Chapter 4
Analog Input
4-2
ni.com
•
NI-PGIA
—The NI programmable gain instr
u
mentation amplifier
(NI-PGIA) is a meas
u
rement and instr
u
ment class amplifier that
minimizes settling times for all inp
u
t ranges. The NI-PGIA can
amplify or atten
u
ate an AI signal to ens
u
re that yo
u
u
se the maxim
u
m
resol
u
tion of the ADC.
USB-621
x
devices
u
se the NI-PGIA to deliver high acc
u
racy even
when sampling m
u
ltiple channels with small inp
u
t ranges at fast rates.
USB-621
x
devices can sample channels in any order at the maxim
u
m
conversion rate, and yo
u
can individ
u
ally program each channel in a
sample with a different inp
u
t range.
•
ADC
—The analog-to-digital converter (ADC) digitizes the AI signal
by converting the analog voltage into a digital n
u
mber.
•
AI FIFO
—USB-621
x
devices can perform both single and m
u
ltiple
A/D conversions of a fixed or infinite n
u
mber of samples. A large
first-in-first-o
u
t (FIFO) b
u
ffer holds data d
u
ring AI acq
u
isitions to
ens
u
re that no data is lost. USB-621
x
devices can handle m
u
ltiple A/D
conversion operations with DMA, interr
u
pts, or programmed I/O.
•
Isolation Barrier and Digital Isolators
—Refer to Chapter 9,
Isolation and Digital Isolators on USB-6215/6216/6218 Devices
for more information.
Analog Input Range
The inp
u
t range affects the resol
u
tion of the USB-621
x
device for an
AI channel. For example, a 16-bit ADC converts analog inp
u
ts into one
of 65,536 (= 2
16
) codes—that is, one of 65,536 possible digital val
u
es.
So, for an inp
u
t range of –10 V to 10 V, the voltage of each code of a 16-bit
ADC is:
USB-621
x
devices
u
se a calibration method that req
u
ires some codes
(typically abo
u
t 5% of the codes) to lie o
u
tside of the specified range. This
calibration method improves absol
u
te acc
u
racy, b
u
t it increases the nominal
resol
u
tion of inp
u
t ranges by abo
u
t 5% over what the form
u
la shown above
wo
u
ld indicate.
Choose an inp
u
t range that matches the expected inp
u
t range of yo
u
r signal.
A large inp
u
t range can accommodate a large signal variation, b
u
t red
u
ces
the voltage resol
u
tion. Choosing a smaller inp
u
t range improves the voltage
resol
u
tion, b
u
t may res
u
lt in the inp
u
t signal going o
u
t of range.
10 V
10 V
–
(
)
–
(
)
2
16
-------------------------------------------
305
μ
V
=
