Cooper Instruments & Systems DCM 465 Voltage Bridge Amplifier - Manual (Page 4)

CF 29 4 4/2001
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