Chapter 3 Connecting Signals
SCB-68 Shielded Connector Block User Manual 3-8 ni.com
Common-mode rejection might be improved by using another bias resistor
between ACH+ or ACH<i>, and AIGND. This connection creates a slight
measurement error caused by the voltage divider formed with the output
impedance of the floating source, but it also gives a more balanced input
for better common-mode rejection.
Single-Ended Connection Considerations
A single-ended connection is one in which the DAQ device AI signal is
referenced to a ground that can be shared with other input signals. The input
signal is tied to the positive input of the instrumentation amplifier, and the
ground is tied to the negative input of the instrumentation amplifier.
You can use single-ended input connections for input signals that meet the
following conditions:
•The input signal is high-level (greater than 1 V).
•The leads connecting the signal to the DAQ device are less than
10 ft (3 m).
•The input signal can share a common reference point with other
signals.
DIFF input connections are recommended for greater signal integrity for
any input signal that does not meet the preceding conditions.
In single-ended modes, more electrostatic and magnetic noise couples into
the signal connections than in differential modes. The coupling is the result
of differences in the signal path. Magnetic coupling isprop ortional to the
area between the two signal conductors. Electrical coupling is a function of
how much the electric field differs between the two conductors.