Chapter 3
Connecting Signals
©
National Instruments Corporation
3-3
SCB-68 Shielded Connector Block User Manual
Nonreferenced or Floating Signal Sources
A floating signal source is a signal source that is not connected in any way
to the building ground system, but has an isolated ground-reference point.
Instruments or devices with isolated outputs are considered floating signal
sources, and they have high-impedance paths to ground. Some examples of
floating signal sources are outputs for thermocouples, transformers,
battery-powered devices, optical isolators, and isolation amplifiers. The
ground reference of a floating source must be tied to the ground of the DAQ
device to establish a local or onboard reference for the signal. Otherwise,
the measured input signal varies as the source floats outside the
common-mode input range.
Differential Inputs
When measuring differential floating sources, you must configure the
device for DIFF input mode. To provide a return path for the
instrumentation amplifier bias currents, differential floating sources must
have a 10 to 100 k
Ω
resistor connected to AIGND on one input if they are
DC coupled or on both inputs if sources are AC coupled. You can install
bias resistors in positions B and D of the SCB-68, as shown in Figure 5-1,
Analog Input Channel Configuration Diagram for ACH<i> and
.
Single-Ended Inputs
When measuring single-ended floating signal sources, you must configure
the DAQ device to supply a ground reference by configuring the DAQ
device for RSE input mode. In this mode, the negative input of the
instrumentation amplifier on the DAQ device is tied to the analog ground.
To use the SCB-68 with single-ended inputs, where ACH<
i
> and
ACH<
i
+8> are used as two single-ended channels, configure the SCB-68
in its factory-default configuration. In the factory-default configuration,
jumpers on the SCB-68 are in the two series positions, F and G, as shown
in Figure 5-1,
Analog Input Channel Configuration Diagram for ACH<i>
. In this configuration, you should connect all signal
grounds to AIGND.
Note
Some versions of the SCB-68 use hardwired 0
Ω
resistors as the factory-default
jumpers. In such cases, to move these jumpers to and from the factory-default positions,
you must solder and desolder on the SCB-68 circuit card assembly. When soldering, refer
to Appendix E,
