National Instruments NI USB-621x User Manual

DAQ M Series

NI USB-621x User Manual

Bus-Powered M Series USB Devices

NI USB-621x User Manual

August 2006 371931A-01

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The USB-6210, USB-6211, USB-6215, and USB-6218 devices are warranted against defects in materials and workmanship for a period of three years from the date of shipment, as evidenced by receipts or other documentation. National Instruments will, at its option, repair or replace equipment that proves to be defective during the warranty period. This warranty includes parts and labor.

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About This Manual

Conventions ...................................................................................................................xiii

Related Documentation..................................................................................................xiv

NI-DAQmx for Windows................................................................................xiv

LabVIEW ........................................................................................................xiv

LabWindows™/CVI™....................................................................................xv

Measurement Studio........................................................................................xv

ANSI C without NI Application Software ......................................................xv

.NET Languages without NI Application Software ........................................xvi

Device Documentation and Specifications......................................................xvi

Training Courses .............................................................................................xvi

Technical Support on the Web ........................................................................xvi

Chapter 1 Getting Started

Installing NI-DAQmx ....................................................................................................1-2

Installing Other Software...............................................................................................1-2

Installing the Hardware..................................................................................................1-3

Device Pinouts ...............................................................................................................1-3

Device Specifications ....................................................................................................1-3

Device Accessories ........................................................................................................1-3

Chapter 2 DAQ System Overview

DAQ Hardware ..............................................................................................................2-1

DAQ-STC2......................................................................................................2-2

Calibration Circuitry........................................................................................2-2

Signal Conditioning .......................................................................................................2-3

Sensors and Transducers .................................................................................2-3

Programming Devices in Software ................................................................................2-4

Chapter 3 Connector Information

I/O Connector Signal Descriptions ................................................................................3-1

+5 V Power ....................................................................................................................3-2

+5 V Power as an Output ................................................................................3-2

+5 V Power as an Input ...................................................................................3-3

Contents

Chapter 4 Analog Input

Analog Input Circuitry .................................................................................................. 4-1

Analog Input Range....................................................................................................... 4-2

Analog Input Ground-Reference Settings ..................................................................... 4-3

Multichannel Scanning Considerations ......................................................................... 4-6

Analog Input Data Acquisition Methods....................................................................... 4-9

Analog Input Digital Triggering.................................................................................... 4-10

Field Wiring Considerations..........................................................................................4-11

Analog Input Timing Signals ........................................................................................ 4-11

Configuring AI Ground-Reference Settings in Software................................ 4-5

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

Software-Timed Acquisitions ......................................................................... 4-9

Hardware-Timed Acquisitions........................................................................ 4-9

Buffered ............................................................................................ 4-10

Non-Buffered.................................................................................... 4-10

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 vi ni.com

AI Pause Trigger Signal ..................................................................................4-23

Using a Digital Source ......................................................................4-23

Getting Started with AI Applications in Software.........................................................4-24

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

Connecting Floating Signal Sources..............................................................................5-3

What Are Floating Signal Sources? ................................................................5-3

When to Use Differential Connections with Floating Signal Sources ............5-3

When to Use Referenced Single-Ended (RSE) Connections with

Floating Signal Sources................................................................................5-3

When to Use Non-Referenced Single-Ended (NRSE) Connections

with Floating Signal Sources........................................................................5-4

Using Differential Connections for Floating Signal Sources ..........................5-5

Using Non-Referenced Single-Ended (NRSE) Connections for

Floating Signal Sources................................................................................5-8

Using Referenced Single-Ended (RSE) Connections for Floating

Signal Sources ..............................................................................................5-9

Connecting Ground-Referenced Signal Sources ...........................................................5-9

What Are Ground-Referenced Signal Sources? ..............................................5-9

When to Use Differential Connections with Ground-Referenced

Signal Sources ..............................................................................................5-10

When to Use Non-Referenced Single-Ended (NRSE) Connections with

Ground-Referenced Signal Sources..............................................................5-10

When to Use Referenced Single-Ended (RSE) Connections with

Ground-Referenced Signal Sources..............................................................5-11

Using Differential Connections for Ground-Referenced Signal Sources........5-12

Using Non-Referenced Single-Ended (NRSE) Connections for

Ground-Referenced Signal Sources..............................................................5-13

Contents

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

Differential Measurements ............................................................................................6-1

Differential Pairs............................................................................................................6-1

