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Chapter 9 Isolation and Digital Isolators on USB-6215/6216/6218 Devices
© National Instruments Corporation 9-2 NI USB-621x User Manual
The non-isolated ground is connected to the chassis ground of the PC.
Each isolated ground is not connected to the chassis ground of the PC.
The isolated ground can be at a higher or lower voltage relative to the
non-isolated ground. All analog measurements are made relative to the
isolated ground signal.
The isolated ground is an input to the USB-6215/6216/6218 device. The
user must connect this ground to the ground of system being measured or
controlled. For more information, refer to the following:
• The Connecting Analog Input Signals on USB-6215/6216/6218
Devices section of Chapter 4, Analog Input
• The Connecting Analog Output Signals section of Chapter 5, Analog
Output
• The Connecting Digital I/O Signals on USB-6210/6211/6215/6218
Devices or Connecting Digital I/O Signals on USB-6212/6216 Devices
section of Chapter 6, Digital I/O
• The Connecting PFI Input Signals section of Chapter 7, PFI
Digital Isolation
USB-6215/6216/6218 devices use digital isolators. Unlike analog isolators,
digital isolators do not introduce any analog error in the measurements
taken by the device. The A/D converter, used for analog input, is on the
isolated side of the device. The analog inputs are digitized before they are
sent across the isolation barrier. Similarly, the D/A converters, used for
analog output, are on the isolated side of the device.
Benefits of an Isolated DAQ Device
With isolation, engineers can safely measure a small signal in the presence
of a large common-mode voltage signal. Some advantages of isolation are
as follows:
• Improved rejection—Isolation increases the ability of the
measurement system to reject common-mode voltages.
Common-mode voltage is the signal that is present or “common”
to both the positive and negative input of a measurement device, but
is not part of the signal to be measured.
• Improved accuracy—Isolation improves measurement accuracy by
physically preventing ground loops. Ground loops, a common source
- DAQ M Series 1
- Important Information 3
- Compliance 4
- Chapter 4 6
- Chapter 5 7
- Contents 10
- About This Manual 12
- Related Documentation 13
- LabWindows/CVI 15
- Measurement Studio 15
- Training Courses 17
- Technical Support on the Web 17
- Getting Started 18
- Device Specifications 19
- USB Cable Strain Relief 20
- Mounting the USB-621x 21
- Panel Mounting 22
- Chapter 1 Getting Started 23
- DAQ System Overview 24
- DAQ-STC2 25
- Calibration Circuitry 25
- Signal Conditioning 26
- Cables and Accessories 27
- Chapter 2 DAQ System Overview 28
- Connector and LED Information 29
- +5 V Power 31
- USB Device Fuse Replacement 31
- 0.75A,125V Fuse, Socketed 32
- PWR/ACT LED Indicator 33
- Analog Input 34
- Analog Input Range 35
- Chapter 4 Analog Input 36
- Figure 4-2. NI-PGIA 37
- NI-DAQmx Create 38
- Virtual Channel.vi 38
- Field Wiring Considerations 43
- Analog Input Timing Signals 44
- AI Sample Clock Signal 47
- Using an Internal Source 47
- Using an External Source 47
- Other Timing Requirements 47
- AI Convert Clock Signal 49
- AI Hold Complete Event Signal 52
- AI Start Trigger Signal 53
- Using a Digital Source 53
- Reference Trigger 54
- Pretrigger Samples 54
- Complete Buffer 54
- Posttrigger Samples 54
- AI Pause Trigger Signal 55
- USB-6210/6211/6212 Devices 56
- (NRSE Configuration) 68
- AO Range 73
- Chapter 5 Analog Output 74
- Analog Output Timing Signals 75
- AO Start Trigger Signal 76
- AO Pause Trigger Signal 77
- Pause Trigger 78
- Sample Clock 78
- Digital I/O 81
- Chapter 6 Digital I/O 82
- Chapter 7 PFI 89
- Connecting PFI Input Signals 91
- PFI Filters 91
- Figure 7-4. Filter Example 92
- Table 7-1. Filters 92
- I/O Protection 93
- Programmable Power-Up States 93
- Counters 94
- Counter Input Applications 95
- Pulse-Width Measurement 97
- Period Measurement 99
- Single Period Measurement 99
- Buffered Period Measurement 100
- Semi-Period Measurement 101
- Frequency Measurement 102
- Figure 8-10. Method 1 103
- Figure 8-11. Method 1b 104
- Figure 8-12. Method 2 105
- Figure 8-13. Method 3 106
- Chapter 8 Counters 107
- Position Measurement 108
- Channel Z Behavior 109
- Counter Output Applications 113
- Pulse Train Generation 115
- Frequency Generation 116
- Using the Frequency Generator 116
- Frequency Division 117
- Pulse Generation for ETS 117
- Counter Timing Signals 118
- Counter n Source Signal 119
- Counter n Gate Signal 120
- Counter n Aux Signal 120
- Counter n Up_Down Signal 121
- Counter n HW Arm Signal 121
- Default Counter/Timer Pinouts 123
- Counter Triggering 124
- Other Counter Features 125
- Cascading Counters 126
- Counter Filters 126
- Prescaling 127
- Duplicate Count Prevention 128
- Duplicate Counting) 128
- (Duplicate Counting) 129
- USB-6215/6216/6218 Devices 131
- Digital Isolation 132
- Reducing Common-Mode Noise 133
- Isolated Systems 134
- Non-Isolated Systems 134
- 80 MHz Timebase 135
- 20 MHz Timebase 135
- 100 kHz Timebase 135
- Bus Interface 136
- Chapter 11 Bus Interface 137
- Triggering 138
- Chapter 12 Triggering 139
- Device-Specific Information 140
- USB-6210 141
- USB-6211/6215 143
- USB-6212/6216 Screw Terminal 145
- USB-6216 Mass Termination 147
- TERMINAL 1 148
- TERMINAL 34 148
- TERMINAL 35 148
- TERMINAL 68 148
- NC = No Connect 148
- SCC Accessories 149
- BNC Accessories 150
- Screw Terminal Accessories 150
- USB-6218 152
- Troubleshooting 154
- Channel 0 155
- Channel 1 155
- Convert Period 155
- Sample Period 155
- Analog Output 156
- Technical Support and 157
- Professional Services 157
- Glossary 159
- Numerics 175
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