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National Instruments NI PXI-4224 Signal Conditioning Functional Overview, Measurement Considerations, Input Impedance

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Chapter 4 Theory of Operation

© National Instruments Corporation 4-3 NI PXI-4224 User Manual

Signal Conditioning Functional Overview

The NI PXI-4224 is part of the PXI-4200 series of DAQ devices with

integrated signal conditioning designed to provide application-specific

signal conditioning, DAQ, and integrated field wiring connectivity on the

same product. The NI PXI-4224 signal conditioning circuitry is designed

to provide attenuation, amplification, and filtering capabilities as described

in Table 4-1.

Measurement Considerations

This section provides more information about the type of signal connection

made to the NI PXI-4224 and important factors that can affect your

measurement.

Input Impedance

Figure 4-2 illustrates the input impedance of an NI PXI-4224 and its effect

on the measurement of a circuit under test. If you know the source

impedance of the circuit under test, you can correct for the attenuation

caused by the NI PXI-4224 in software. Since RIN is relatively large

(1 GΩ), it requires a large source impedance, RS, to cause a significant

change in the measured voltage, VMEAS. In general, a source impedance of

less than 200 kΩ does not interfere with the accuracy of the measurement.

For example, a 200 kΩ source impedance results in a 0.02% gain error.

Table 4-1. Signal Conditioning Functional Blocks

Signal Conditioning Component Description

Input Protection Each NI PXI-4224 channel has overvoltage protection in the

event that a channel is improperly wired.

PGA Each channel has a programmable gain amplifier. The

available gains on the NI PXI-4224 are 1 and 10, which

covers the input range of ±1 V to ±10 V. The DAQ device can

provide a gain of up to 200 in order to maximize the ADC

resolution for signals below 1 V.

Isolation Amplifier Each channel has an isolation amplifier that creates true

channel-to-channel isolation.

Post Filter A post filter is provided to clean up noise spikes created by

the isolation amplifier.

Contents

Main Page Important Information Warranty Copyright Trademarks Patents Conventions Contents Page Figures Tables About the NI PXI-4224 What You Need to Get Started National Instruments Documentation Installing the Application Software, NI-DAQ, and the DAQ Device Installing the NI PXI-4224 LED Pattern Descriptions Connecting Signals Connecting Signals to the NI PXI-4224 Front Signal Connector Page Analog Input Connections Page Figure 2-3. Unshielded Grounded Signal Source Connection Using a D-SUB Connector Figure 2-4. Shielded Floating Signal Source Connection Using a D-SUB Connector Figure 2-5. Shielded Grounded Signal Source Connection Using a D-SUB Connector Figures 2-6 through 2-9 illustrate connecting signals using a terminal block. Figure 2-6. Unshielded Floating Signal Source Connection Using a Terminal Block Figure 2-7. Unshielded Grounded Signal Source Connection Using a Terminal Block Figure 2-8. Shielded Floating Signal Source Connection Using a Terminal Block Floating Signal Source Connection Ground-Referenced Signal Connection Shielded Ground-Referenced Signal Connection (Recommended) Configuring and Testing Verifying and Self-Testing the Signals Using Test Panels Configuring the NI PXI-4224 in MAX Creating a Voltage Task or Global Channel Using NI-DAQmx Verifying and Self-Testing an NI-DAQmx Task or Global Channel Page Chapter 4 Theory of Operation Figure 4-1. Block Diagram of NI PXI-4224 NI PXI-4224 User Manual 4-2 ni.com Signal Conditioning Functional Overview Measurement Considerations Input Impedance Common-Mode Rejection Ratio Effective CMR Timing and Control Functional Overview Programmable Function Inputs Device and PXI Clocks Figure 4-4 shows this signal connection scheme. Figure 4-4. NI PXI-4224 PXI Trigger Bus Signal Connection Page Using the NI PXI-4224 Developing Your Application Typical Program Flow Chart Figure 5-1. Typical Program Flowchart Overview of Typical Flow Chart Creating a Task Using DAQ Assistant or Programmatically Adjusting Timing and Triggering Configuring Channel Properties Acquiring, Analyzing, and Presenting Completing the Application Developing an Application Using LabVIEW Page Using a DAQmx Channel Property Node in LabVIEW Synchronization and Triggering Synchronizing the NI PXI-4224 Figure 5-2. General Synchronizing Flowchart Synchronizing the NI PXI-4224 Using LabVIEW Other Application Documentation and Material Calibrating the NI PXI-4224 Loading Calibration Constants Self-Calibration External Calibration A Specifications Overvoltage Protection Analog Input Accuracy Transfer Characteristics Calibration Pre-Calibration Errors Memory Digital Triggers PXI Trigger Bus PCI Bus Interface Power Requirements Physical Maximum Working Voltage Isolation Voltages Environmental Safety Electromagnetic Compatibility CE Compliance Online Product Certification Environmental Management RoHS B Timing Signal Information Connecting Timing Signals Programmable Function Input Connections DAQ Timing Connections AI START TRIG Signal AI REF TRIG Signal AI SAMP CLK Signal Page AI CONV CLK Signal AI PAUSE TRIG Signal AI SAMPLE CLK TIMEBASE Signal AI HOLD COMPLETE Signal C Removing the NI PXI-4224 Removing the NI PXI-4224 from a PXI or PXI/SCXI Combination Chassis Removing the NI PXI-4224 from MAX D Common Questions Page Glossary Symbols A B C D E F G H I K L M N O P R S T V Page Index A B C D E F G H I L O P Q R S T V