Referenced Single-Ended (RSE) Measurements ...........................................................6-3

Non-Referenced Single-Ended (NRSE) Measurements ................................................6-4

Contents

Chapter 7 Analog Output

Analog Output Circuitry................................................................................................ 7-1

AO Range ...................................................................................................................... 7-2

Minimizing Glitches on the Output Signal.................................................................... 7-2

Analog Output Data Generation Methods ..................................................................... 7-2

Software-Timed Generations .......................................................................... 7-2

Hardware-Timed Generations......................................................................... 7-2

Analog Output Digital Triggering................................................................................. 7-4

Connecting Analog Output Signals ............................................................................... 7-4

Analog Output Timing Signals...................................................................................... 7-5

AO Start Trigger Signal .................................................................................. 7-5

AO Pause Trigger Signal ................................................................................ 7-6

AO Sample Clock Signal ................................................................................ 7-7

AO Sample Clock Timebase Signal................................................................ 7-8

Getting Started with AO Applications in Software....................................................... 7-9

Using a Digital Source...................................................................... 7-5

Routing AO Start Trigger Signal to an Output Terminal ................. 7-6

Using a Digital Source...................................................................... 7-6

Using an Internal Source .................................................................. 7-7

Using an External Source ................................................................. 7-7

Routing AO Sample Clock Signal to an Output Terminal ............... 7-7

Other Timing Requirements ............................................................. 7-7

Chapter 8 Digital I/O

Static DIO......................................................................................................................8-2

I/O Protection ................................................................................................................ 8-2

Increasing Current Drive ............................................................................................... 8-2

Connecting Digital I/O Signals ..................................................................................... 8-3

Getting Started with DIO Applications in Software...................................................... 8-4

Chapter 9 Counters

Counter Input Applications ........................................................................................... 9-2

Counting Edges ............................................................................................... 9-2

Single Point (On-Demand) Edge Counting ...................................... 9-2

Buffered (Sample Clock) Edge Counting......................................... 9-3

Non-Cumulative Buffered Edge Counting ....................................... 9-4

Controlling the Direction of Counting.............................................. 9-4

NI USB-621x User Manual viii ni.com

Contents

Pulse-Width Measurement ..............................................................................9-5

Single Pulse-Width Measurement.....................................................9-5

Buffered Pulse-Width Measurement.................................................9-5

Period Measurement........................................................................................9-6

Single Period Measurement ..............................................................9-7

Buffered Period Measurement ..........................................................9-7

Semi-Period Measurement ..............................................................................9-9

Single Semi-Period Measurement.....................................................9-9

Buffered Semi-Period Measurement.................................................9-9

Frequency Measurement .................................................................................9-10

Method 1—Measure Low Frequency with One Counter..................9-10

Method 1b—Measure Low Frequency with One Counter

(Averaged)......................................................................................9-11

Method 2—Measure High Frequency with Two Counters...............9-11

Method 3—Measure Large Range of Frequencies Using

Two Counters .................................................................................9-12

Choosing a Method for Measuring Frequency .................................9-13

Position Measurement .....................................................................................9-15

Measurements Using Quadrature Encoders......................................9-15

Measurements Using Two Pulse Encoders.......................................9-17

Two-Signal Edge-Separation Measurement....................................................9-18

Single Two-Signal Edge-Separation Measurement ..........................9-18

Buffered Two-Signal Edge-Separation Measurement ......................9-19

Counter Output Applications .........................................................................................9-20

Simple Pulse Generation .................................................................................9-20

Single Pulse Generation ....................................................................9-20

Single Pulse Generation with Start Trigger ......................................9-20

Retriggerable Single Pulse Generation .............................................9-21

Pulse Train Generation ....................................................................................9-22

Continuous Pulse Train Generation ..................................................9-22

Frequency Generation .....................................................................................9-23

Using the Frequency Generator ........................................................9-23

Frequency Division .........................................................................................9-24

Pulse Generation for ETS................................................................................9-24

Counter Timing Signals.................................................................................................9-25

Counter n Source Signal..................................................................................9-26

Routing a Signal to Counter n Source...............................................9-26

Routing Counter n Source to an Output Terminal ............................9-26

Counter n Gate Signal .....................................................................................9-27

Routing a Signal to Counter n Gate ..................................................9-27

Routing Counter n Gate to an Output Terminal................................9-27

Counter n Aux Signal ......................................................................................9-27

