Use Low Impedance Sources; Use Short High-Quality Cabling - National Instruments NI 6221 User Manual

Chapter 4 Analog Input

Use Low Impedance Sources

Use Short High-Quality Cabling

M Series User Manual
ADC. The specifications document for your DAQ device lists its settling
time.
M Series devices are designed to have fast settling times. However, several
factors can increase the settling time which decreases the accuracy of your
measurements. To ensure fast settling times, you should do the following
(in order of importance):
• Use Low Impedance Sources
• Use Short High-Quality Cabling
• Carefully Choose the Channel Scanning Order
• Avoid Scanning Faster Than Necessary
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
ni.com/info
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
and entering the info code
4-8
.
rdbbis
section for more information.
ni.com