Chapter 4
Connecting the Signals
©
National Instruments Corporation
4-9
In NRSE mode, the AISENSE signal internally connects to the negative
input of the PGIA when the corresponding channels are selected. In DIFF
and RSE modes, the signal is unconnected.
AIGND is an AI common signal that is directly routed to the ground
connection point on the device. You can also use this signal for a general
analog ground connect point to the device.
Connect the AI signals to the device according to the configuration of the
AI channels you are using and the type of input signal source. With each
configuration, you can use the PGIA in a different way. Figure 4-4 shows a
diagram of the PGIA.
Figure 4-4.
PGIA
The PGIA applies gain and common-mode voltage rejection and presents
high-input impedance to the AI signals connected to the device. Signals are
routed to the positive and negative inputs of the PGIA through input
multiplexers on the device. The PGIA converts two input signals to a signal
that equals the difference between the two input signals multiplied by the
gain setting of the amplifier. The amplifier output voltage is referenced to
the device ground. The ADC measures this voltage when it performs A/D
conversions.
You must reference all signals to ground either at the signal source or at
the device. If you have a floating source, reference the signal to ground
using the RSE input mode or the DIFF input mode with bias resistors.
Refer to the
Differential Connections for Nonreferenced or Floating Signal
–
Instrumentation
Amplifier
–
Measured
Voltage
V
m
+
+
PGIA
V
in+
V
in–
V
m
= [V
in+
– V
in–
]
×
Gain