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National Instruments NI USB-621x User Manual

National Instruments NI USB-621x
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Chapter 4 Analog Input
NI USB-621x User Manual 4-8 ni.com
You can connect channel 2 to AI GND (or you can use the internal ground
signal; refer to Internal Channels in the NI-DAQmx Help). Set the input
range of channel 2 to –200 mV to 200 mV to match channel 1. Then scan
channels in the order: 0, 2, 1.
Inserting a grounded channel between signal channels improves settling
time because the NI-PGIA adjusts to the new input range setting faster
when the input is grounded.
Minimize Voltage Step between Adjacent Channels
When scanning between channels that have the same input range, the
settling time increases with the voltage 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.
For example, suppose all channels in a system use a –5 to 5 V input range.
The signals on channels 0, 2, and 4 vary between 4.3 V and 5 V. The signals
on channels 1, 3, and 5 vary between –4 V and 0 V. Scanning channels in
the order 0, 2, 4, 1, 3, 5 produces more accurate results than scanning
channels in the order 0, 1, 2, 3, 4, 5.
Avoid Scanning Faster Than Necessary
Designing your system to scan at slower speeds gives the NI-PGIA more
time to settle to a more accurate level. Here are two examples to consider.
Example 1
Averaging many AI samples can increase the accuracy of the reading by
decreasing noise effects. In general, the more points you average, the more
accurate the final result. However, you may choose to decrease the number
of points you average and slow down the scanning rate.
Suppose you want to sample 10 channels over a period of 20 ms and
average the results. You could acquire 250 points from each channel at a
scan rate of 125 kS/s. Another method would be to acquire 500 points from
each channel at a scan rate of 250 kS/s. Both methods take the same amount
of time. Doubling the number of samples averaged (from 250 to 500)
decreases the effect of noise by a factor of 1.4 (the square root of 2).
However, doubling the number of samples (in this example) decreases the
time the NI-PGIA has to settle from 8 µs to 4 µs. In some cases, the slower
scan rate system returns more accurate results.

Table of Contents

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National Instruments NI USB-621x Specifications

General IconGeneral
ModelNI USB-621x
CategoryComputer Hardware
InterfaceUSB
Analog Input Resolution16 bits
Analog Input Sample Rate250 kS/s (aggregate)
Analog Output Channels2
Analog Output Resolution16 bits
Digital I/O Channels8
Counter/Timers2
Operating System CompatibilityWindows, Linux
Power SupplyUSB powered
Device TypeData Acquisition Device
Analog Input Channels16 single-ended or 8 differential
Analog Input Range±10 V
Operating Temperature0 °C to 50 °C

Summary

Chapter 1 Getting Started

Installing NI-DAQmx

Details on installing the NI-DAQmx software, including step-by-step instructions for setup and configuration.

Installing the Hardware

Non-software-specific information for installing USB DAQ devices.

Chapter 2 DAQ System Overview

Programming Devices in Software

How to program DAQ devices using NI-DAQ driver software, functions, and VIs.

Chapter 3 Connector Information

I/O Connector Signal Descriptions

Details the signals found on the I/O connectors, including AI, AO, and PFI signals.

Chapter 4 Analog Input

Analog Input Ground-Reference Settings

Describes supported ground-reference settings (DIFF, RSE, NRSE) for analog input.

Multichannel Scanning Considerations

Issues to consider for accurate multichannel scanning, such as settling time and channel order.

Analog Input Data Acquisition Methods

Explains software-timed vs. hardware-timed acquisitions and buffered vs. non-buffered modes.

Analog Input Digital Triggering

Details the support for start, reference, and pause triggers for analog input.

Analog Input Timing Signals

Summarizes the various timing signals used for analog input operations.

Getting Started with AI Applications in Software

Guidance on using M Series devices for analog input applications in software.

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

Connecting Floating Signal Sources

How to connect floating signal sources, including when to use differential or single-ended configurations.

Connecting Ground-Referenced Signal Sources

How to connect ground-referenced signal sources and common connection configurations.

Using Differential Connections for Floating Signal Sources

Diagrams and explanations for differential connections with floating signal sources.

Chapter 6 Connecting AI Signals on the USB-6215/6218 Devices

Chapter 7 Analog Output

Analog Output Data Generation Methods

Explains software-timed vs. hardware-timed generations for analog output.

Analog Output Digital Triggering

Details the support for start and pause triggers for analog output operations.

Chapter 8 Digital I/O

Chapter 9 Counters

Frequency Measurement

Explains various methods for measuring frequency using the device's counters.

Counter Output Applications

Details applications for counter outputs, including pulse and train generation.

Counter Triggering

Describes arm start, start, and pause triggers for counter operations.

Chapter 10 PFI

Chapter 11 Isolation and Digital Isolators

Chapter 14 Triggering

Triggering with a Digital Source

How to generate triggers using digital signals, specifying source and edge.

Appendix B Troubleshooting

Appendix C Technical Support and Professional Services

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