Choosing the Right Mixed-Signal Test Equipment

At one time the boundaries between analog and digital designs were clearly defined. As the complexity and performance levels of systems have increased, maintaining those boundaries has become more difficult. Today it is believed that significant majorities of designers are working with boards that include a substantial mix of analog and digital components. In addition, high-speed digital signals require far more insight into their analog characteristics as they pertain to signal-integrity margins.

Unfortunately, the blurring of these environments can create complexity when it comes to selecting test equipment. It has become increasingly important for designers to be able to either time-correlate the signals between analog and digital devices in their designs, or to be able to view time-correlated analog and digital views of the same signal.

The traditional solution of two standalone instruments operating independently—an oscilloscope for analog analysis and a logic analyzer for digital analysis—has become cumbersome and insufficient. The new age of mixed-signal analysis requires solutions that bring the analog and digital worlds together in a seamless fashion.

There are currently three solutions that address the mixed-signal environment by providing views of both analog and digital data in a time-correlated fashion.

• The Scope-Centric Solution—The Mixed Signal Oscilloscope (MSO) is a scope-centric solution because it utilizes the physical form factor and system software of a traditional digital oscilloscope. The instrument is enhanced for mixed-signal analysis with the addition of some basic logic-analyzer functionality.
  
  
• The Logic-Centric Solution—A logic analyzer's oscilloscope module is considered a logic-centric solution because it utilizes the physical form factor and system software of the logic analyzer. You get mixed-signal capabilities via the module's basic analog acquisition and analysis capabilities. In addition, newer logic analyzers are also capable of acquiring analog eye diagrams with a feature called eye scan.
  
  
• The Extended Logic-Centric Solution—A standalone oscilloscope and standalone logic analyzer, connected together and time-correlated, comprise the extended logic-centric solution. The user has access to all of the features and functionality of each instrument. In addition, the instruments are automatically de-skewed and data is shared between them to provide time-correlated analog and digital data.

On the surface, each of these solutions provides the same fundamental benefit—the ability to capture and view time-correlated analog and digital data in the same display. However, it is important to look deeper into the specifications, functionality, and intended use models of each solution to fully understand how they differ and how each is best suited for specific types of applications.

In addition to this customary functionality, the MSO adds basic digital-timing analysis and deep memory. The net effect of this combined functionality is that you can easily observe the complexity of current technology because an MSO provides more channels, more memory, and more triggering capability than any traditional DSO available on the market. As an example, the screen shown in Figure 1 depicts a deep trace of five digital waveforms correlated to a single analog signal. The combined functionality is controlled by the same front-panel controls and user interface found in a traditional DSO, thus maintaining a DSO's familiarity and ease-of-use.

Figure 1:  MSO display correlating analog and digital waveforms

The MSO lends itself very well to two primary use models. The first is a true mixed-signal environment that typically combines slower analog signals with faster digital-control signals from microcontrollers or DSP's. The MSO's ability to trigger across all channels allows the designer to use the extra digital channels to qualify the trigger and the analog channels to perform the necessary debug analysis. In addition, deep memory allows for long capture capability of the slower analog signals and also enables high-resolution capture at a fast sample rate of the faster digital signals. The second use model is a scenario in which a designer is interested in viewing the analog characteristics of a digital signal for signal-integrity purposes. In this case, a designer could view a bus using the digital channels and see a time-correlated analog view of any of the signals by probing it with an analog channel. Figure 1 illustrates an excellent example application of the first use model. In this example, a 32-bit embedded processor is being used to control a wireless LAN board design. The MSO used five of the digital control lines to qualify the trigger event and correlate it to the analog Ethernet signal. After acquiring this data, all of the traditional DSO capabilities are available to analyze the Ethernet signal.

The strengths of the scope-centric MSO solution consist of a tightly time-correlated display of analog and digital waveforms in a single display, triggering across all channels, and the familiar and easy-to-use DSO user interface and front-panel controls. In addition, the single-instrument approach saves bench space and greatly simplifies the setup tasks. For many simple 8-bit and 16-bit MCU/DSP-based designs, the 16 digital channels are all that are needed to debug the system and no additional logic-analyzer functionality is necessary. The MSO cannot be considered a replacement for a full-featured logic analyzer because it does not have state analysis, complex multi-sequence triggering, more than 16 digital channels, or the performance specifications of higher end analyzers. However, it is an excellent compliment to this high-end functionality.

Figure 2:  Oscilloscope module's display on the logic analyzer

You can display the data associated with these analog channels on the instrument in two different windows, depending on the type of analysis the user is trying to accomplish. The first display, shown in Figure 2, is a traditional oscilloscope view of the data that provides very basic scope functionality such as time and voltage measurements, triggering, and the vertical voltage resolution necessary for analyzing analog signals. While this display does not integrate the analog and digital data into the same view, the scope viewer is time-correlated to the logic analyzer's waveform viewer and you can use global markers to mark correlated positions in time on both displays. The display shown in Figure 3 is the second option, which is similar to the approach used by the MSO to display the signals. This display is an integrated view of both analog and digital waveforms using the logic analyzer's waveform display. While it sacrifices vertical voltage resolution on the analog waveforms, it clearly displays the analog signals time-correlated to the digital signals of interest.

