Data Translation DT300 Series, User Manual, Page: 49 / 164

Wiring Signals
49
Connecting Differential Voltage Inputs
Figure 10A illustrates how to connect a floating signal source to the STP300 screw terminal
panel using differential inputs. (A floating signal source is a voltage source that has no
connection 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 +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 10B , 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 10: Connecting Differential Voltage Inputs (Shown for Channel 0)
STP300 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
STP300 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)