Glossary
©
National Instruments
G-7
X Series User Manual
encoder
A device that converts linear or rotary displacement into digital or p
u
lse
signals. The most pop
u
lar type of encoder is the optical encoder, which
u
ses
a rotating disk with alternating opaq
u
e areas, a light so
u
rce, and a
photodetector.
external trigger
A voltage p
u
lse from an external so
u
rce that ca
u
ses a DAQ operation to
begin.
F
FIFO
First-In-First-O
u
t memory b
u
ffer—A data b
u
ffering techniq
u
e that
f
u
nctions like a shift register where the oldest val
u
es (first in) come o
u
t
first. Many DAQ prod
u
cts and instr
u
ments
u
se FIFOs to b
u
ffer digital data
from an A/D converter, or to b
u
ffer the data before or after b
u
s
transmission.
The first data stored is the first data sent to the acceptor. FIFOs are often
u
sed on DAQ devices to temporarily store incoming or o
u
tgoing data
u
ntil
that data can be retrieved or o
u
tp
u
t. For example, an analog inp
u
t FIFO
stores the res
u
lts of A/D conversions
u
ntil the data can be retrieved into
system memory, a process that often req
u
ires programming the DMA
controller. This process can take several milliseconds in some cases. D
u
ring
this time, data acc
u
m
u
lates in the FIFO for f
u
t
u
re retrieval. With a larger
FIFO, longer latencies can be tolerated. In the case of analog o
u
tp
u
t, a FIFO
permits faster
u
pdate rates, beca
u
se the waveform data can be stored on the
FIFO ahead of time. This again red
u
ces the effect of latencies associated
with getting the data from system memory to the DAQ device.
filter
A physical device or digital algorithm that selectively removes noise from
a signal, or emphasizes certain freq
u
ency ranges and de-emphasizes others.
Electronic filters incl
u
de lowpass, band-pass, and highpass types. Digital
filters can operate on n
u
meric data to perform eq
u
ivalent operations on
digitized analog data or to enhance video images.
floating
The condition where a common mode voltage exists, or may exist, between
earth gro
u
nd and the instr
u
ment or circ
u
it of interest. Neither the high, nor
the low side of a circ
u
it is at earth potential.
floating signal so
u
rces
Signal so
u
rces with voltage signals that are not connected to an absol
u
te
reference of system gro
u
nd. Also called non-referenced signal so
u
rces.
Some common examples of floating signal so
u
rces are batteries,
transformers, and thermoco
u
ples.
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