Cooper Instruments & Systems DCM 465 Voltage Bridge Amplifier - Manual (Page 2)

CF 29 2 4/2001
Features
• Rugged, Compact and Fully Encapsulated
• Complete System - Just Add AC Power
• Ready to Use with Screwdriver Wiring
• Stable and Accurate
Applications
• Weighing with Load Cells
• Long Term Structural Monitoring
• Process Control Pressure Transducers
• Low Frequency Strain Measurements
Description
The DCM 465 is a self-contained, AC powered, signal conditioning module for bridge type instrumentation. It contains a
precision differential instrumentation amplifier with filtered output and a highly regulated, low noise, adjustable output
bridge excitation source. The unit is completely encapsulated for use in rugged environments.
Transducer Excitation
Transducer bridge excitation is provided by an AC line powered, adjustable, well regulated, low noise power supply. The
excitation voltage is adjusted by means of a molded-in potentiometer, which allows the output voltage to be varied from 4
to 15 Volts. The output ripple is extremely low and the line and load regulation are 0.05%.
The sense lines minimize variations in output voltage with changes in load current or lead resistance. It should be noted
that if the sense feature is not being used, terminal 1 must be connected to terminal 2. Terminals 3 and 4 must be
connected also. The supply will provide up to 150mA of output current. These features make the unit ideal for use with
most common strain gage bridge circuits of 120, 350, and 500 Ohms.
The supply has short circuit protection to protect it against short-term faults. The output recovers automatically from short
circuit conditions once the short is removed.
Figure 1. Bridge Supply-Safe Operation
Safe Operation in the Unsafe Area
The curve (Figure 1) describes the region of safe operating output current at each voltage setting level. If excitation is
desired at the 4 Volt level with 150mA, this can be accomplished by using a resistor of at least 2 Watts in size in series
with the bridge. The right value resistor would drop 11 Volts allowing 4 Volts at 150mA across the bridge while minimizing
internal heating of the DCM 465. The + sense line would regulate the supply to the required 4 Volts. Normal operation of
single bridges at 10 Volts and under 100mA (all 120, 350, and 500 ohm bridges) can be accomplished within the safe
operating area.
Instrumentation Amplifier
The built-in amplifier is a true differential input, low noise, low drift, instrumentation amplifier. It has a high common mode
rejection ratio (CMRR) and is provided with an output offset that is potentiometer adjustable. The minimum gain setting of
the amplifier is 40 and the maximum gain using the built-in potentiometer is 250. The gain may be adjusted up to a