Keithley KPCI-3101 Series, User Manual

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KPCI-3101 — KPCI-3104 Series User’s Manual Installation and Configuration 3-17
Connecting differential voltage inputs
Figure 3-7A illustrates how to connect a floating signal source to the STA-300 screw terminal
panel using differential inputs. (A floating signal source is a voltage source that has no connec-
tion with earth ground.) You need to provide a bias return path by adding resistors R1 to R8 for
channels 0 to 7, respectively, for floating signal sources.
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 3-7B , 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)
STA-300 Panel
Floating
Signal
Source
+
-
TB1
TB2
TB18
Analog In 0
Analog In 0
Return
Analog Ground
R1
R
s
TB1
TB2
TB18
Analog In 0
Analog In 0
Return
Analog Ground
STA-300 Panel
R
v
R
s
Bridge
+ -
DC Supply
You can use resistor R1 to
connect the low side of
channel 0 to analog
A)
B)