Chapter 4
Analog Input
©
National Instruments
4-7
X Series User Manual
NI-PGIA to amplify the inp
u
t signal to the desired acc
u
racy before it is
sampled by the ADC. The specifications doc
u
ment for yo
u
r DAQ device
lists its settling time.
MIO X Series devices are designed to have fast settling times. However,
several factors can increase the settling time which decreases the acc
u
racy
of yo
u
r meas
u
rements. To ens
u
re fast settling times, yo
u
sho
u
ld do the
following (in order of importance):
1.
Use Low Impedance Sources
—To ens
u
re fast settling times, yo
u
r
signal so
u
rces sho
u
ld have an impedance of <1 k
Ω
. Large so
u
rce
impedances increase the settling time of the NI-PGIA, and so decrease
the acc
u
racy at fast scanning rates.
Settling times increase when scanning high-impedance signals d
u
e to
a phenomenon called charge injection. M
u
ltiplexers contain switches,
u
s
u
ally made of switched capacitors. When one of the channels, for
example channel 0, is selected in a m
u
ltiplexer, those capacitors
acc
u
m
u
late charge. When the next channel, for example channel 1, is
selected, the acc
u
m
u
lated charge leaks backward thro
u
gh channel 1. If
the o
u
tp
u
t impedance of the so
u
rce connected to channel 1 is high
eno
u
gh, the res
u
lting reading of channel 1 can be partially affected by
the voltage on channel 0. This effect is referred to as ghosting.
If yo
u
r so
u
rce impedance is high, yo
u
can decrease the scan rate to
allow the NI-PGIA more time to settle. Another option is to
u
se a
voltage follower circ
u
it external to yo
u
r DAQ device to decrease the
impedance seen by the DAQ device. Refer to the KnowledgeBase
doc
u
ment,
Decreasing the Source Impedance of an Analog Input
Signal
, by going to
ni.com/info
and entering the Info Code
rdbbi
s
.
2.
Use Short High-Quality Cabling
—Using short high-q
u
ality cables
can minimize several effects that degrade acc
u
racy incl
u
ding crosstalk,
transmission line effects, and noise. The capacitance of the cable also
can increase the settling time.
National Instr
u
ments recommends
u
sing individ
u
ally shielded,
twisted-pair wires that are 2 m or less to connect AI signals to the
device. Refer to the
Connecting Analog Input Signals
section for more
information.
3.
Carefully Choose the Channel Scanning Order
•
Avoid Switching from a Large to a Small Inp
u
t
Range—Switching from a channel with a large inp
u
t range to a
channel with a small inp
u
t range can greatly increase the settling
time.
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