National Instruments Data Acquisition Device E Series - Manual (Page 43)

Chapter 2 Analog Input
E Series User Manual 2-10 ni.com
E Series devices are designed to have fast settling times. Several factors
can increase the settling time, which decreases the accuracy of your
measurements. To ensure fast settling times, you should (in order of
importance):
• Use Low Impedance Sources
• Use Short High-Quality Cabling
• Carefully Choose the Channel Scanning Order
• Avoid Scanning Faster than Necessary
All E Series devices can acquire data in the interval-scanning mode, which
fully accommodates multichannel acquisition in both round-robin and
pseudo-simultaneous fashions. In multichannel scanning mode, the
maximum conversion rate of the device is distributed among the number of
channels scanned. With the addition of external signal conditioners, such as
the SCXI-1140 or the SC-2040, you can perform true simultaneous
sample-and-hold acquisition of eight channels.
Use Low Impedance Sources
To ensure fast settling times, your signal sources should have an
impedances of <1 k
Ω. The settling time specifications for your device
assume a 1 k
Ω source. Large source impedances increase the settling time
of the 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 that channel. If
the output impedance of the source connected to channel 1 is high enough,
the resulting reading of channel 1 can partially reflect the voltage on
channel 0. This is referred to as ghosting, or crosstalk.
If your source impedance is high, you can decrease the scan rate to allow
the 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?, for
more information.