Chapter 4
Analog Input
4-6
ni.com
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):
•
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
.
•
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 on
Connecting Analog Input Signals on
section for more information.
•
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.
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
