Chapter 3
Device Overview and Theory of Operation
©
National Instruments Corporation
3-3
AI channels has no overvoltage protection. Do
not
apply a signal that varies
by more than ±2.5 V from the ground of the NI 447
X
.
Analog Input Signal Connections
Figure 3-3 shows a diagram of one of the eight (NI PCI/PXI-4472) or four
(NI PCI-4474) AI stages.
Figure 3-3.
Analog Input Stage
The AI stage presents high-input impedance to the AI signals connected to
the NI 447
X
. Signals are routed to the positive inputs of the AI stage, and
their returns are routed to AIGND through a common-mode choke. The
NI 447
X
ADCs measure these signals when they perform A/D conversions.
These input channels have 24-bit resolution and are simultaneously
sampled at software-programmable rates from 102.4 kS/s down to 1.0 kS/s
in 190.7
µ
S/s increments for
f
s
> 51.2 kS/s or 95.37
µ
S/s increments for
f
s
≤
51.2 kS/s. This flexibility in sample rates makes the device well-suited
for a wide variety of applications, including audio, acoustics, and vibration
analysis.
+
–
Differential
Buffer
Common-Mode
Choke
Calibration
Multiplexer
1 M
Ω
DC/AC
Coupling
Analog-
Lowpass
Filter
A/D
Converter
CH0
Gain = –12.77 dB
0.01
µ
F
50
Ω
IEPE
On/Off
24 V Compliant
IEPE Constant Current
Source
4 mA
0.01
µ
F
