Cooper DCM 465 User Manual

DCM 465

VOLTAGE BRIDGE AMPLIFIER

USER’S GUIDE

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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

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maximum of 1,000 by means of an external resistor connected across terminals 12 and 13. The size of the external
resistor can be calculated using the gain formula in the specifications. When doing this, the coarse gain potentiometer
should be turned fully clockwise. The fine gain pot can then be used for final gain adjustment. The output offset adjustment
range is ±0.5V.

The amplifier can withstand input voltages up to 15 Volts without damage. The output of the amplifier is filtered to be 3 dB
down at 3 Hz using a double pole Butterworth response filter to minimize the effects of high frequency electrical and
physical noise on the system. The output of the amplifier is ±10 Volts at 5mA making it compatible with modern data
acquisition techniques and systems.

Specifications

(Typical @ 25°C unless noted)

Amplifier

Gain Range
With external R
Gain Temperature Coefficient 200 ppm/°C
Gain Potentiometer Hysteresis 0.2% of span
Gain Equation

Note: G is the desired voltage gain in V/V

Drift (RTI) 2±20/G µV/°C
Input Bias ±30 nA

Input Impedance

Differential
Common Mode

Output Noise (RTO)

At gain=100
1 Hz to 100 Hz 120 µV RMS

Common Mode Rejection

Gain=40 (DC) 96 dB
Rated Output (2k load) ±10V
Common Mode Voltage ±6.5V
Output Impedance (DC) 0.01 ohms
Dynamic Response 0.3s to 0.1%
DC to –3 dB two pole Butterworth filter 3 Hz
Max. Input Voltage ±15V
Bridge Supply
Input 115 VAC ±10% 50 to 60 Hz

Output Voltage 4 to 15 Volts
Output Current 5 to 150 mA

(see output voltage vs. current curve)
Load & Line Regulation 0.05%
VOUT=12V, I
Output Noise 0.5 mV RMS
Drift 200 ppm/°C max.
B+ Potentiometer Hysteresis 0.3% of output max.
Short Circuit Current 750 mA
Line Isolation 1,500 VDC
Mechanical
Operating Temperature 0°C to 70°C
Storage Temperature -25°C to +85°C
Weight 18 oz. (510) grams
Size 3.75”L x 2.0”W x 2.87”H

=5 to 100 mA

L

40-250
to 1,000

____1_____
Rg = G - 1
40 kΩ 150Ω

3,000 megohms
6 megohms

(100, 220 & 230 VAC available)

9.53 x 5.1 x 7.62 cm

- 1

Getting Started with the DCM 465

I. Hook Up Procedure

A. Connect the +out of your load cell to the +INPUT, pin 10.
B. Connect the -out of your load cell to the -INPUT, pin 11.

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Note: If the ± SENSE are not used in your load cell application, the connections in step C & D need to be
followed. If the ±SENSE are going to be used, do not jumper them as described in steps C & D.
C. Connect B +, pin 4, to the +excitation of your load cell and jumper the + SENSE, pin 3, to

B+, pin 4.

D. Connect B-, pin 2, to the -excitation of your load cell and jumper the - SENSE, pin 1, to B-, pin 2.
E. Connect the VAC power supply to the AC input lines, pins 6 and 7.

II. Turn On Procedure

A. Verify that the hook up procedure is complete.
B. Verify the correct AC voltage is applied to the 465; i.e. 100, 115, 220, 230.
C. Turn on the AC source supply to the DCM 465.
D. Set the required EXCITATION supply voltage to the load cell by adjusting B+ ADJUST, Pot B

III. Calibration Procedure for Zero Adjustment

A. Jumper the + and - input terminals, pins 10 and 11, together.
B. Connect a voltmeter across the output, pins 8 and 9.
C. Adjust the OUTPUT OFFSET, Pot A, potentiometer for zero.

IV. Full Scale Voltage Adjustment

A. Remove the jumper between the + and - input terminals and apply a known load to your load cell, in

most cases it would be 100% of full scale.

B. Adjust the COARSE GAIN, Pot D, and FINE GAIN, Pot C, potentiometers for the desired FULL SCALE

output.

C. Calibration is now complete. However, the user should recheck the ZERO & FULL SCALE output before

continuing.

