MPXM2102AS datasheet(515/670 Pages) MOTOROLA

515 / 670 page

AN1586

3–369

Motorola Sensor Device Data

www.motorola.com/semiconductors

After the discussion of the circuit components, the following

system–related issues will be discussed simultaneously using

the design example:

• How the system works

• Defining and designing the digital output for a desired sig-

nal resolution

• A step–by–step procedure that shows you how to digitize

the signal

• A procedure to show you how to software calibrate the digi-

tal output

• Related software examples

This system, in addition to the benefits of a digital output

(noise immunity, etc.), also has the following additional

inherent benefits. These benefits will be addressed in more

detail in the systems topics.

• The circuit topology and method of “digitizing” the sensor’s

analog output is very stable and accurate. The system uses

the microcontroller’s precise, internal, digital time base to

digitize the analog signal.

• The signal resolution is user–programmable via software

— i.e. the user can program whether the resolution is 8–bit,

10–bit, etc.

• The digital output is calibrated in software so that compo-

nent tolerances can be nullified.

• The software required to digitize the signal requires very

little CPU time and overhead.

• The required circuitry is minimal, simple, and cost–effec-

tive.

THE PRESSURE SENSOR

Motorola’s MPX2000 series sensors are temperature

compensated and calibrated (i.e. offset and span are

precision trimmed) pressure transducers. These sensors are

available in full scale pressure ranges from 10 kPa (1.5 psi) to

700 kPa (100 psi). Although the specifications (see Table 1)

in the data sheets apply to a 10 V supply voltage, the output

of these devices is ratiometric with the supply voltage. For

example, at the absolute maximum supply voltage rating,

16 V, the sensor will typically produce a differential output

voltage of 64 mV at the rated full scale pressure of the given

sensor. One exception to this is that the span of the MPX2010

(10 kPa sensor) will be only 40 mV due to the device’s slightly

lower sensitivity. Since the maximum supply voltage produces

the largest output signal, it is evident that even the best case

scenario will require some signal conditioning to obtain a

usable signal (input to an A/D, etc.). For this specific design,

an MPX2100 and 5.0 V supply are used, yielding a typical

maximum sensor output of 20 mV (typical zero pressure offset

is 0.0 mV and typical span is 20 mV). The sensor’s output is

then signal conditioned (amplified and level shifted) to provide

a four volt span with a zero pressure offset of 0.5 V.

Table 1. MPX2100 Electrical Characteristics for

VS = 10 V, TA = 25°C

Characteristic

Symbol

Min

Typ

Max

Unit

Pressure Range

Pop

100

kPa

Supply Voltage

Vdc

Full Scale Span

VFSS

38.5

41.5

Zero Pressure Off-

set

Voff

–1.0

1.0

Sensitivity

∆V/∆P

0.4

mV/kPa

Linearity

—

–0.25

0.25

%VFSS

Temperature Effect

on Span

TCVFSS –1.0

1.0

%VFSS

Temperature Effect

on Offset

TCVoff

–1.0

1.0

AMPLIFIER STAGE

The amplifier circuitry, shown in Figure 1. , is composed of

two op amps. This interface circuit has a much lower

component

count

than

conventional

quad

amp

instrumentation amplifiers. The two op amp design offers the

high input impedance, low output impedance, and high gain

desired for a transducer interface, while performing a

differential to single–ended conversion. The amplifier

incorporates level shifting capability. The amplifier has the

following transfer function:

Vo + 1 )

• (Vsensor) + V + shift

where R1 = R4, R2 = R3, the gain is 1

) R4

, Vsensor is the

sensor’s differential output (S+ – S–), and V+shift is the

positive dc level shift voltage created by the resistor divider

comprised of R+shift1 and R+shift2. V+shift is used to position

the zero pressure offset at the desired level.

Table 2 summarizes the 1% resistor values used to obtain

a four volt span with a zero pressure offset of 0.5 V (assuming

the typical sensor offset and span values of 0.0 mV and 20 mV,

respectively).

Table 2. Resistor Values for the MPX2100

Amplifier Design

R+shift1

R+shift2

4.99 k

Ω

549

Ω

20.0 k

Ω

100

Ω

100

Ω

20.0 k

Ω

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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