Measurement Computing Data Translation DT3034, User Manual

Page: 47 / 178

Chapter 4
46
Connecting Pseudo-Differential Voltage Inputs
Figure 6 shows how to connect pseudo-differential voltage inputs to the DT740 screw terminal
panel.
Figure 6: Connecting Pseudo-Differential Voltage Inputs to the DT740
(Shown for Channels 0 and 1)
Connecting Differential Voltage Inputs
Figure 7A illustrates how to connect a floating signal source to the DT740 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

.
However, when you add noise from external sources to the high impedance, a resistor value
of 100

to 100 k

is more practical.
In Figure 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.
DT740 Panel
Signal Source
TB1
TB3
TB33
Analog In 0
Analog In 1
V
CM
Remove Jumper W1 to use Amp
Low as a remote ground sense.
TB34
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.
TB2
+

Vsource
0
+

Vsource
1