© National Instruments
|
4-7
M Series User Manual
Settling times increase when scanning high-impedance signals due to a phenomenon called
charge injection. Multiplexers contain switches, usually made of switched capacitors.
When one of the channels, for example channel 0, is selected in a multiplexer, those
capacitors accumulate charge. When the next channel, for example channel 1, is selected,
the accumulated charge leaks backward through channel 1. If the output impedance of the
source connected to channel 1 is high enough, the resulting reading of channel 1 can be
partially affected by the voltage on channel 0. This effect is referred to as ghosting.
If your source impedance is high, you can decrease the scan rate to allow the NI-PGIA more
time to settle. Another option is to use a voltage follower circuit external to your DAQ
device to decrease the impedance seen by the DAQ device. Refer to the KnowledgeBase
document,
Decreasing the Source Impedance of an Analog Input Signal
, by going to
ni.com/info
and entering the Info Code
rdbbis
.
2.
Use Short High-Quality Cabling
—Using short high-quality cables can minimize several
effects that degrade accuracy including crosstalk, transmission line effects, and noise. The
capacitance of the cable also can increase the settling time.
National Instruments recommends using individually 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 Input Range—Switching from a channel
with a large input range to a channel with a small input range can greatly increase the
settling time.
Suppose a 4 V signal is connected to channel 0 and a 1 mV signal is connected to
channel 1. The input range for channel 0 is -10 V to 10 V and the input range of
channel 1 is -200 mV to 200 mV.
When the multiplexer switches from channel 0 to channel 1, the input 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 full-scale range. For a 16-bit device to settle within 0.0015% (15 ppm or
1 LSB) of the ±200 mV full-scale range on channel 1, the input circuitry must 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 circuitry to settle this much.
To avoid this effect, you should arrange your channel scanning order so that transitions
from large to small input ranges are infrequent.
In general, you do not need this extra settling time when the NI-PGIA is switching
from a small input range to a larger input range.
–
Insert Grounded Channel between Signal Channels—Another technique to improve
settling time is to connect an input channel to ground. Then insert this channel in the
scan list between two of your signal channels. The input range of the grounded channel
should match the input range of the signal after the grounded channel in the scan list.
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