National Instruments NI USB-621x 6

Contents

Analog Input Circuitry ..................................................................................................	4-1
Analog Input Range.......................................................................................................	4-2
Analog Input Ground-Reference Settings .....................................................................	4-3
Configuring AI Ground-Reference Settings in Software................................	4-5
Multichannel Scanning Considerations.........................................................................	4-6
Use Low Impedance Sources..........................................................................	4-6
Carefully Choose the Channel Scanning Order ..............................................	4-7
Avoid Switching from a Large to a Small Input Range ...................	4-7
Insert Grounded Channel between Signal Channels ........................	4-7
Minimize Voltage Step between Adjacent Channels .......................	4-8
Avoid Scanning Faster Than Necessary .........................................................	4-8
Example 1 .........................................................................................	4-8
Example 2 .........................................................................................	4-9
Analog Input Data Acquisition Methods.......................................................................	4-9
Software-Timed Acquisitions .........................................................................	4-9
Hardware-Timed Acquisitions........................................................................	4-9
Buffered ............................................................................................	4-10
Non-Buffered....................................................................................	4-10
Analog Input Digital Triggering....................................................................................	4-10
Field Wiring Considerations..........................................................................................	4-11
Analog Input Timing Signals ........................................................................................	4-11
AI Sample Clock Signal..................................................................................	4-14
Using an Internal Source ..................................................................	4-15
Using an External Source .................................................................	4-15
Routing AI Sample Clock Signal to an Output Terminal.................	4-15
Other Timing Requirements .............................................................	4-15
AI Sample Clock Timebase Signal .................................................................	4-16
AI Convert Clock Signal.................................................................................	4-16
Using an Internal Source ..................................................................	4-17
Using an External Source .................................................................	4-17
Routing AI Convert Clock Signal to an Output Terminal................	4-17
Using a Delay from Sample Clock to Convert Clock ......................	4-17
Other Timing Requirements .............................................................	4-18
AI Convert Clock Timebase Signal ................................................................	4-20
AI Hold Complete Event Signal .....................................................................	4-21
AI Start Trigger Signal....................................................................................	4-21
Using a Digital Source......................................................................	4-21
Routing AI Start Trigger to an Output Terminal ..............................	4-21
AI Reference Trigger Signal ...........................................................................	4-22
Using a Digital Source......................................................................	4-23
Routing AI Reference Trigger Signal to an Output Terminal ..........	4-23

NI USB-621x User Manual

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