Cooper Instruments & Systems DCM 460 Voltage Bridge Amplifier - Manual (Page 4)

CF 28 4 4/2001
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 120 mA of output current. These features make the unit ideal
for 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.
Safe Operating in the Unsafe Area
The curve (Figure 2) describes the region of safe operating output current at each voltage setting level. If excitation
is desired at the 4 Volt level with 120mA, 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 120mA across the bridge
while minimizing internal heating of the DCM 460. The + sense line would regulate the supply to the required 4
Volts. Normal operation of single bridges at 10 Volts and under 100mA (all 125, 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 is 250. The amplifier can withstand input voltages up to 15
Volts without damage. The output of the amplifier is filtered to be 3dB down at 2kHz using a double pole Bessel
response filter. The output of the amplifier is 10 Volts at 5mA making it compatible with modern data acquisition
techniques and systems. Capacitors may be added to terminals 10, 11, 12, and 13 to lower the filter cutoff
frequency. This will reduce the DCM 460 output noise and system noise where the high frequency response is not
required.
Typical Bridge Application
Figure 3 shows a typical bridge application using a standard 350-Ohm bridge. This could be a strain gage, load cell,
pressure transducer, accelerometer, or other bridge based measurement.
Typical bridge output is 2 or 3 mV/Volt of excitation. With the power supply excitation voltage at 10 Volts an output
of 20 to 30 mV 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.
The built-in potentiometer set to a gain of 200 would achieve an output voltage of 4 to 6 Volts.
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 0.286 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 bridge rather than at the output of the supply. The decision 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 460 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 heatsinking of the
transducer whenever possible. For optimum stability a one-hour warm-up is recommended. Avoid large
temperature changes or stray magnetic fields.
The DCM 460 Bridgesensor is ready to wire into your system, have power applied, and start making measurements
for you.