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A G R E AT E R M E A S U R E O F C O N F I D E N C E USB modules offer good noise immunity,
with performance benefits for noise-sensitive
measurements. USB cables are typically 1 to
5 meters long, so the I/O circuitry is locat-
ed further away from the computer's noisy
motherboard and power supplies, and closer
to the signals they will be measuring.
Most USB modules offer full- and high-
speed data transfer rates. Computers config-
ured with USB 1.1 ports can transfer data to
and from a USB DAQ module at up to 12
Mbits/second. The full-speed rate is good
for data streaming applications and supports
data acquisition rates of up to 400kHz. For
high-performance applications, USB mod-
ules that comply with USB 2.0 and are con-
nected to PCs with a high-speed USB 2.0
port can attain data transfer rates of up to
480 Mbits/second. The increased bandwidth
The Pluses and Pitfalls
enables multiple I/O operations simultane-
ously at throughput rates up to 500kHz in
each direction, similar to PCI measurement
of USB DAQ
systems.
Easy assembly and portability are also
beneficial for most test and measurement ap-
plications. Many USB DAQ modules include
removable terminal blocks or BNC connec-
Jonathan Tucker, Lead Industry Consultant tors that conveniently handle all user I/O
Keithley Instruments, Inc. connections. This design is convenient and
cost-effective, eliminating optional screw
terminal accessories. Modules are also com-
Using USB-based data acquisition for test one of the fastest growing buses within the pact and portable, allowing users to move
and measurement applications has many computer and electronics industry today. even the most sophisticated test and meas-
benefitsbutalsosomepotentialpitfalls. urement applications out of the lab and into
USB Benefits for Test and the field.
Introduction Measurement The ready availability of accessories
The Universal Serial Bus (USB) is gain- For test and measurement applications, makes USB DAQ modules easily expand-
ing favor as an alternative interface for data USB data acquisition modules offer a num- able. Using low-cost expansion hubs and
acquisition and measurement applications. ber of significant advantages. USB provides USB cables, a single USB port can connect
USB is already used in computer peripherals users with a simple means of developing test up to 127 data acquisition modules. Most
such as printers, mice, and digital cameras and measurement applications by offering USB modules are also hot-swappable, which
as a quick and easy way to connect to a PC. advantages over PC plug-in boards. These means they can be installed or removed
USB connections simplify peripheral instal- include plug-and-play capability, better noise while the computer is running, with no need
lation by eliminating the need to crack open immunity, cost savings, and portability, to power down the PC.
the PC to install a plug-in board. among others. Power connections on USB DAQ mod-
USB DAQ modules offer true plug-and- ules are simple and easy to use, with mod-
What is USB? play capability. Modules connect to a PC ules being powered either directly by the
Today, virtually all desktop and laptop USB port using a standard, low-cost cable. bus or through a simple connection to an
PCs on the market come equipped with USB When plugged in, the PC automatically iden- external power source. Low-powered mod-
ports. These ports have full software support tifies the module and installs device driver ules draw less than 100mA at 5V and use the
under common operating systems such as software, simplifying setup and reducing power supplied by the USB port. Self-pow-
Windows 2000 and XP. start up time. This eliminates the need to ered modules draw up to 500mA at 5V.
Originally created to simplify the con- open a PC to add a board, configure DIP
nection of external peripherals to personal switches and IRQ settings, search for the Potential Hazards of USB
computers, USB's ease of use has made it right device driver, and reboot the system. With all the benefits of USB come a few
The Pluses and Pitfalls of USB DAQ June2005
hidden hazards, which, if unchecked, can
lead to inaccurate measurement results. Be-
ing aware of the hazards can help prevent
problems.
For starters, unlike PCI boards, which
have short substantial ground systems built
into the backplane of the PC, USB modules
have a long ground connection (up to 5 me-
ters) and active circuitry at both ends. If a Figure 1. Typical application setup.
USB module is not designed properly, this
can cause system lockups, erratic perfor-
mance, and electromagnetic transients