National Instruments USB-621 Series User Manual page 184

F
FIFO
filter
filtering
floating signal sources
frequency
ft
function
© National Instruments Corporation
First-In-First-Out memory buffer—A data buffering technique that
functions like a shift register where the oldest values (first in) come out
first. Many DAQ products and instruments use FIFOs to buffer digital data
from an A/D converter, or to buffer the data before or after bus
transmission.
The first data stored is the first data sent to the acceptor. FIFOs are often
used on DAQ devices to temporarily store incoming or outgoing data until
that data can be retrieved or output. For example, an analog input FIFO
stores the results of A/D conversions until the data can be retrieved into
system memory, a process that requires the servicing of interrupts and often
the programming of the DMA controller. This process can take several
milliseconds in some cases. During this time, data accumulates in the FIFO
for future retrieval. With a larger FIFO, longer latencies can be tolerated. In
the case of analog output, a FIFO permits faster update rates, because the
waveform data can be stored on the FIFO ahead of time. This again reduces
the effect of latencies associated with getting the data from system memory
to the DAQ device.
A physical device or digital algorithm that selectively removes noise from
a signal, or emphasizes certain frequency ranges and de-emphasizes others.
Electronic filters include lowpass, band-pass, and highpass types. Digital
filters can operate on numeric data to perform equivalent operations on
digitized analog data or to enhance video images.
A type of signal conditioning that allows you to filter unwanted frequency
components from the signal you are trying to measure.
Signal sources with voltage signals that are not connected to an absolute
reference of system ground. Also called non-referenced signal sources.
Some common examples of floating signal sources are batteries,
transformers, and thermocouples.
The number of alternating signals that occur per unit time.
Feet.
1. A built-in execution element, comparable to an operator, function, or
statement in a conventional language.
2. A set of software instructions executed by a single line of code that may
have input and/or output parameters and returns a value when executed.
G-7
Glossary
NI USB-621x User Manual