie signal ground to power

ground at only one point.

Rule 5: Use ground planes selectively. Although ground

planes are highly beneficial when used with digital circuitry, in

the analog world they are better used selectively. A single

ground plane on an analog board puts parasitic capacitance

in places where it is not desired, such as at the inverting inputs

of op amps. Ground planes also limit efforts to take advantage

of field cancellation, since the return is distributed.

ANALOG LAYOUT

In analog systems, both minimizing loop areas and field

cancellation are useful design techniques. Field cancellation

is applicable to power and ground traces, where currents are

equal and opposite. Running these two traces directly over

each other provides field cancellation for unwanted noise, and

minimum loop area.

Figure 8 illustrates the difference between a power supply

de–coupling loop that has been routed correctly and one that

has not. In this figure, the circles represent pads, the sche-

matic symbols show the components that are connected to the

pads, and the routing layers are shown as dark lines (top

trace) or grey lines (bottom trace). Note that by routing the two

traces one over the other that the critical loop area is mini-

mized. In addition, it is important to keep de–coupling capaci-

tors close to active devices such as MPX5000–series sensors

and operational amplifiers. As a rule of thumb, when 50 mil

ground and Vcc traces are used, it is not advisable to have

more than 1 inch between a de–coupling capacitor and the

active device that it is intended to be de–coupled.

Figure 8. Minimizing Loop Areas

SENSOR

RECOMMENDED

AVOID

+5 V

GND

+5 V

GND

TOP TRACE

BOT TRACE

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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