Figure 3:  Integrated display of analog and digital data in a logic analyzer waveform display

Like the MSO, the logic-analyzer scope module fits into two primary mixed-signal-analysis use models. The first is an application that requires an analog view of a signal in addition to the digital waveform provided by the logic analyzer. This would typically be done to check or verify certain analog characteristics of a digital waveform such as rise time or voltage levels. Additionally, the second use model is a broader debug application that requires correlation between the analog signal to a digital waveform, state-mode listing display, or even to source code. An example application for the second use model would be an embedded processor running software that disrupts analog signal quality when calling an interrupt routine. To identify and debug this sort of problem, the designer needs a time-correlated view of the analog signal acquired by the oscilloscope module, the state listing of events on the processor acquired by the logic analyzer, and the source code. Without this tightly correlated "signals to source" view of the data, the root cause of hardware-software integration issues would be difficult to identify.

The strengths of logic-centric solutions include tightly time-correlated data, high channel counts for digital data, both integrated waveform displays and a separate scope display, the ability to correlate analog characteristics to events in a state listing or in actual source code, easy cross triggering, and the possibility of having up to 8 analog channels. Like the MSO, this single instrument form-factor saves bench space and requires minimal setup for mixed-signal analysis. It will also be familiar and comfortable to any digital designer familiar with a logic analyzer. The oscilloscope module cannot fully replace a stand-alone DSO because the feature set is limited and does not include more advanced analog analysis tools such as waveform math, FFTs, and histograms. In addition, the scope module does not have the benefit of the high resolution, fast update displays that are used in most DSOs. This makes the scope module really only adequate for single-shot measurements.

Eye Scan
In addition to the scope module, newer logic analyzers offer an option for performing additional analog analysis on high-speed signals without any additional acquisition hardware or probing. The Eye Scan measurement is a clock-synchronous measurement that generates eye diagrams across many signals. While eye scan is not a time-correlated measurement in the traditional sense of mixed-signal analysis, it is an analog representation of voltage transitions across time for digital signals. An eye scan measurement is shown in Figure 4. This eye diagram is a composite of all channels comprising a 16-bit bus.

Figure 4:  Eye scan eye diagram

This feature expands the logic analyzer into the signal-integrity territory generally left to oscilloscopes and other instruments by allowing a designer to easily validate such things as setup/hold margins, eye openings, and threshold voltages. By generating eye diagrams for up to 339 signals in a single run, eye scan can quickly identify errant signals that require further analysis. The feature offers the user a variety of tools to measure the eye opening, generate histograms, overlay the individual signals, or alter the color grading to better highlight problems. An example application to utilize this functionality would be a high-speed bus design that has errors in its digital data. Since high-speed designs have very tight specifications and are highly susceptible to signal-integrity problems, eye scan offers a natural starting point in the debug process. In a matter of minutes this measurement can generate eye diagrams of every signal in a bus and either verify that it is working as specified or reveal problems such as skew, jitter, or noise that would distort the eye opening.

Eye scan's strength lies in the fact that it can generate eye diagrams for hundreds of signals in a single run in a fraction of the time it would take to make the same measurements with a scope. The resulting eye diagrams are very similar to those generated by an oscilloscope, as shown in Figure 5. Eye scan is limited in terms of its voltage resolution and its ability to detect activity when no data transitions are occurring. It can quickly flag problems, but precise signal-integrity measurements still require an oscilloscope.

Figure 5a:  800 Mbps PRBS measured with eye scan

Figure 5b:  800 Mbps PRBS measured with an oscilloscope

Figure 6a:  Global markers are placed in the logic analyzer's waveform display

Figure 6b:  Markers Ax and Bx in the oscilloscope display are locked to the same positions in time as the logic analyzer's global markers

The dual instrument use-model applies very well to high-end applications that need the full-feature, high-performance logic synchronized to full-featured high-performance analog capabilities. Like the logic-centric scope-module solution, you can use the dual instrument use-model to view the analog characteristics of a digital waveform, or to correlate signals between analog and digital components. In addition, because you use a full-featured DSO, all of the traditional analog-analysis capabilities are available to the user. An example application of this solution would require the high-end functionality of both instruments. A high speed packetized bus, for instance, might require the logic analyzer's channel count, state mode, and complex trigger functionality to acquire and trigger on a specific packet event and then cross trigger to an oscilloscope for analog analysis. The bandwidth and sample-rate of a high-end scope are necessary for accurately acquiring the high-speed analog signal.

The dual instrument solution's strengths is that it combines full-featured instruments into a fully time-correlated mixed-signal analysis solution and gives the user a much broader range of selections in terms of performance and capabilities for both analog and digital analysis. This solution does come with a few drawbacks. First, the time correlation is not as tight as the MSO and scope-module solutions whose acquisition hardware is built into the same instrument as the complementary analysis hardware. Second, setup is more time consuming and cumbersome and requires far more bench space than does a single-instrument solution. Third, the triggering functionality is not across all digital and analog channels. In other words, you can either use the logic analyzer channels to trigger the scope or the scope channels to trigger the analyzer but not a combination of both as in the MSO. Finally, the functionality is split between two separate and very different displays and software user-interface environments with primary control in the logic analyzer. This is typically not a problem when used infrequently, but can be cumbersome as an everyday solution.

The second step is to evaluate the desired functionality for analyzing data. Table 2 rates each solution in terms of various features and functions. In addition to the first two steps, there is an intangible decision based on user preferences. Individual designers often prefer, or are more comfortable with, one instrument over another. If multiple solutions meet a designer's needs, then these preferences must be strongly considered when determining the appropriate solution. It is certain that unavailable mixed-signal solutions are broad and deep enough to address a large variety of needs.