Omega Engineering OMB-DBK-34A, User Manual

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Differential Circuit with Shunt-reference
A way around this problem is to use a differential measurement for each shunt, with the instrument
common connected to the supply common. Each input channel will measure the shunt voltage and will
reject any voltage in the common wire (common-mode rejection).
Differential Mode, Floating (Isolated from Ground)
CH0H
CH0L
(+)
(-)
10 k
Ω
MUX
BOARD
DBK12/13/15
Analog
Com.
T/C
Floating Differential Circuit
Floating-differential measurements are made when low-level signals must
be measured in the presence of high levels of common-mode noise (e.g., a
non-grounded thermocouple). When the signal source has no direct
connection to the system analog common, one must be provided. This can
be done by connecting a resistor between one of the two signal lines;
usually the lower in potential and common. A resistor of 10 to 100 k
Ω
is
satisfactory (less noise with the lower values).
CAUTION
Do not use differential signal hookups with the intent of achieving isolation or circuit
protection. Differential signal hookups do not provide isolation, or any other kind of
circuit protection.
Connecting Differential Amplifiers
Wire connections must be solid. Loose wires will add noise to the circuit. Low grade unshielded cables
will act as antennas, inducing more noise into the system. For this reason, all applications using a
differential amplifier require the use of quality signal cables and connectors. The signal cable used should
be constructed with:
Insulated Outer Jacket
Foil Shield
Drain Wire
Stranded Copper
Conductors
Twisted Signal Pair
Twisted Signal Pair
Shielded Signal Cable
•
Insulated outer jacket
•
Twisted signal pairs
•
Foil shield
•
Drain wire (copper stranded)
The twisted signal pairs should make use of
low impedance, stranded copper conductors;
and the foil shield should be of the type using
multiple folds.
The copper-stranded drain wire should be considered as part of the shield, and should be connected as
described later in this section. Proper use of a quality signal cable will result in a dramatic reduction of
noise.
The signal circuit must be connected with only one ground from the shield, as indicated in the left side of
the figure below. A mistake, which is often made, is having two grounds (one at each end of the signal
shield). Having two grounds, as shown in the right side of the figure, creates a “ground loop.” The ground
loop provides a path for current to circulate, causing the induction of noise that can affect the signal.
DBK Option Cards and Modules
886995
Signal Management 1-3