National Instruments SCB-68 - Manual (Page 33)

Chapter 3 Connecting Signals
© National Instruments Corporation 3-5 SCB-68 Shielded Connector Block User Manual
Analog Input Channel Configuration Diagram for ACH<i> and
ACH<i+8>
). Any signal conditioning circuitry requiring a ground
reference should be built in the custom breadboard area using AISENSE as
the ground reference instead of building the circuitry in the open
component positions. Referencing the signal to AIGND can cause
inaccurate measurements resulting from an incorrect ground reference.
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 .
Differential Connection Considerations (DIFF Input Mode)
A differential connection is one in which the DAQ device AI signal has its
own reference signal, or signal return path. These connections are available
when the selected channel is configured in DIFF input mode. The input
signal is tied to the positive input of the instrumentation amplifier, and its
reference signal, or return, is tied to the negative input of the
instrumentation amplifier. On DAQ devices that support both single-ended
and DIFF input modes, using DIFF input mode commits two channels,
ACH<i> and ACH<i+8>, to each signal.
You should use differential input connections for any channel that meets
any of the following conditions:
• The input signal is low-level (less than 1 V).
• The leads connecting the signal to the DAQ device are longer than
10 ft (3 m).
• The input signal requires a separate ground-reference point or return
signal.
• The signal leads travel through noisy environments.
Differential signal connections reduce noise pickup and increase
common-mode noise rejection. Differential signal connections also
allow input signals to float within the common-mode limits of the
instrumentation amplifier.