Measurement Computing Data Translation DT3010 Series, User Manual, Page: 67 / 232

Chapter 4
66
Figure 17: Connecting Pseudo-Differential Voltage Inputs to the STP268-EC
(Shown for Channels 0 and 1)
Connecting Differential Voltage Inputs
Figure 18A 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 18B , 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 19 illustrates how to connect a floating signal source to the STP268-EC screw terminal
panel.
TB18
TB52
STP268-EC Panel
Signal Source
V
CM
Analog Ground
*Make this connection as close to V
sources
as
possible to reduce ground loop errors.
V
cm
is the common-mode voltage for all 32
analog inputs.
+
−
Vsource
0
+
−
Vsource
1
TB34
TB33
Analog In 0
Analog In 1
Amp Low
*