Chapter 4
Analog Input
X Series User Manual
4-8
ni.com
S
u
ppose a 4 V signal is connected to channel 0 and a 1 mV signal
is connected to channel 1. The inp
u
t range for channel 0 is –10 V
to 10 V and the inp
u
t range of channel 1 is –200 mV to 200 mV.
When the m
u
ltiplexer switches from channel 0 to channel 1, the
inp
u
t to the NI-PGIA switches from 4 V to 1 mV. The
approximately 4 V step from 4 V to 1 mV is 1,000% of the new
f
u
ll-scale range. For a 16-bit device to settle within 0.0015%
(15 ppm or 1 LSB) of the ±200 mV f
u
ll-scale range on channel 1,
the inp
u
t circ
u
itry m
u
st settle to within 0.000031% (0.31 ppm or
1/50 LSB) of the ±10 V range. Some devices can take many
microseconds for the circ
u
itry to settle this m
u
ch.
To avoid this effect, yo
u
sho
u
ld arrange yo
u
r channel scanning
order so that transitions from large to small inp
u
t ranges are
infreq
u
ent.
In general, yo
u
do not need this extra settling time when the
NI-PGIA is switching from a small inp
u
t range to a larger inp
u
t
range.
•
Insert Gro
u
nded Channel between Signal Channels—Another
techniq
u
e to improve settling time is to connect an inp
u
t channel
to gro
u
nd. Then insert this channel in the scan list between two of
yo
u
r signal channels. The inp
u
t range of the gro
u
nded channel
sho
u
ld match the inp
u
t range of the signal after the gro
u
nded
channel in the scan list.
Consider again the example above where a 4 V signal is connected
to channel 0 and a 1 mV signal is connected to channel 1. S
u
ppose
the inp
u
t range for channel 0 is –10 V to 10 V and the inp
u
t range
of channel 1 is –200 mV to 200 mV.
Yo
u
can connect channel 2 to AI GND (or yo
u
can
u
se the internal
gro
u
nd; refer to
Internal Channels
in the
NI-DAQmx Help
). Set
the inp
u
t range of channel 2 to –200 mV to 200 mV to match
channel 1. Then scan channels in the order: 0, 2, 1.
Inserting a gro
u
nded channel between signal channels improves
settling time beca
u
se the NI-PGIA adj
u
sts to the new inp
u
t range
setting faster when the inp
u
t is gro
u
nded.
•
Minimize Voltage Step between Adjacent Channels—When
scanning between channels that have the same inp
u
t range, the
settling time increases with the voltage step between the channels.
If yo
u
know the expected inp
u
t range of yo
u
r signals, yo
u
can
gro
u
p signals with similar expected ranges together in yo
u
r scan
list.
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