Routing a Signal to Counter n Aux ...................................................9-27

Contents

Counter n A, Counter n B, and Counter n Z Signals ...................................... 9-28

Routing Signals to A, B, and Z Counter Inputs................................ 9-28

Counter n Up_Down Signal............................................................................ 9-28

Counter n HW Arm Signal.............................................................................. 9-28

Routing Signals to Counter n HW Arm Input.................................. 9-28

Counter n Internal Output and Counter n TC Signals..................................... 9-29

Routing Counter n Internal Output to an Output Terminal .............. 9-29

Frequency Output Signal ................................................................................ 9-29

Routing Frequency Output to a Terminal......................................... 9-29

Default Counter/Timer Pinouts ..................................................................................... 9-29

Counter Triggering ........................................................................................................ 9-31

Arm Start Trigger............................................................................................ 9-31

Start Trigger .................................................................................................... 9-31

Pause Trigger .................................................................................................. 9-31

Other Counter Features.................................................................................................. 9-32

Sample Clock .................................................................................................. 9-32

Cascading Counters......................................................................................... 9-33

Counter Filters................................................................................................. 9-33

Prescaling ........................................................................................................ 9-34

Duplicate Count Prevention ............................................................................ 9-35

Example Application That Works Correctly

(No Duplicate Counting) ............................................................... 9-36

Example Application That Works Incorrectly

(Duplicate Counting) ..................................................................... 9-37

Example Application That Prevents Duplicate Count...................... 9-37

Enabling Duplicate Count Prevention in NI-DAQmx...................... 9-38

Chapter 10 PFI

Using PFI Terminals as Timing Input Signals .............................................................. 10-2

Exporting Timing Output Signals Using PFI Terminals............................................... 10-3

Using PFI Terminals as Static Digital I/Os ................................................................... 10-3

Connecting PFI Input Signals........................................................................................ 10-3

PFI Filters ...................................................................................................................... 10-4

I/O Protection ................................................................................................................ 10-6

Programmable Power-Up States.................................................................................... 10-6

NI USB-621x User Manual x ni.com

Chapter 11 Isolation and Digital Isolators

Digital Isolation .............................................................................................................11-2

Benefits of an Isolated DAQ Device .............................................................................11-2

Reducing Common-Mode Noise ...................................................................................11-2

Creating an AC Return Path............................................................................11-3

Isolated Systems................................................................................11-3

Non-Isolated Systems .......................................................................11-3

Chapter 12 Digital Routing and Clock Generation

80 MHz Timebase..........................................................................................................12-1

20 MHz Timebase..........................................................................................................12-1

100 kHz Timebase .........................................................................................................12-2

Chapter 13 Bus Interface

USB Signal Streams.......................................................................................................13-1

Data Transfer Methods ..................................................................................................13-1

USB Signal Stream..........................................................................................13-1

Programmed I/O ..............................................................................................13-2

Changing Data Transfer Methods ...................................................................13-2

Contents

Chapter 14 Triggering

Triggering with a Digital Source ...................................................................................14-1

Appendix A Device-Specific Information

USB-6210 ......................................................................................................................A-1

USB-6211/6215 .............................................................................................................A-4

USB 6218.......................................................................................................................A-7

Contents

Appendix B Troubleshooting

Appendix C Technical Support and Professional Services

Glossary

Index

Device Pinouts

Figure A-1. USB-6210 Pinout .................................................................................. A-2

Figure A-2. USB-6211/6215 Pinout ......................................................................... A-5

Figure A-3. USB 6218 Pinout .................................................................................. A-8

NI USB-621x User Manual xii ni.com

About This Manual

The NI 621x User Manual contains information about using the National Instruments USB-621x data acquisition (DAQ) devices with NI-DAQmx 8.3 and later. NI 621x devices feature up to 32 analog input (AI) channels, up to two analog output (AO) channels, up to eight lines of digital input (DI), up to eight lines of digital output (DO), and two counters.

Conventions

The following conventions are used in this manual:

<> Angle brackets that contain numbers separated by an ellipsis represent

a range of values associated with a bit or signal name—for example, AO <3..0>.

» The » symbol leads you through nested menu items and dialog box options

to a final action. The sequence File»Page Setup»Options directs you to pull down the File menu, select the Page Setup item, and select Options from the last dialog box.

This icon denotes a note, which alerts you to important information.

