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Chapter 2 Hardware Overview of the NI 78x x © National Instruments Corporation 2-11 Multifunction Reconfigurabl e I/O User Manual Differential Connections for Ground-Referenced Signal Sources Figure 2-6 shows how to co n…
Chapter 2 Hardware Overview of the NI 78xx
Multifunction Reconfigurable I/O User Manual 2-10 ni.com
Differential Connection Considerations (DIFF Input Mode)
In DIFF input mode, the NI 783x/784x/785x device measures the
difference between the positive and negative inputs. DIFF input mode is
ideal for measuring ground-referenced signals from other devices. When
using DIFF input mode, the input signal connects to the positive input of
the instrumentation amplifier and its reference signal, or return, connects to
the negative input of the instrumentation amplifier.
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 device are greater than
3m(10ft).
• 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.
Chapter 2 Hardware Overview of the NI 78xx
© National Instruments Corporation 2-11 Multifunction Reconfigurable I/O User Manual
Differential Connections for Ground-Referenced
Signal Sources
Figure 2-6 shows how to connect a ground-referenced signal source to a
channel on the NI 783x/784x/785x configured in DIFF input mode.
Figure 2-6. Differential Input Connections for Ground-Referenced Signals
With this connection type, the instrumentation amplifier rejects both the
common-mode noise in the signal and the ground potential difference
between the signal source and the NI 783x/784x/785x ground, shown as
V
cm
in Figure 2-6. In addition, 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.
DIFF Input Mode Selected
–
+
–
+
–
+
–
+
V
cm
V
s
I/O Connector
AISENSE
AIGND
V
m
AI+
AI–
Ground-
Referenced
Signal
Source
Common-
Mode
Noise and
Ground
Potential
Measured
Voltage
Instrumentation
Amplifier
Chapter 2 Hardware Overview of the NI 78xx
Multifunction Reconfigurable I/O User Manual 2-12 ni.com
Differential Connections for Nonreferenced or
Floating Signal Sources
Figure 2-7 shows how to connect a floating signal source to a channel on
the NI 783x/784x/785x configured in DIFF input mode.
Figure 2-7. Differential Input Connections for Nonreferenced Signals
Figure 2-7 shows two bias resistors connected in parallel with the signal
leads of a floating signal source. If you do not use the resistors and the
source is truly floating, the source might not remain within the
common-mode signal range of the instrumentation amplifier, causing
erroneous readings. You must reference the source to AIGND by
connecting the positive side of the signal to the positive input of the
instrumentation amplifier and connecting the negative side of the signal to
AIGND and to the negative input of the instrumentation amplifier without
resistors. This connection works well for DC-coupled sources with low
source impedance, less than 100 Ω.
For larger source impedances, this connection leaves the differential signal
path significantly out of balance. Noise that couples electrostatically onto
the positive line does not couple onto the negative line because it is
connected to ground. Hence, this noise appears as a differential-mode
signal instead of a common-mode signal, and the instrumentation amplifier
does not reject it. In this case, instead of directly connecting the negative
DIFF Input Mode Selected
–
+
–
+
AISENSE
AIGND
V
m
AI+
AI–
–
+
V
s
I/O Connector
Floating
Signal
Source
Bias
Current
Return
Paths
Measured
Voltage
Instrumentation
Amplifier
Bias
Resistors
(see text)