MPXM2102AS datasheet(419/670 Pages) MOTOROLA

419 / 670 page

AN1318

3–273

Motorola Sensor Device Data

www.motorola.com/semiconductors

Figure 6. Application Example

RETURN

B+

RC1

RV1

S+

RP1

RP2

RV2

S–

RC2

BIAS

—

B+

INTERFACE

AMPLIFIER

OUTPUT

GND

VREFH

VREFL

+5 V

15.0 OHMS

30.1 OHMS

X4.85 V

X.302 V

453 OHMS

PORT E

MC68HC11

VSS

MPX2000

SERIES

PRESSURE

SENSOR

Beginning with the ramp generator, a timing ramp is

generated with current source U5 and capacitor C3.

Initialization is provided by Q1 which sets the voltage on C3

at approximately ground. With the values shown, 470

µA

flowing into 0.47

µF provide approximately a 5 msec ramp time

from zero to 5 V. Assuming zero pressure on the sensor, inputs

to both comparators U2A and U2B are at the same voltage.

Therefore, as the ramp voltage sweeps from zero to 5 V, both

PA0 and PA1 will go low at the same time when the ramp

voltage exceeds the common mode voltage. The processor

counts the number of clock cycles between the time that PA0

and PA1 go low, reading zero for zero pressure.

In this circuit, U4A and U4B form the front end of an

instrument amplifier. They differentially amplify the sensor’s

output. The resulting amplified differential signal is then

sampled and held in U1 and U3. The sample and hold function

is performed in order to keep input data constant during the

conversion process. The stabilized signals coming out of U1

and U3 feed a higher output voltage to U2A

than U2B,

assuming that pressure is applied to the sensor. Therefore,

the ramp will trip U2B before U2A is tripped, creating a time

difference between PA0 going low and PA1 going low. The

processor reads the number of clock cycles between these

two events. This number is then linearly scaled with software

to represent the amplified output voltage, accomplishing the

analog to digital conversion.

When the ramp reaches the reference voltage established

by R9 and R10, comparator U2C is tripped, and a reset

command is generated. To accomplish reset, Q1 is turned on

with an output from PA7, and the sample and hold circuits are

delatched with an output from PB1. Resolution is limited by

clock frequency and ramp linearity. With the ramp generator

shown in Figure 7 and a clock frequency of 2 MHz; resolution

is 11 bits.

From a software point of view, the A/D conversion consists

of latching the sample and hold, reading the value of the

microcomputer’s free running counter, turning off Q1, and

waiting for the three comparator outputs to change state from

logic 1 to logic 0. The analog input voltage is determined by

counting, in 0.5

µsec steps, the number of clock cycles

between PA0 and PA1 going low.

LONG DISTANCE INTERFACES

In applications where there is a significant distance between

the sensor and microcomputer, two types of interfaces are

typically used. They are frequency output and 4–20 mA loops.

In the frequency output topology, pressure is converted into a

zero to 5 V digital signal whose frequency varies linearly with

pressure. A minimum frequency corresponds to zero pressure

and above this, frequency output is determined by a Hz/unit

pressure scaling factor. If minimizing the number of wires to a

remote sensor is the most important design consideration,

4–20 mA current loops are the topology of choice. These loops

utilize power and ground as the 4–20 mA signal line and

therefore require only two wires to the sensor. In this topology

4 mA of total current drain from the sensor corresponds to zero

pressure, and 20 mA to full scale.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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