This icon denotes a caution, which advises you of precautions to take to avoid injury, data loss, or a system crash. When this symbol is marked on a product, refer to the Read Me First: Safety and Radio-Frequency Interference document which can be found at information about precautions to take.

bold Bold text denotes items that you must select or click in the software, such

as menu items and dialog box options. Bold text also denotes parameter names.

italic Italic text denotes variables, emphasis, a cross-reference, or an introduction

to a key concept. Italic text also denotes text that is a placeholder for a word or value that you must supply.

monospace Text in this font denotes text or characters that you should enter from the

keyboard, sections of code, programming examples, and syntax examples. This font is also used for the proper names of disk drives, paths, directories, programs, subprograms, subroutines, device names, functions, operations, variables, filenames, and extensions.

ni.com/manuals, for

About This Manual

Figure 1-1. USB-6210/6211

Chapter 1 Getting Started

Figure 1-2. USB-6215/6218

NI 621x devices feature up to 32 analog input (AI) channels, up to two analog output (AO) channels, 8 lines of digital input (DI), 8 lines of digital output (DO), and two counters. If you have not already installed your device, refer to the NI-DAQmx for USB Devices Getting Started Guide. For specifications, refer to the NI 621x Specifications document on

ni.com/manuals.

Before installing your DAQ device, you must install the software you plan to use with the device.

Installing NI-DAQmx

The NI-DAQmx for USB Devices Getting Started Guide, which you can download at instructions for installing software and hardware, configuring channels and tasks, and getting started developing an application.

ni.com/manuals, offers NI-DAQmx users step-by-step

Installing Other Software

If you are using other software, refer to the installation instructions that accompany your software.

NI USB-621x User Manual 1-2 ni.com

Installing the Hardware

The NI-DAQmx for USB Devices Getting Started Guide contains non-software-specific information about how to install USB devices.

Device Pinouts

Refer to Appendix A, Device-Specific Information, for NI 621x device pinouts.

Device Specifications

Refer to the NI 621x Specifications, available on the NI-DAQ Device Document Browser or about NI 621x devices.

Device Accessories

NI offers a variety of accessories to use with your DAQ device. Refer to Appendix A, Device-Specific Information, or information.

Chapter 1 Getting Started

ni.com/manuals, for more detailed information

ni.com for more

DAQ System Overview

Figure 2-1 shows a typical DAQ system, which includes sensors, transducers, signal conditioning devices, cables that connect the various devices to the accessories, the M Series device, programming software, and PC. The following sections cover the components of a typical DAQ system.

DAQ Hardware

DAQ

Hardware

Figure 2-1. Components of a Typical DAQ System

DAQ hardware digitizes signals, performs D/A conversions to generate analog output signals, and measures and controls digital I/O signals. Figure 2-2 features components common to all USB M Series devices.

DAQ

Software

Personal Computer

or Laptop

Chapter 2 DAQ System Overview

Analog Input

Analog Output

Digital I/O

I/O Connector

Counters

PFI

DAQ-STC2

The DAQ-STC2 implements a high-performance digital engine for M Series data acquisition hardware. Some key features of this engine include the following:

• Flexible AI and AO sample and convert timing

• Many triggering modes

• Independent AI, AO, and CTR FIFOs

• Generation and routing of internal and external timing signals

• Two flexible 32-bit counter/timer modules with hardware gating

• Static DI and static DO signals

• USB Hi-Speed 2.0 interface

• Up to four USB Signal Streams for acquisition and generation

and Clock

Generation

functions

Digital

Routing

Isolation

Barrier

(USB-6215

and USB-6218

devices only)

Digital

Isolators

Bus

Interface

Figure 2-2. USB-621x Block Diagram

Bus

Calibration Circuitry

The M Series analog inputs and outputs have calibration circuitry to correct gain and offset errors. You can calibrate the device to minimize AI and AO errors caused by time and temperature drift at run time. No external circuitry is necessary; an internal reference ensures high accuracy and stability over time and temperature changes.

NI USB-621x User Manual 2-2 ni.com

Factory-calibration constants are permanently stored in an onboard EEPROM and cannot be modified. When you self-calibrate the device, software stores new constants in a user-modifiable section of the EEPROM. To return a device to its initial factory calibration settings, software can copy the factory-calibration constants to the user-modifiable section of the EEPROM. Refer to the NI-DAQmx Help or the LabVIEW 8.x Help for more information about using calibration constants.

