National Instruments NI USB-621x - Page 6

Contents
NI USB-621x User Manual vi ni.com
Chapter 4
Analog Input
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