Keithley KPCI-3110, User Manual

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3-16 Installation and Configuration KPCI-3110 and KPCI-3116 User’s Manual
Connecting differential voltage inputs
Figure 3-7 A illustrates how to connect a floating signal source to the STP-3110 screw terminal
panel using differential inputs. (A floating signal source is a voltage source that has no connec-
tion 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 +10V, then the common-mode voltage could be 1V. Theoretically, the resis-
tor value (R
b
) should be 1V divided by the input bias current (20nA) or 50m
Ω
. However, when
you add noise from external sources to the high impedance, a resistor value of 100
Ω
to 100k
Ω
is
more practical.
In Figure 3-7 B, 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 3-7
Connecting differential voltage inputs (shown for Channel 0)
STP-3110
Panel
Floating
Signal
Source
+
-
TB1
TB2
TB34
Analog In 0
Analog In 0
Return
Analog Ground
R1
R
s
TB1
TB2
TB34
Analog In 0
Analog In 0
Return
Analog Ground
STP-3110
Panel
R
v
R
s
Bridge
+
-
DC Supply
You can use resistor R1 to
connect the low side of
channel 0 to analog ground.
A)
B)