Typical Bridge Application

Figure 2 shows a typical load cell application using a standard 350-Ohm bridge. Typical bridge output is 2 or 3mV/Volt of
excitation. With the power supply excitation voltage at 10 Volts an output of 20 or 30mV from the bridge can be obtained.
The common mode voltage of the bridge (the instrumentation amplifier input signal level) is 5 Volts. This is well within the

6.5 Volt common mode voltage range of the amplifier.

Figure 2. Typical Load Cell Application

The gain must be set between 300 and 600 depending on the output of the bridge. The built-in potentiometer set to a gain
of 200 would achieve an output voltage of 4 to 6 Volts. For a higher-level output an external resistor must be used. The
value of that resistor can be calculated using the gain formula. It can also be arrived at empirically using a calibrated input
signal equal to the maximum input signal expected and a resistance substitution box to adjust the gain until the desired full
scale output voltage is achieved.

Assuming a standard 350-Ohm bridge is used, the current required from the excitation supply (set at 10 Volts) would be

28.6mA. If the leads were long enough to have 10 Ohms of internal resistance there would be a drop of over 0.25 Volts in
both the plus and common side of the bridge. To eliminate this potential error (especially where the current might vary
during the course of a measurement) the sense lines are connected to measure and regulate the voltage right at the

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bridge rather than at the output of the supply thereby eliminating this potential source of error. The decision of whether to
use the sense leads or not depends entirely on the lead length, its resistance and the effect of that error on the
measurement.

Application Suggestions

The DCM 465 is designed to eliminate many of the ordinary problems associated with bridge type measurements. Since
the whole system is in one case the common problem of ground loops or circulating currents caused by poor wiring
practices is eliminated. It is recommended that lead lengths be kept to a minimum. The use of shielded twisted pairs for
the input leads is recommended for most applications.

To minimize self-heating errors the use of minimum excitation power is suggested as is sufficient heat sinking of the
transducer wherever possible. For optimum stability a one hour warm-up is recommended. Avoid large temperature
changes or stray magnetic fields.

The output leads should be kept short to minimize capacitive loading on the output of the amplifier. The DCM 465
Bridgesensor is ready to wire into your system, have power applied, and to start making measurements for you.

Mechanical Specifications

Specifically designed for rugged field use the DCM 465 is completely encapsulated in epoxy using a vacuum potting
system to insure a complete seal against corrosive environments. It is similarly protected against shock and vibration and
will provide years of reliable and accurate operation.

Figure 3. Mechanical Specifications

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Terminal Strip Assignments

Screw

Terminal

Function

Screw

Terminal

Function

1 -SENSE 8 OUTPUT
2 B- 9 AMPLIFIER CMN
3 +SENSE 10 +INPUT
4 B+ 11 -INPUT
5 NOT USED 12 EXT. GAIN
6 AC 13 EXT. GAIN
7 AC

WARRANTY REPAIR POLICY

Limited Warranty on Products

Any Cooper Instruments product which, under normal operating conditions, proves defective in material or in workmanship
within one year of the date of shipment by Cooper will be repaired or replaced free of charge provided that a return
material authorization is obtained from Cooper and the defective product is sent, transportation charges prepaid, with
notice of the defect, and it is established that the product has been properly installed, maintained, and operated within the
limits of rated and normal usage. Replacement or repaired product will be shipped F.O.B. from our plant. The terms of
this warranty do not extend to any product or part thereof which, under normal usage, has an inherently shorter useful life
than one year. The replacement warranty detailed here is the buyer’s exclusive remedy, and will satisfy all obligations of
Cooper whether based on contract, negligence, or otherwise. Cooper is not responsible for any incidental or
consequential loss or damage which might result from a failure of any and all other warranties, express or implied,
including implied warranty of merchantability or fitness for particular purpose. Any unauthorized disassembly or attempt to
repair voids this warranty.

Obtaining Service under Warranty

Advance authorization is required prior to the return to Cooper Instruments. Before returning the item, contact the Repair
Department c/o Cooper Instruments at [phone] for a Return Material Authorization number. Shipment to Cooper
shall be at buyer’s expense and repaired or replacement items will be shipped F.O.B. from our plant in Warrenton,
Virginia. Non-verified problems or defects may be subject to a $100 evaluation charge. Please return the original
calibration data with the unit.

Repair Warranty

All repairs of Cooper products are warranted for a period of 90 days from date of shipment. This warranty applies only to
those items that were found defective and repaired; it does not apply to products in which no defect was found and
returned as is or merely recalibrated. It may be possible for out-of-warranty products to be returned to the exact original
specifications or dimensions.

* Technical description of the defect: In order to properly repair a product, it is absolutely necessary for Cooper to receive
information specifying the reason the product is being returned. Specific test data, written observations on the failure and
the specific corrective action you require are needed.

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