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Chapter 4 Signal Connections
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National Instruments Corporation 4-9 PCI-4451/4452 User Manual
Analog Input Signal Connections
The analog input signals for the PCI-4451/4452 devices are +ACH<0..3>,
−ACH<0..3>, and AIGND. The +
ACH<0..1> signals are tied to the two
analog input channels of your PCI-4451, and ±ACH<0..3> are tied to the
four analog input channels of your PCI-4452 device.
Caution
Exceeding the differential and common-mode input ranges distorts your input
signals.
AIGND is an analog input common signal that connects directly to the
ground system on the PCI-4451/4452 devices. You can use this signal for a
general analog ground tie point to your PCI-4451/4452 device if necessary,
but connecting AIGND to other earth-connected grounds is not
recommended. AIGND is not directly available if you are using a
BNC-2140 accessory.
Figure 4-3 shows a diagram of your PCI-4451/4452 device analog input
stage.
Figure 4-3.
Analog Input Stage
The analog input stage applies gain and common-mode voltage rejection
and presents high input impedance to the analog input signals connected to
your PCI-4451/4452 device. Signals are routed directly to the positive and
negative inputs of the analog input stage on the device. The analog input
stage converts two input signals to a signal that is the difference between
the two input signals multiplied by the gain setting of the amplifier. The
!
+ACH
x
-ACH
x
AIGND
Calibration
Multiplexer
DC/AC
Coupling
20 dB
Attenuator
Analog
Lowpass
Filter
Differential
Amplifier
+
–
Gain = 0 dB
Gain = 10 dB
Gain = 20 dB
Gain = 30 dB
Gain = 40 dB
Gain = 50 dB
Gain = 60 dB
Gain = 0 dB
Gain = -20 dB
f
c
= 3.4 Hz
1 GΩ
1 GΩ
900 kΩ
100 kΩ
100 kΩ
900 kΩ
0.047 F
0.047 F
A/D
Converter
AIGND
User.book Page 9 Tuesday, April 14, 1998 10:20 AM