MPXM2102AS datasheet(358/670 Pages) MOTOROLA

358 / 670 page

AN1303

3–212

Motorola Sensor Device Data

www.motorola.com/semiconductors

INPUT TERMINALS

A schematic of the 4 – 20 mA Pressure Transducer topology

is shown in Figure 2. Connections to this topology are made

at the terminals labeled (+) and (–). Because this system

utilizes a current signal, the power supply, the load and any

current meter must be put in series with the (+) to (–) terminals

as indicated in the block diagram. The load for this type of

system is typically a few hundred ohms. As described above,

a typical use of a 4 – 20 mA current transmission signal is the

transfer of information over long distances. Therefore, a long

transmission line can be connected between the (+) and (–)

terminals on the evaluation board and the power supply/load.

2 14

1N4565A

6.4V @ 0.5mA

XDCR1

MPX7100

2 mA

17 13

10 11

750

1/2 W

MPSA06

XTR101

100K

1N4002

0.01

µF

+ 4 – 20 mA OUTPUT

– RETURN

Figure 2. Schematic Diagram

4 – 20 mA PRESSURE TRANSDUCER

PRESSURE INPUT

The device supplied on this topology is an MPX5100DP,

which provides two ports. P1, the positive pressure port, is on

top of the sensor and P2, the vacuum port, is on the bottom of

the sensor. The system can be supplied up to 15 PSI of

positive pressure to P1 or up to 15 PSI of vacuum to P2 or a

differential pressure up to 15 PSI between P1 and P2. Any of

these pressure applications will create the same results at the

sensor output.

CIRCUIT DESCRIPTION

The XTR101 current transmitter provides two one–milliamp

current sources for sensor excitation when its bias voltage is

between 12 V and 40 V. The MPX5100 series sensors are

constant voltage devices, so a zener, D2, is placed in parallel

with the sensor input terminals. Because the MPX5100 series

parts have a high impedance the zener and sensor

combination can be biased with just the two milliamps

available from the XTR101.

The offset adjustment is composed of R4 and R6. They are

used to remove the offset voltage at the differential inputs to

the XTR101. R6 is set so a zero input pressure will result in

the desired output of 4 mA.

R3 and R5 are used to provide the full scale current span of

16 mA. R5 is set such that a 15 PSI input pressure results in

the desired output of 20 mA. Thus the current signal will span

16 mA from the zero pressure output of 4 mA to the full scale

output of 20 mA. To calculate the resistor required to set the

full scale output span, the input voltage span must be defined.

The full scale output span of the sensor is 24.8 mV and is

∆VIN

to the XTR101. Burr–Brown specifies the following equation

for Rspan. The 40 and 16 mΩ values are parameters of the

XTR101.

Rspan + 40

16 mA

DVin) * 0.016 mhos]

+ 64 W

The XTR101 requires that the differential input voltage at

pins 3 and 4, V2 – V1 be less than 1V and that V2 (pin 4)

always be greater than V1 (pin 3).

Furthermore, this

differential voltage is required to have a common mode of 4–6

volts above the reference (pin 7). The sensor produces the

differential output with a common mode of approximately 3.1

volts above its reference pin 1. Because the current of both 1

mA sources will go through R2, a total common mode voltage

of about 5.1 volts (1 k

Ω x 2 mA + 3.1 volts = 5.1 volts) is

provided.

CONCLUSION

This circuit is an example of how the MPX5000 series

sensors can be utilized in an industrial application. It provides

a simple design alternative where remote pressure sensing is

required.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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