What is the difference between an oscilloscope and a digitizer?
There are some key variations between these instruments, and, chances are, you have applications that can benefit from both. If you consider your need for automated versus interactive measurements in tasks throughout the design process, you can employ the strengths of the appropriate instrument and reduce costs throughout the product development cycle – from design to validation to volume production. By selecting the right software architecture, you can integrate different instruments at various stages of the development flow within the same software framework.
Interactivity is common in the product design phase where multiple prototypes are necessary to correct design flaws or improve performance among product revisions. If you can quickly change probe points and acquire waveforms with a high-bandwidth oscilloscope, you can more efficiently solve these problems. Stand-alone oscilloscopes are designed for interactive use, focusing on features such as time to first measurement, waveform display update rate and visualization, and connectivity to a wide array of probes.
For example, consider the design phase of a baseband-to-intermediate frequency (IF) converter circuit. You cannot foresee possible problems with a prototype. The design may exhibit unanticipated harmonics, excessive jitter, or flatness problems. Using a high-bandwidth instrument with good probing ability, you can quickly find issues and determine their sources. However, sometimes you need additional features. Tasks such as testing a variety of analog and digital inputs and outputs or generating previously acquired data are difficult with a stand-alone oscilloscope. With NI LabVIEW SignalExpress software, you can use the oscilloscope to perform standard as well as custom measurements, easily integrate other instruments, and drag and drop data into Microsoft Excel with a few simple mouse clicks and no programming.
While oscilloscopes are often used in the design phase, a stand-alone oscilloscope may not be ideal for all design applications. What if you need to acquire data at a resolution greater than 8 bits or on more than four channels? The wide breadth of available modular digitizers, sometimes called PC-based oscilloscopes, can perform oscilloscope measurements through user-defined measurement and control software and meet additional needs by providing resolutions ranging from 8 to 24 bits and the ability to easily synchronize multiple modules to create an instrument with hundreds of channels.
Figure 1. Modular digitizers and stand-alone oscilloscopes provide different benefits. For example, modular digitizers are ideal for automated use, but you might need a stand-alone oscilloscope for automated measurements that require very high bandwidth.
In automated design validation and manufacturing, the primary goal is to shorten the time to test critical specifications. Both phases require numerous repeated measurements and usually involve integration with several other instruments including waveform generators, digital I/O devices, and digital multimeters (DMMs). These applications are typically served best by a modular digitizer, which is designed for automated use. Digitizer designs focus on features such as measurement and data throughput, synchronization for higher-channel-count and mixed-signal applications, and ease of automation. Digitizers draw on the benefits of a PC-based platform such as PXI, which combines a modular architecture with the high-throughput PCI and PCI Express buses.
In design verification, the need to run through thousands of measurements often makes it impractical to use an interactive manual process. An automated approach using a high-throughput, modular digitizer is ideal for these tasks. Consider again the baseband-to-IF converter. Correct behavior has been verified in the lab; additional testing must confirm the functionality with real-world conditions such as fading or echoing signals. You can use the LabVIEW SignalExpress measurement sequence you created for integration in the design stage and automatically generate LabVIEW graphical code for additional customization. For example, you might create an automated test that generates multiple sets of simulation data and logs acquired data while flagging any deviations from expected limits so those tests can be repeated manually.
In manufacturing test, where measurements can be taken on thousands of units per day, shorter test times mean lower test costs per unit. In the baseband-to-IF converter example, the device has been characterized under a myriad of conditions and design problems have been corrected. Now, you must ensure proper operation. Using LabVIEW software and PXI hardware to create an automated, high-throughput test system with a digitizer and other instruments, you can customize the test system to meet your requirements, thereby shortening test times. Of course, there are some automated applications requiring very high bandwidths, which are available only from stand-alone oscilloscopes. For these applications, you can meet the requirements using a hybrid system that combines modular devices and the oscilloscope within a common test software framework.
Figure 2. Some tasks are difficult with a stand-alone oscilloscope. With LabVIEW SignalExpress, you can use a digitizer or an oscilloscope to perform standard or custom measurements, easily integrate other instruments, and drag and drop data into Microsoft Excel with a few simple mouse clicks and no programming required.
Choosing the Right Instrument
While you can use both oscilloscopes and digitizers to acquire voltages, these instruments provide different advantages throughout the development cycle. Stand-alone oscilloscopes can quickly display and measure signals, while modular digitizers offer a user-defined architecture with high throughput and tight integration with other instruments. LabVIEW delivers a common software framework for both measurement approaches and helps you efficiently transition through the design process. When you break the design process into tasks that require interactive versus automated measurements, you can confidently choose the instrument that makes your job easier.
John Hottenroth is a product engineer for NI modular instrumentation. He holds a bachelor’s degree in electrical engineering from the Colorado School of Mines.
This article first appeared in the Q4 2007 issue of Instrumentation Newsletter.
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