Chapter 4
Analog Input
©
National Instruments Corporation
4-5
Configuring AI Ground-Reference Settings in Software
Yo
u
can program channels on an USB-621
x
device to acq
u
ire with
different gro
u
nd references.
To enable m
u
ltimode scanning in LabVIEW,
u
se
NI-DAQmx Create
Virtual Channel.vi
of the NI-DAQmx API. Yo
u
m
u
st
u
se a new VI for
each channel or gro
u
p of channels config
u
red in a different inp
u
t mode. In
Fig
u
re 4-3, channel 0 is config
u
red in differential mode, and channel 1 is
config
u
red in referenced single-ended mode.
Figure 4-3.
Enabling Multimode Scanning in LabVIEW
To config
u
re the inp
u
t mode of yo
u
r voltage meas
u
rement
u
sing the DAQ
Assistant,
u
se the
Terminal Configuration
drop-down list. Refer to the
DAQ Assistant Help
for more information abo
u
t the DAQ Assistant.
To config
u
re the inp
u
t mode of yo
u
r voltage meas
u
rement
u
sing the
NI-DAQmx C API, set the
terminalConfig
property. Refer to the
NI-DAQmx C Reference Help
for more information.
Multichannel Scanning Considerations
USB-621
x
devices can scan m
u
ltiple channels at high rates and digitize the
signals acc
u
rately. However, yo
u
sho
u
ld consider several iss
u
es when
designing yo
u
r meas
u
rement system to ens
u
re the high acc
u
racy of yo
u
r
meas
u
rements.
In m
u
ltichannel scanning applications, acc
u
racy is affected by settling
time. When yo
u
r USB-621
x
device switches from one AI channel to
another AI channel, the device config
u
res the NI-PGIA with the inp
u
t range
of the new channel. The NI-PGIA then amplifies the inp
u
t signal with the
gain for the new inp
u
t range. Settling time refers to the time it takes the
NI-PGIA to amplify the inp
u
t signal to the desired acc
u
racy before it is
sampled by the ADC. The
NI USB-621x Specifications
lists settling time.
USB-621
x
devices are designed to have fast settling times. However,
several factors can increase the settling time which decreases the acc
u
racy
