Chapter 2
DAQ System Overview
©
National Instruments Corporation
2-3
Factory-calibration constants are permanently stored in an onboard
EEPROM and cannot be modified. When yo
u
self-calibrate the device,
software stores new constants in a
u
ser-modifiable section of the EEPROM.
To ret
u
rn a device to its initial factory calibration settings, software can
copy the factory-calibration constants to the
u
ser-modifiable section
of the EEPROM. Refer to the
NI-DAQmx Help
or the
LabVIEW Help
in
version 8.0 or later for more information abo
u
t
u
sing calibration constants.
For a detailed calibration proced
u
re for USB-621
x
devices, refer to the
B/E/M/S Series Calibration Procedure for NI-DAQmx
by clicking
Manual Calibration Procedures
on
ni.com/calibration
.
Signal Conditioning
Many sensors and transd
u
cers req
u
ire signal conditioning before a
meas
u
rement system can effectively and acc
u
rately acq
u
ire the signal.
The front-end signal conditioning system can incl
u
de f
u
nctions s
u
ch
as signal amplification, atten
u
ation, filtering, electrical isolation,
sim
u
ltaneo
u
s sampling, and m
u
ltiplexing. In addition, many transd
u
cers
req
u
ire excitation c
u
rrents or voltages, bridge completion, linearization,
or high amplification for proper and acc
u
rate operation. Therefore, most
comp
u
ter-based meas
u
rement systems incl
u
de some form of signal
conditioning in addition to pl
u
g-in data acq
u
isition DAQ devices.
Sensors and Transducers
Sensors can generate electrical signals to meas
u
re physical phenomena,
s
u
ch as temperat
u
re, force, so
u
nd, or light. Some commonly
u
sed sensors
are strain ga
u
ges, thermoco
u
ples, thermistors, ang
u
lar encoders, linear
encoders, and resistance temperat
u
re detectors (RTDs).
To meas
u
re signals from these vario
u
s transd
u
cers, yo
u
m
u
st convert them
into a form that a DAQ device can accept. For example, the o
u
tp
u
t voltage
of most thermoco
u
ples is very small and s
u
sceptible to noise. Therefore,
yo
u
may need to amplify or filter the thermoco
u
ple o
u
tp
u
t before digitizing
it. The manip
u
lation of signals to prepare them for digitizing is called
signal conditioning.
For more information abo
u
t sensors, refer to the following doc
u
ments:
•
For general information abo
u
t sensors, visit
ni.com/
s
en
s
or
s
.
•
If yo
u
are
u
sing LabVIEW, refer to the
LabVIEW Help
by selecting
Help»Search the LabVIEW Help
in LabVIEW and then navigate to
the
Taking Measurements
book on the
Contents
tab.
