NI sbRIO-960x/962x/963x OEM Instructions & Specifications 28 ni.com
Best Practices for Scanning Multiple Channels
NI sbRIO devices can scan multiple channels at high rates and digitize the
signals accurately. If your application scans multiple channels, settling
error can affect the accuracy of your measurements. Settling error is a
difference between the input value and the value that the ADC digitizes,
and is the result of switching between multiplexed input channels. National
Instruments defines settling error as the difference between a
single-channel measurement of a signal and a multiple-channel
measurement of the same signal. Settling errors are directly proportional to
channel-to-channel voltage step size and the time between acquisitions.
NI sbRIO devices scan at a fixed rate equal to the maximum aggregate rate.
To ensure the lowest possible settling errors, design your application
according to the following best practices:
1. Use Low-Impedance Sources—Ensure that the impedance of signal
sources is less than 1 kΩ. High-impedance sources increase settling
errors and decrease accuracy at fast scanning rates. You can reduce
impedance by connecting a voltage-follower circuit between the signal
source and the AI pin of the channel. For more information about
reducing impedance, go 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 can also increase the settling error. National Instruments
recommends using individually shielded twisted-pair wires shorter
than 2 m to connect AI signals to the device. Refer to the Connecting
Analog Input Signals section for more information.
3. Minimize Voltage Step between Adjacent Channels—Settling error
increases with the voltage step between channels. If you know the
expected input ranges of your signals, you can group signals with
similar expected ranges together on adjacent channels.
Note When you program your I/O node with a scan list, the NI sbRIO device scans
channels in numerical order at a fixed rate. Calling a second I/O node adds an extra delay
before the first channel but does not reduce settling error.
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