Signal Conditioning

Many sensors and transducers require signal conditioning before a measurement system can effectively and accurately acquire the signal. The front-end signal conditioning system can include functions such as signal amplification, attenuation, filtering, electrical isolation, simultaneous sampling, and multiplexing. In addition, many transducers require excitation currents or voltages, bridge completion, linearization, or high amplification for proper and accurate operation. Therefore, most computer-based measurement systems include some form of signal conditioning in addition to plug-in data acquisition DAQ devices.

Sensors and Transducers

Sensors can generate electrical signals to measure physical phenomena, such as temperature, force, sound, or light. Some commonly used sensors are strain gauges, thermocouples, thermistors, angular encoders, linear encoders, and resistance temperature detectors (RTDs).

Chapter 2 DAQ System Overview

To measure signals from these various transducers, you must convert them into a form that a DAQ device can accept. For example, the output voltage of most thermocouples is very small and susceptible to noise. Therefore, you may need to amplify or filter the thermocouple output before digitizing it. The manipulation of signals to prepare them for digitizing is called signal conditioning.

For more information about sensors, refer to the following documents.

• For general information about sensors, visit

• If you are using LabVIEW, refer to the LabVIEW Help by selecting Help»Search the LabVIEW Help in LabVIEW and then navigate to the Taking Measurements book on the Contents tab.

• If you are using other application software, refer to Common Sensors in the NI-DAQmx Help or the LabVIEW 8.x Help.

ni.com/sensors.

Chapter 2 DAQ System Overview

Programming Devices in Software

National Instruments measurement devices are packaged with NI-DAQ driver software, an extensive library of functions and VIs you can call from your application software, such as LabVIEW or LabWindows/CVI, to program all the features of your NI measurement devices. Driver software has an application programming interface (API), which is a library of VIs, functions, classes, attributes, and properties for creating applications for your device.

NI-DAQ 7.3 and later includes two NI-DAQ drivers—Traditional NI-DAQ (Legacy) and NI-DAQmx. M Series devices use the NI-DAQmx driver. Each driver has its own API, hardware configuration, and software configuration. Refer to the NI-DAQmx for USB Devices Getting Started Guide for more information about the two drivers.

NI-DAQmx includes a collection of programming examples to help you get started developing an application. You can modify example code and save it in an application. You can use examples to develop a new application or add example code to an existing application.

To locate LabVIEW and LabWindows/CVI examples, open the National Instruments Example Finder.

• In LabVIEW, select Help»Find Examples.

• In LabWindows/CVI, select Help»NI Example Finder.

Measurement Studio, Visual Basic, and ANSI C examples are located in the following directories:

• NI-DAQmx examples for Measurement Studio-supported languages are in the following directories:

–

MeasurementStudio\VCNET\Examples\NIDaq

– MeasurementStudio\DotNET\Examples\NIDaq

• NI-DAQmx examples for ANSI C are in the

NI-DAQ\Examples\DAQmx ANSI C Dev directory

For additional examples, refer to

NI USB-621x User Manual 2-4 ni.com

zone.ni.com.

Connector Information

The I/O Connector Signal Descriptions and +5 V Power sections contain information about NI 621x connectors. Refer to Appendix A,

Device-Specific Information, for device I/O connector pinouts.

I/O Connector Signal Descriptions

Table 3-1 describes the signals found on the I/O connectors. Not all signals are available on all devices.

Table 3-1. I/O Connector Signals

Signal Name Reference Direction Description

AI GND — — Analog Input Ground—These terminals are the

reference point for single-ended AI measurements in RSE mode and the bias current return point for DIFF measurements. All three ground references—AI GND, AO GND, and D GND—are connected on the device.

AI <0..31> Va r ie s Input Analog Input Channels 0 to 31—For single-ended

measurements, each signal is an analog input voltage channel. In RSE mode, AI GND is the reference for these signals. In NRSE mode, the reference for each AI <0..31> signal is AI SENSE.

For differential measurements, AI 0 and AI 8 are the positive and negative inputs of differential analog input channel 0. Similarly, the following signal pairs also form differential input channels:

AI SENSE — Input Analog Input Sense—In NRSE mode, the reference for

each AI <0..31> signal is AI SENSE.

AO <0..1> AO GND Output Analog Output Channels 0 to 1—These terminals

supply the voltage output of AO channels 0 to 1.

