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Chapter 2 Hardware Overview of the NI 78x x © National Instruments Corporation 2-15 Multifunction Reconfigurabl e I/O User Manual Single-Ended Connections for Grounded Signal Sources (NRSE Input Mode) To measure a ground…

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Chapter 2 Hardware Overview of the NI 78xx
Multifunction Reconfigurable I/O User Manual 2-14 ni.com
You can configure the NI 783x/784x/785x device channels in software for
RSE or NRSE input modes. Use the RSE input mode for floating signal
sources. In this case, the device provides the reference ground point for the
external signal. Use the NRSE input mode for ground-referenced signal
sources. In this case, the external signal supplies its own reference ground
point and the device should not supply one.
In single-ended input modes, electrostatic and magnetic noise couples into
the signal connections more than in differential input modes. The coupling
is the result of differences in the signal path. Magnetic coupling
is proportional to the area between the two signal conductors. Electrical
coupling is a function of how much the electric field differs between the
two conductors.
Single-Ended Connections for Floating Signal
Sources (RSE Input Mode)
Figure 2-8 shows how to connect a floating signal source to a channel on
the NI 783x/784x/785x configured for RSE input mode.
Figure 2-8. Single-Ended Input Connections for Nonreferenced or Floating Signals
–
+
–
+
–
+
I/O Connector
AISENSE
AIGND
V
m
AI+
AI–
V
s
Floating
Signal
Source
Instrumentation
Amplifier
Measured
Voltage
RSE Input Mode Selected
Chapter 2 Hardware Overview of the NI 78xx
© National Instruments Corporation 2-15 Multifunction Reconfigurable I/O User Manual
Single-Ended Connections for Grounded Signal
Sources (NRSE Input Mode)
To measure a grounded signal source with a single-ended input mode, you
must configure the NI 783x/784x/785x in the NRSE input mode. Then
connect the signal to the positive input of the NI 783x/784x/785x
instrumentation amplifier and connect the signal local ground reference to
the negative input of the instrumentation amplifier. The ground point of the
signal should be connected to AISENSE. Any potential difference between
the NI 783x/784x/785x ground and the signal ground appears as a
common-mode signal at both the positive and negative inputs of the
instrumentation amplifier. The instrumentation amplifier rejects this
difference. If the input circuitry of a NI 783x/784x/785x is referenced to
ground in RSE input mode, this difference in ground potentials appears as
an error in the measured voltage.
Figure 2-9 shows how to connect a grounded signal source to a channel on
the NI 783x/784x/785x configured for NRSE input mode.
Figure 2-9. Single-Ended Input Connections for Ground-Referenced Signals
–
+
–
+
–
+
–
+
V
cm
V
s
I/O Connector
AISENSE
AIGND
V
m
AI+
AI–
Instrumentation
Amplifier
Measured
Voltage
Ground-
Referenced
Signal
Source
Common-
Mode
Noise and
Ground
Potential
NRSE Input Mode Selected
Chapter 2 Hardware Overview of the NI 78xx
Multifunction Reconfigurable I/O User Manual 2-16 ni.com
Common-Mode Signal Rejection Considerations
Figure 2-6 and Figure 2-9 show connections for signal sources that
are already referenced to some ground point with respect to the
NI 783x/784x/785x. In these cases, the instrumentation amplifier can reject
any voltage caused by ground potential differences between the signal
source and the device. With differential input connections, the
instrumentation amplifier can reject common-mode noise pickup in the
leads connecting the signal sources to the device. The instrumentation
amplifier can reject common-mode signals when V+
in
and V–
in
(input signals) are both within their specified input ranges. Refer to
the Multifunction Reconfigurable I/O Specifications, available at
ni.com/
docs
, for more information about input ranges.
Analog Output
The bipolar output range of the NI 783x/784x/785x AO channels is fixed at
±10 V. Some applications require that the AO channels power on to known
voltage levels. To set the power-on levels, you can configure the
NI 783x/784x/785x to load and run a VI when the system powers on. The
VI can set the AO channels to the desired voltage levels. The VI interprets
data written to the DAC in two’s complement format. Table 2-3 shows the
ideal AO voltage generated for a given input code.
Note If your VI does not set the output value for an AO channel, then the AO channel
voltage output will be undefined.
Table 2-3. Ideal Output Voltage and Input Code Mapping
Output Description AO Voltage
Input Code (Hex)
(Two’s Complement)
Full-scale range –1 LSB 9.999695 7FFF
Full-scale range –2 LSB 9.999390 7FFE
Midscale 0.000000 0000
Negative full-scale range, +1 LSB –9.999695 8001
Negative full-scale range –10.000000 8000
Any output voltage —