National Instruments Data Acquisition Device NI USB-621x User Manual page 39

Chapter 4 Analog Input
NI USB-621x User Manual
of your measurements. To ensure fast settling times, you should do the
following (in order of importance):
• Use Low Impedance Sources—To ensure fast settling times, your
signal sources should have an impedance of <1 kΩ. Large source
impedances increase the settling time of the NI-PGIA, and so decrease
the accuracy at fast scanning rates.
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, How Do I Create a Buffer to Decrease the Source
Impedance of My Analog Input Signal?, by going to
and entering the info code
• 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
USB-6210/6211/6212 Devices
USB-6215/6216/6218 Devices
• 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
.
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Connecting Analog Input Signals on
or Connecting Analog Input Signals on
section for more information.
4-6
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