Chapter 3 Connector Information

Table 3-1. I/O Connector Signals (Continued)

Signal Name Reference Direction Description

AO GND — — Analog Output Ground—AO GND is the reference for

AO <0..1>. All three ground references—AI GND, AO GND, and D GND—are connected on the device.

D GND — — Digital Ground—D GND supplies the reference for

PFI <0..15>/P0/P1 and +5 V. All three ground references—AI GND, AO GND, and D GND—are connected on the device.

+5 V D GND Input or

Output

PFI <0..3>, PFI <8..11>/P0.<0..7>

PFI <4..7>, PFI <12..15>/P1.<0..7>

NC — — No connect—Do not connect signals to these terminals.

D GND Input Programmable Function Interface or Static Digital

D GND Output Programmable Function Interface or Static Digital

+5 V Power—These terminals provide a +5 V power source or can be used to externally power the PFI outputs.

Input Channels 0 to 7—Each PFI terminal can be used to supply an external source for AI, AO, or counter/timer inputs.

You also can use these terminals as static digital input lines.

Output Channels 0 to 7—You can route many different internal AI, AO, or counter/timer outputs to each PFI terminal.

You also can use these terminals as static digital output lines.

+5 V Power

The +5 V terminals on the I/O connector can be use as either an output or an input. Both terminals are internally connected on the USB-621x.

+5 V Power as an Output

Because the USB-621x devices are bus powered, there is a 50 mA limit on the total current that can be drawn from the +5 V terminals and the digital outputs PFI <4..7> and PFI <12..15>/P1.<0..7>. The USB-621x monitors the total current and will drop the voltage on all of the digital outputs and the +5 V terminals if the 50 mA limit is exceeded.

NI USB-621x User Manual 3-2 ni.com

+5 V Power as an Input

If you have high current loads for the digital outputs to drive, you can exceed the 50 mA internal limit by connecting an external +5 V power source to the +5 V terminals. These terminals are protected against undervoltage and overvoltage, and they have a 350 mA self-resetting fuse to protect them from short circuit conditions. If your USB-621x device has more than one +5 V terminal, you can connect the external power supply to one terminal and use the other as a power source.

Chapter 3 Connector Information

Analog Input

Figure 4-1 shows the analog input circuitry of NI 621x devices.

AI <0..n>

MUX

DIFF, RSE,

AI SENSE

I/O Connector

AI GND

AI Terminal

Configuration

or NRSE

Selection

NI-PGIA

Input Range

Selection

ADC

AI FIFO

Isolation

Barrier

(USB-6215

and USB-6218

devices only)

Digital

Isolators

AI Data

Figure 4-1. M Series Analog Input Circuitry

Analog Input Circuitry

I/O Connector

You can connect analog input signals to the M Series device through the I/O connector. The proper way to connect analog input signals depends on the analog input ground-reference settings, described in the Analog Input

Ground-Reference Settings section. Also refer to Appendix A, Device-Specific Information, for device I/O connector pinouts.

MUX

Each M Series device has one analog-to-digital converter (ADC). The multiplexers (MUX) route one AI channel at a time to the ADC through the NI-PGIA.

Chapter 4 Analog Input

Ground-Reference Settings

The analog input ground-reference settings circuitry selects between differential, referenced single-ended, and non-referenced single-ended input modes. Each AI channel can use a different mode.

Instrumentation Amplifier (NI-PGIA)

The NI programmable gain instrumentation amplifier (NI-PGIA) is a measurement and instrument class amplifier that minimizes settling times for all input ranges. The NI-PGIA can amplify or attenuate an AI signal to ensure that you use the maximum resolution of the ADC.

M Series devices use the NI-PGIA to deliver high accuracy even when sampling multiple channels with small input ranges at fast rates. M Series devices can sample channels in any order at the maximum conversion rate, and you can individually program each channel in a sample with a different input range.

A/D Converter

The analog-to-digital converter (ADC) digitizes the AI signal by converting the analog voltage into a digital number.

AI FIFO

M Series devices can perform both single and multiple A/D conversions of a fixed or infinite number of samples. A large first-in-first-out (FIFO) buffer holds data during AI acquisitions to ensure that no data is lost. M Series devices can handle multiple A/D conversion operations with DMA, interrupts, or programmed I/O.

