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National Instruments SCB-68 Differential Connections for Nonreferenced or Floating Signal Sources

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Chapter 3 Connecting Signals

© National Instruments Corporation 3-7 SCB-68 Shielded Connector Block User Manual

Differential Connections for Nonreferenced or Floating Signal Sources

Figure3 -3 shows how to connect a floating signal source to a channel on

the DAQ device configured in DIFF input mode.

Figure 3-3. Differential Input Connections for Nonreferenced Signals

Using Bias Resistors

Figure 3-3 shows a bias resistor connected between ACH– or ACH<i+8>,

and AIGND. This resistor provides a return path for the ±200 pA bias

current. A value of 10 kΩ to 100 kΩ is usually sufficient. Ifyou do not use

the resistor and the source is truly floating, the source is not likely to remain

within the common-mode signal range of the PGIA, and the PGIA

saturates, causing erroneous readings. You must reference the source to the

respective channel ground.

Measurement Device Configured in DIFF Input Mode

PGIA

–

+

–

+

–

+

Floating

Signal

Source

Vs

Bias

Resistor

(see text)

I/O Connector

AISENSE*

AIGND

Instrumentation

Amplifier

Measured

Voltage

Vm

ACH+ or ACH<

i

>

ACH– or ACH<

i

+8>

*AISENSE is not present on all devices.

Contents

Main Page Important Information Warranty Copyright Trademarks Patents Compliance FFCC/Canada Radio Frequency Interference Compliance Determining FCC Class FCC/DOC Warnings Class A Compliance to EU Directives Contents Chapter 4 Using Thermocouples Chapter 5 Adding Components for Special Functions Page About This Manual Conventions Page Introduction What You Need to Get Started Quick Reference Label Page Table 1-1. Device-Specific Hardware Configuration (Continued) Installing Cables Using 68-Pin Cables Using 100-Pin Cables Page Figure 1-4. SCB-68 E Series I/O Connector Pinout (Full) Figure 1-5. SCB-68 E Series I/O Connector Pinout (Extended AI) Figure 1-6. SCB-68 E Series I/O Connector Pinout (Extended Digital) Configuring the SCB-68 Safety Information Page Page Parts Locator and Wiring Guide Figure 2-1. SCB-68 Printed Circuit Diagram SCB-68 Switch Configuration Page Page Connecting Signals Connecting Analog Input Signals Input Modes Figure 3-1. Summary of AI Connections Using Bias Resistors Nonreferenced or Floating Signal Sources Ground-Referenced Signal Sources Differential Connection Considerations (DIFF Input Mode) Differential Connections for Ground-Referenced Signal Sources i Figure 3-2. Differential Input Connections for Ground-Referenced Signals Differential Connections for Nonreferenced or Floating Signal Sources Using Bias Resistors Single-Ended Connection Considerations Single-Ended Connections for Floating Signal Sources (RSE Input Mode) Single-Ended Connections for Grounded Signal Sources (NRSE Input Mode) Connecting Analog Output Signals Connecting Digital Signals Connecting Timing Signals Noise Considerations Page Using Thermocouples Switch Settings and Temperature Sensor Configuration Special Considerations Adding Components for Special Functions Channel Pad Configurations Conditioning Analog Input Channels Conditioning Analog Output Channels Figure 5-3. Analog Output Channel Configuration Diagram for DAC0OUT Conditioning PFI0/TRIG1 Figure 5-5. Digital Input Channel Configuration Diagram for PFI0/TRIG1 Figure 5-4. Digital Input Channel Configuration Diagram Accuracy and Resolution Considerations Open Thermocouple Detection Differential Open Thermocouple Detection Single-Ended Open Thermocouple Detection Sources of Error Lowpass Filtering Page Page One-Pole Lowpass RC Filter Selecting Components Single-Ended Lowpass Filter Differential Lowpass Filter Analog Output and Digital Input Lowpass Filtering Lowpass Filtering Applications Noise Filtering Antialiasing Filtering Special Consideration for Analog Input Channels Special Consideration for Analog Output Signals Special Consideration for Digital Trigger Input Signals Measuring a 4 to 20 mA Current Selecting a Resistor Attenuating Voltage Page Selecting Components Accuracy Considerations Single-Ended Input Attenuators Differential Input Attenuators Analog Output and Digital Input Attenuators Special Considerations for Analog Input Special Considerations for Analog Output Special Considerations for Digital Inputs A Specifications Analog Input Power Requirement Fuse Physical Maximum Working Voltage Environmental Safety Electromagnetic Compatibility CE Compliance Page Figure B-1. ESeries Device s SCB-68 Shielded Connector Block User Manual B-2 ni.com E SERIES DEVICES NI 670 Figure B-2. NI670X Devices National Instruments Corporation B-3 SCB-68 Shielded Connector Block User Manual DEVICES NI 671 /673 Figure B-3. NI671X/673X Devices SCB-68 Shielded Connector Block User Manual B-4 ni.com Figure B-4. S Series Devices S SERIES DEVICES NI 660 Figure B-5. NI660X Devices X S5 X NI 653 Figure B-6. NI653X Devices National Instruments Corporation B-7 SCB-68 Shielded Connector Block User Manual DEVICES Figure B-7. NI7811R/7831R Devices NI 7811R/7831R DEVICES C Fuse and Power D SCB-68 Circuit Diagrams Figure D-1. +5 V Power Supply This appendix contains illustrations of circuit diagrams for the SCB-68. Figure D-2. Cold-Junction Compensation Circuitry Figure D-3. Digital Trigger Circuitry Figure D-4. Analog Output Circuitry E Soldering and Desoldering on the SCB-68 Page F Technical Support and Professional Services Page Glossary Numbers/Symbols A C D Page I L M N O P R S T U V W Index Numbers A B C Page D E F G I L M N O P Q R S T V