A Simple 4-20 Ma Pressure Transducer Evaluation Board

 INTRODUCTION
The two wire 4-20 mA current loop is one of the most
widely utilized transmission signals for use with transducers in
industrial applications. A two wire transmitter allows signal
and power to be supplied on a single wire-pair. Because the
information is transmitted as current, the signal is relatively
immune to voltage drops from long runs and noise from
motors, relays, switches and industrial equipment. The use of
additional power sources is not desirable because the
usefulness of this system is greatest when a signal has to be
transmitted over a long distance with the Sensor at a remote
location. Therefore, the 4 mA minimum current in the loop is
the maximum usable current to power the entire control
circuitry.
Figure 1 is a block diagram of a typical 4-20 mA current
loop system which illustrates a simple two chip solution to
converting pressure to a 4-20 mA signal. This system is
designed to be powered with a 24 Vdc supply. Pressure is
converted to a differential voltage by the MPX5100 pressure
sensor. The voltage signal proportional to the monitored
pressure is then converted to the 4-20 mA current signal with
the Burr-Brown XTR101 Precision Two-Wire Transmitter. The
current signal CAN be monitored by a meter in series with the
supply or by measuring the voltage drop across RL. A key
advantage to this system is that circuit performance is not
affected by a long transmission line.
 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.
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4-20 mA Pressure Transducer Schematic Diagram
4-20 mA Pressure Transducer   Schematic Diagram



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