Analog Input Range

The input range affects the resolution of the M Series device for an AI channel. For example, a 16-bit ADC converts analog inputs into one of 65,536 (= 2 an input range of –10 V to 10 V, the voltage of each code of a 16-bit ADC is:

M Series devices use a calibration method that requires some codes (typically about 5% of the codes) to lie outside of the specified range. This

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) codes—that is, one of 65,536 possible digital values. So, for

(10 V – (–10 V))

= 305 μV

Chapter 4 Analog Input

calibration method improves absolute accuracy, but it increases the nominal resolution of input ranges by about 5% over what the formula shown above would indicate.

Choose an input range that matches the expected input range of your signal. A large input range can accommodate a large signal variation, but reduces the voltage resolution. Choosing a smaller input range improves the voltage resolution, but may result in the input signal going out of range.

For more information about setting ranges, refer to the NI-DAQmx Help or the LabVIEW 8.x Help.

Table 4-1 shows the input ranges and resolutions supported by NI 621x devices.

Table 4-1. Input Ranges for NI 621x

Nominal Resolution Assuming

Input Range

–10 V to 10 V 320 μV

–5 V to 5 V 160 μV

5% Over Range

–1 V to 1 V 32 μV

–200 mV to 200 mV 6.4 μV

Analog Input Ground-Reference Settings

NI 621x devices support the analog input ground-reference settings shown in Table 4-2.

Table 4-2. Analog Input Ground-Reference Settings

AI Ground-Reference

Settings

DIFF In differential (DIFF) mode, NI 621x devices measure the difference in

voltage between two AI signals.

RSE In referenced single-ended (RSE) mode, NI 621x devices measure the

voltage of an AI signal relative to AI GND.

NRSE In non-referenced single-ended (NRSE) mode, NI 621x devices measure

the voltage of an AI signal relative to the AI SENSE input.

Description

Chapter 4 Analog Input

The AI ground-reference setting determines how you should connect your AI signals to the NI 621x device. Refer to Chapter 5, Connecting AI Signals

on the USB-6210/6211 Devices, section for more information.

Ground-reference settings are programmed on a per-channel basis. For example, you might configure the device to scan 12 channels—four differentially-configured channels and eight single-ended channels.

NI 621x devices implement the different analog input ground-reference settings by routing different signals to the NI-PGIA. The NI-PGIA is a differential amplifier. That is, the NI-PGIA amplifies (or attenuates) the difference in voltage between its two inputs. The NI-PGIA drives the ADC with this amplified voltage. The amount of amplification (the gain), is determined by the analog input range, as shown in Figure 4-2.

Instrumentation

in+

Amplifier

AI Ground-Reference

Settings

RSE AI <0..31> AI GND

NRSE AI <0..31> AI SENSE

DIFF AI <0..7> AI <8..15>

] × Gain

Measured

Voltage

–

PGIA

in–

Vm = [V

in+

– V

in–

Figure 4-2. NI-PGIA

Table 4-3 shows how signals are routed to the NI-PGIA.

Table 4-3. Signals Routed to the NI-PGIA

Signals Routed to the Positive

Input of the NI-PGIA (V

in+

Signals Routed to the Negative

)

Input of the NI-PGIA (V

in–

)

AI <16..23> AI <24..31>

For differential measurements, AI 0 and AI 8 are the positive and negative inputs of differential analog input channel 0. For a complete list of signal

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National Instruments NI USB-621x User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

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Contents

About This Manual

Conventions

Related Documentation

NI-DAQmx for Windows

LabVIEW

Measurement Studio

ANSI C without NI Application Software

.NET Languages without NI Application Software

Device Documentation and Specifications

Training Courses

Technical Support on the Web

Figure 1-1. USB-6210/6211

Figure 1-2. USB-6215/6218

Installing NI-DAQmx

Installing Other Software

Installing the Hardware

Device Pinouts

Device Specifications

Device Accessories

DAQ Hardware

Figure 2-1. Components of a Typical DAQ System

DAQ-STC2

Figure 2-2. USB-621x Block Diagram

Calibration Circuitry

Signal Conditioning

Sensors and Transducers

Programming Devices in Software

I/O Connector Signal Descriptions

Table 3-1. I/O Connector Signals

+5 V Power

+5 V Power as an Output

+5 V Power as an Input

Figure 4-1. M Series Analog Input Circuitry

Analog Input Circuitry

Analog Input Range

Table 4-1. Input Ranges for NI 621x

Analog Input Ground-Reference Settings

Table 4-2. Analog Input Ground-Reference Settings

Figure 4-2. NI-PGIA

Table 4-3. Signals Routed to the NI-PGIA