Chapter 3 Connecting Signals
SCB-68 Shielded Connector Block User Manual 3-4 ni.com
Ground-Referenced Signal Sources
A grounded signal source is connected in some way to the building system
ground; therefore, the signal source is already connected to a common
ground point with respect to the DAQ device (assuming that the host
computer is plugged into the same power system). Nonisolated outputs of
instruments and devices that plug into the building power system fall into
this category.
The difference in ground potential between two instruments connected to
the same building power system is typically between 1 and 100V, but the
difference can be much greater if the power distribution circuits are
improperly connected. If a grounded signal source is incorrectly measured,
this difference may appear as a measurement error. The connection
instructions for grounded signal sources are designed to eliminate this
ground potential difference from the measured signal.
Differential Inputs
If the DAQ device is configured for DIFF input mode, where ACH<i> and
ACH<i+8> are used as a single differential channel pair, ground-referenced
signal sources connected to the SCB-68 need no special components. You
can leave the inputs of the SCB-68 in the factory configuration with the
jumpers in the two series positions, F and G. Refer to Figure 5-1, Analog
Input Channel Configuration Diagram for ACH<i> and ACH<i+8>, for a
diagram of this configuration.
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, Soldering and Desoldering on the SCB-68.
Single-Ended Inputs
When you measure ground-referenced single-ended signals, the external
signal supplies its own reference ground point, and the DAQ device should
not supply one. Therefore, you should configure the DAQ device for NRSE
input mode. In this input mode, connect all the signal grounds to AISENSE
pin, which connects to the negative input of the instrumentation amplifier
on the DAQ device. RSE input mode is not recommended for grounded
signal sources.
To leave the SCB-68 inputs in the factory configuration with jumpers in the
series position (F or G, depending on the channel), do not use the open
positions that connect the input to AIGND, A, and C (refer toFigure 5-1,