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Creating a Differential Output to Cancel Environmental Noise
When the coaxial cable connecting the preamplifier output to the
shaping amplifier input is long and the cable runs through an
electrically noisy environment, it is advantageous to employ
differential signal transmission. Several amplifiers (Models 450,
671, 672, 973, and 973U) offer differential inputs for this purpose.
A few preamplifiers include differential outputs to accommodate this
function. If the preamplifier does not provide differential outputs,
the box depicted in Figure 7 can be used to create a differential
output.
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Fig. 7. An Add-On Box to Convert a
Single Preamplifier Output to a Differential Output.
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All the components shown in Figure 7 are mounted in a small metal
box located close to the preamplifier. Care must be exercised to
ensure low-impedance grounds. The input on the left side of the box
is connected to the normal preamplifier signal output with a coaxial
cable that is as short as possible. This short cable must provide a
low impedance path from the preamplifier ground to the metal box.
The center conductor of this short cable transmits the normal
preamplifier output signal through the box to the normal input of
the shaping amplifier. The 93-Ω resistor
in the box is used to transmit the preamplifier ground signal to the
differential reference input of the amplifier. Both the “normal” and
the “differential reference” cables are RG-62A/U, 93-Ω coaxial cables. Thus, the 93-Ω
resistor in the box provides reverse termination of the differential
reference cable. This termination matches the 93-Ω reverse termination included inside the preamplifier
for the normal output signal.
To ensure that both cables are affected in the same way by
electrical interference, the two cables are twisted together in a
spiral as they are routed to the amplifier. When connected to the
amplifier inputs, the normal signal includes the desired
preamplifier signal plus any interfering noise from ground loops or
the environment. The differential reference signal includes only the
interfering noise. Hence, when the amplifier subtracts the
differential reference from the normal signal, the interfering noise
is removed from the signal. Amplifiers with differential inputs
usually incorporate a differential gain balancing adjustment to
allow matching of the gains on the two inputs for exact cancellation
of the interfering noise. |
