Data Translation DT3010 Series, Getting Started Manual

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Wiring Signals
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Connecting Differential Voltage Inputs
Figure 19A illustrates how to connect a floating signal source to the
DT740 or STP268 screw terminal panel using differential inputs. (A
floating signal source is a voltage source that has no connection with
earth ground.) For floating signal sources, you need to provide a bias
return path by adding resistors R1 to R16 for channels 0 to 15,
respectively.
If the input signal is +10 V, then the common-mode voltage could be
1 V. Theoretically, the resistor value (R
b
) should be 1 V divided by the
input bias current (20 nA) or 50 m
Ω
. However, when you add noise
from external sources to the high impedance, a resistor value of 100
Ω
to 100 k
Ω
is more practical.
In Figure 19B , the signal source itself provides the bias return path;
therefore, you do not need to use bias return resistors. R
s
is the signal
source resistance while R
v
is the resistance required to balance the
bridge. Note that the negative side of the bridge supply must be
returned to analog ground.
Figure 20 illustrates how to connect a floating signal source to the
STP268-EC screw terminal panel.