©
National Instruments Corporation
4-1
4
Analog Input
Fig
u
re 4-1 shows the analog inp
u
t circ
u
itry of USB-621
x
devices.
Figure 4-1.
USB-621
x
Analog Input Circuitry
The main blocks feat
u
red in the USB-621
x
analog inp
u
t circ
u
itry are as
follows:
•
I/O Connector
—Yo
u
can connect analog inp
u
t signals to the
USB-621
x
device thro
u
gh the I/O connector. The proper way
to connect analog inp
u
t signals depends on the analog inp
u
t
gro
u
nd-reference settings, described in the
section. Also refer to Appendix A,
, for device I/O connector pino
u
ts.
•
Mux
—Each USB-621
x
device has one analog-to-digital converter
(ADC). The m
u
ltiplexers (m
u
x) ro
u
te one AI channel at a time to the
ADC thro
u
gh the NI-PGIA.
•
AI Ground-Reference Settings
—The analog inp
u
t gro
u
nd-reference
settings circ
u
itry selects between differential (DIFF), referenced
single-ended (RSE), and non-referenced single-ended (NRSE) inp
u
t
modes. Each AI channel can
u
se a different mode.
DIFF, R
S
E,
or NR
S
E
I/O Connector
AI <0..
n
>
M
u
x
AI
S
EN
S
E
AI GND
NI-PGIA
AI Gro
u
nd-Reference
S
etting
s
Inp
u
t R
a
nge
S
election
ADC
AI FIFO
AI D
a
t
a
Digit
a
l
I
s
ol
a
tor
s
I
s
ol
a
tion
B
a
rrier
(U
S
B-6215/
6216/6218
device
s
only)
