As a hybrid test instrument, a mixed-signal oscilloscope (MSO) combines the measurement capabilities of a logic analyzer with those of a digital storage oscilloscope (DSO). This creates a powerful tool for data acquisition.
Features of an MSO include:
- Autoscale
- Trigger holdoff
- Infinite persistence on analog and digital channels
- Probe/channel de-skew
Such oscilloscopes display an analog and digital waveform that is time-aligned; an MSO allows designers to see all of these on one common display. In many modern embedded design debugging applications, MSO offers distinct benefits over both conventional oscilloscopes and logic analyzers, even though the former may lack the latter's substantial number of digital acquisition channels and more sophisticated digital measurement capabilities.
How does a mixed-signal oscilloscope work?
Stage one: Input
The input stage is the starting point of the MSO's trip. In this setup, the oscilloscope receives signals through both digital and analog channels from the device under test (DUT).
Stage two: Conditioning of signals
The receiving signals are next subjected to signal conditioning, which involves adjusting their level, passing them through filters or amplifying them if needed, in order to ensure precise viewing and measurement.
Stage three: Converting from analog to digital
The next step is to use an analog-to-digital converter (ADC) to transform the conditioned analog signals into digital. Since digital signals are already in the proper format, they do not need this step.
Stage four: Display and digital signal processing
Advanced digital signal processing techniques are applied to all signals after conversion, whether they are digital or analog-turned-digital. The MSO's screen then shows the processed signals, which include both digital data and analog waveforms. Many different types of visual representations of the data are available, such as histograms, eye diagrams and fast-forward charts.
Advantages of mixed-signal oscilloscope
The MSO's use model is one of its main advantages. The operation of an MSO is quite like that of an oscilloscope. Because of the time and effort needed to learn or relearn how to use a logic analyzer, many design and test engineers opt not to use one, even if it may be necessary to debug a complex design.
Setting up a logic analyzer to take specific measurements typically takes significantly more time than setting up oscilloscope measurements, even for engineers who are proficient with their use. Finally, many modern systems that rely on microcontroller units (MCUs), field-programmable gate arrays (FPGAs) or digital signal processors (DSPs) lack the capabilities of a logic analyzer, which adds complexity to the design.
Debugging a power management system
Consider a battery-operated device whose power management system incorporates both digital and analog circuits, such as voltage regulators and microcontrollers and communication interfaces.
Simultaneous signal analysis:
With an MSO, you can capture and analyze both the analog signals (like the output from a voltage regulator) and the digital signals (like the control signals from the microcontroller) simultaneously. This is crucial for understanding how the digital control logic affects the analog performance.
Time-correlated triggers:
You can set up time-correlated triggers to capture events that occur across both domains. For instance, if a specific digital signal transitions (like a PWM signal to control a switch), you can capture the corresponding change in the analog voltage output in real time. This helps identify timing issues or glitches.
Bus decoding:
MSOs often have built-in capabilities to decode common digital communication protocols (like I2C, SPI or UART). This means you can visualize the data being sent over these buses alongside the analog signals, making it easier to spot communication errors or timing mismatches.
Power integrity analysis:
You can measure ripple voltage and current on the power lines while simultaneously monitoring the performance of digital logic. This is critical for ensuring that the power supply is stable and that the digital components are receiving clean power, which is essential for reliable operation.
Complex triggering options:
MSOs offer advanced triggering options that allow you to set complex conditions, such as triggering on a specific sequence of digital events followed by a change in an analog signal. This is useful for diagnosing intermittent issues that are difficult to capture with standard oscilloscopes.
How to select the right mixed-signal oscilloscope?
When selecting the right mixed-signal oscilloscope, first ensure the bandwidth is sufficient for your highest frequency signals, ideally two to five times greater than that frequency. A high sample rate, more than twice the bandwidth, is important for accurately capturing fast transients. Determine the number of analog and digital channels you need, as MSOs typically combine both; common configurations include two or four analog channels with eight or more digital channels.
Look for adequate memory depth to capture long signals without sacrificing sample rate and check for advanced triggering capabilities to isolate specific events. Ensure the MSO has built-in decoding for relevant digital protocols (e.g., I2C, SPI). Finally, balance your budget with your needs, while considering user interface and manufacturer support. By evaluating these factors, you can select an MSO that effectively meets your debugging and analysis requirements.
Conclusion
The mixed-signal oscilloscope has evolved into an invaluable instrument for contemporary engineers. Because of their ability to analyze digital and analog signals simultaneously, they are crucial for comprehending the intricate relationships in modern electronic systems. Making the right choice of MSO according to criteria like bandwidth, channel count, sampling rate and probe compatibility can greatly simplify testing, system design and troubleshooting.
