MPXM2102AS datasheet(436/670 Pages) MOTOROLA

436 / 670 page

AN1326

3–290

Motorola Sensor Device Data

www.motorola.com/semiconductors

Table 3. MPX2100AP Electrical Characteristics

Characteristic

Symbol

Minimum

Typical

Max

Unit

Pressure Range

POP

100

kPa

Supply Voltage

Vdc

Full Scale Span

VFSS

38.5

41.5

Zero Pressure Offset

Voff

±1.0

Sensitivity

0.4

mv/kPa

Linearity

0.05

%FSS

Temperature Effect on Span

0.5

%FSS

Temperature Effect on Offset

0.2

%FSS

Pressure Sensor

The first and most important subsystem is the pressure

transducer. This device converts the applied pressure into a

proportional, differential voltage signal. This output signal will

vary linearly with pressure. Since the applied pressure in this

application will approach a maximum level of 30.5 in–Hg. (100

kPa) at sea level, the sensor output must have a linear output

response over this pressure range. Also, the applied pressure

must be measured with respect to a known reference pressure,

preferably absolute zero pressure (vacuum). The device should

also produce a stable output over the entire operating

temperature range.

The desired sensor for this application is a temperature

compensated

and

calibrated,

semiconductor

pressure

transducer, such as the Motorola MPXM2102A series sensor

family. The MPX2000 series sensors are available in

full–scale pressure ranges from 10 kPa (1.5 psi) to 200 kPa

(30 psi). Furthermore, they are available in a variety of

pressure configurations (gauge, differential, and absolute)

and porting options. Because of the pressure ranges involved

with barometric pressure measurement, this system will

employ an MPXM2102AS (absolute with single port). This

device will produce a linear voltage output in the pressure

range of 0 to 100 kPa. The ambient pressure applied to the

single port will be measured with respect to an evacuated

cavity (vacuum reference). The electrical characteristics for

this device are summarized in Table 3.

As indicated in Table 3, the sensor can be operated at

different supply voltages. The full–scale output of the sensor,

which is specified at 40 mV nominally for a supply voltage of

10 Vdc, changes linearly with supply voltage. All non–digital

circuitry is operated at a regulated supply voltage of 8 Vdc.

Therefore, the full–scale sensor output (also the output of the

sensor at sea level) will be approximately 32 mV.

40 mV

The sensor output voltage at the systems minimum range

(15 in–Hg.) is approximately 16.2 mV. Thus, the sensor output

over the intended range of operations is expected to vary from

32 to 16.2 mV. These values can vary slightly for each sensor

as the offset voltage and full–scale span tolerances indicate.

Signal Conditioning Circuitry

In order to convert the small–signal differential output signal

of the sensor to MCU compatible levels, the next subsystem

includes

signal

conditioning

circuitry.

The

operational

amplifier circuit is designed to amplify, level–shift, and ground

reference the output signal. The signal is converted to a

single–ended, 0.5 – 4.5 Vdc range. The schematic for this

amplifier is shown in Figure 3.

This particular circuit is based on classic instrumentation

amplifier design criteria. The differential output signal of the

sensor is inverted, amplified, and then level–shifted by an

adjustable offset voltage (through Roffset1). The offset voltage

is adjusted to produce 0.5 volts at the maximum barometric

pressure (30.5 in–Hg.). The output voltage will increase for

decreasing pressure. If the output exceeds 5.1 V, a zener

protection diode will clamp the output. This feature is included

to protect the A/D channel input of the MCU. Using the transfer

function for this circuit, the offset voltage and gain can be

determined to provide 0.1 in–Hg of system resolution and the

desired output voltage level. The calculation of these

parameters is illustrated below.

In determining the amplifier gain and range of the trimmable

offset voltage, it is necessary to calculate the number of steps

used in the A/D conversion process to resolve 0.1 in–Hg.

(30.5

* 15.0)in-Hg * 10

steps

+ 155 steps

The span voltage can now be determined. The resolution

provided by an 8–bit A/D converter with low and high voltage

references of zero and five volts, respectively, will detect 19.5

mV of change per step.

RH +

5V, V

RL +

Sensor Output at 30.5 in–Hg = 32.44 mV

Sensor Output at 15.0 in–Hg = 16.26 mV

∆Sensor Output = ∆SO = 16.18 mV

Gain

+ 3.04 V

DSO +

187

Note: 30.5 in–Hg and 15.0 in–Hg are the assumed

maximum and minimum absolute pressures, respectively.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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