The line-driver circuit is used to provide a precise voltage from a source with a noisy ground reference to a remote load that also has a noisy ground reference (see the figure). In this application, the circuit is used to interface a computer’s digital-to-analog converter to a servodrive amplifier in a remote piece of machinery. This machinery has 400-Hz and 60-Hz primary power. Its ground noise is produced by a number of active ac motors and loads. A computer notorious for noise on its ground bus provides the input signal. The common ground connection is a heavy-gauge wire, which runs back to a power safety ground from the computer, the driver circuit, and the remote machine.
A Burr-Brown 3627 differential amplifier receives the input signal from the computer. Output generated by the 3627 is referenced to the ground on the circuit board. The diff-amp is typically used for receiving signals with commonmode noise. Laser-trimmed resistors in the 3627 provide high CMRR with no adjustment required.
The output op amp is also configured as a differential amplifier. But the reference is connected to the ground point at the terminating end. Superposition theory helps to explain the operation of the circuit. When the terminating end’s ground reference is zero, the output is:
VO = VI(1+R2/R1)[R3/(R3+R4)]
which for the values shown in the schematic results in:
VO = 6 VI
When the input signal is zero, the output is a function of the reference voltage. It is given by:
VO = VR(1+R2/R1)[R4/(R3+R4)]
which for the values in the schematic is:
VO = VR
The output is the amplified input signal riding on top of the reference signal. If measured at the terminating end where the reference signal is ground, the signal has very little ground noise.
The output op amp must be stable when driving the capacitance of the twisted-shielded pair. In this application, the cable length was about 150 ft. with a capacitance of about 0.01 µF. The 10-pF capacitor in parallel with R2 provides zero-compensation feedback to keep the PA82J op amp stable.
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