National Instruments NI PXI-7831R - page 31

Chapter 2 Hardware Overview ofthe NI PXI-7831R

NI PXI-7831R User Manual 2-10 ni.com

Figure 2-6. Differential Input Connections for Nonreferenced Signals

Figure 2-6 shows two bias resistors connected in parallel with the signal

leads of a floating signal source. If you do not use the resistors and the

source is truly floating, the source is not likely to remain within the

common-mode signal range of the instrumentation amplifier, and the

instrumentation amplifier will saturate, causing erroneous readings. You

must reference the source to AIGND, which you can do by connecting the

positive side of the signal to the positive input of the instrumentation

amplifier and connecting the negative side of the signal to AIGND and to

the negative input of the instrumentation amplifier, without any resistors at

all. This connection works well for DC-coupled sources with low source

impedance (less than 100 Ω).

However, for larger source impedances, this connection leaves the

differential signal path significantly out of balance. Noise that couples

electrostatically onto the positive line does not couple onto the negative

line because it is connected to ground. Hence, this noise appears as a

differential-mode signal instead of a common-mode signal, and the

instrumentation amplifier does not reject it. In this case, instead of directly

connecting the negative line to AIGND, connect it to AIGND through a

resistor that is about 100 times the equivalent source impedance. The

resistor puts the signal path nearly in balance, so about the same amount

ofn oise couples onto both connections, which yields better rejection of

electrostatically coupled noise. Also, this input mode does not load down

Bias

Resistors

(see text)

DIFF Input Mode Selected

–

+

–

+

AISENSE

AIGND

VmMeasured

Voltage

Instrumentation

Amplifier

x8 Channels

AI+

AI–

–

+

Floating

Signal

Source

Bias

Current

Return

Paths

Vs

I/O Connector

Contents

Main Page Important Information Warranty Copyright Trademarks Patents Compliance FCC/Canada Radio Frequency Interference Compliance Determining FCC Class FCC/DOC Warnings Class A Contents About This Manual Chapter 1 Introduction Chapter 2 Hardware Overview of the NI PXI-7831R Page About This Manual Conventions Reconfigurable I/O Documentation Related Documentation Introduction About the Reconfigurable I/O Devices Using PXI with CompactPCI What You Need to Get Started Getting Started with Windows 2000/XP Getting Started with the RT Module Overview of Reconfigurable I/O Reconfigurable I/O Concept Flexible Functionality User-Defined I/O Resources Device-Embedded Logic and Processing Reconfigurable I/O Architecture Page Reconfigurable I/O Applications Software Development FPGA Module RT Module Cables and Optional Equipment Custom Cabling Unpacking Safety Information Page Page National Instruments Corporation 2-1 NI PXI-7831R User Manual Hardware Overview of the NI PXI-7831R PXI/CompactPCI Bus Figure 2-1. NIPXI-7831R Block Diagram + Calibration DACs Analog Input Input Modes Input Range Connecting Analog Input Signals Types of Signal Sources Floating Signal Sources Ground-Referenced Signal Sources Input Modes Figure 2-4. Summary of Analog Input Connections Differential Connection Considerations (DIFF Input Mode) Differential Connections for Ground-Referenced Signal Sources Differential Connections for Nonreferenced or Floating Signal Sources Page Single-Ended Connection Considerations Single-Ended Connections for Floating Signal Sources (RSE Input Mode) Single-Ended Connections for Grounded Signal Sources (NRSE Input Mode) Common-Mode Signal Rejection Considerations Analog Output Connecting Analog Output Signals Digital I/O Connecting Digital I/O Signals Page Page PXI Trigger Bus PXI Local Bus Switch Settings Power Connections Field Wiring Considerations Page Calibration Loading Calibration Constants Internal Calibration External Calibration A Specifications Analog Input Input Characteristics Accuracy Information DC Transfer Characteristics Dynamic Characteristics Analog Output Output Characteristics Accuracy Information DC Transfer Characteristics Voltage Output Dynamic Characteristics Digital I/O Reconfigurable FPGA Calibration Bus Interface Power Requirement Physical Maximum Working Voltage Environmental Safety Electromagnetic Compatibility CE Compliance B Connecting I/O Signals Page Appendix B Connecting I/O Signals National Instruments Corporation B-3 NI PXI-7831R User Manual Figure B-2. NIPXI-7831R I/O Connector Pin Assignments DIO Connector Pin Assignment MIO Connector Pin Assignment Caution Connections that exceed any of the maximum ratings of input or output signals Table B-1. I/O Connector Signal Descriptions Connecting to 5B and SSR Signal Conditioning Table B-2. NIPXI-7831R I/O Signal Summary Page Figure B-3. Connector Pinouts When Using NSC68-262650 Cable AO 07 Connector AI 07 Connector DIO 015 Connector Figure B-4. Connector Pinouts When Using the NSC68-5050 Cable DIO 023 Connector DIO 2439 Connector C Using the SCB-68 Shielded Connector Block Appendix C Using the SCB-68 ShieldedConnector Block NI PXI-7831R User Manual C-2 ni.com Quick Reference Label S5 S4 S1 Figure C-2. SCB-68 Quick Reference Label for the NIPXI-7831R Technical Support and Professional Services Page Glossary Numbers/Symbols A B C D Page I L M N O P R S T V W