National Instruments PCI-6238, User Manual

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Chapter 4 Analog Input
NI 6238/6239 User Manual 4-8 ni.com
Multichannel Scanning Considerations
M Series devices can scan multiple channels at high rates and digitize the
signals accurately. However, you should consider several issues when
designing your measurement system to ensure the high accuracy of your
measurements.
In multichannel scanning applications, accuracy is affected by settling
time. When your M Series device switches from one AI channel to another
AI channel, the device configures the NI-PGIA with the input range of the
new channel. The NI-PGIA then amplifies the input signal with the gain for
the new input range. Settling time refers to the time it takes the NI-PGIA to
amplify the input signal to the desired accuracy before it is sampled by the
ADC. The
NI 6238/6239 Specifications shows the device 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 short high-quality cabling
• Minimize current step between adjacent channels
• Avoid scanning faster than necessary
Refer to the following sections for more information on these factors.
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 Current Input Signals section for more
information.
Minimize Current Step between Adjacent Channels
When scanning between channels that have the same input range, the
settling time increases with the current step between the channels. If you
know the expected input range of your signals, you can group signals with
similar expected ranges together in your scan list.