A family of instruments for testing mixed-signal circuits and systems

Hewlett-Packard Journal, April, 1997 by Robert A. Witte

This entirely new product category combines elements of oscilloscopes and logic analyzers, but unlike previous combination products, these are "oscilloscope first" and logic analysis is the add-on.

Electronic circuits have been a part of modern life for so long that most people take them for granted. Some devices are inherently electronic in nature such as telephones, radio receivers, calculators, and personal computers. Other devices started out as mostly mechanical and have gradually had more electronics incorporated into them over time. Electronics has made its way into automobiles, cameras, water heaters, home appliances, elevators, thermometers and weighing scales. This "electronics everywhere" trend has gradually filled modern life so that it is difficult to imagine what life would be like without all those electrons running around doing their jobs.

Most early electronic systems were analog, especially if they interfaced to real-world physical phenomena. The emergence of digital technology resulted in the gradual diffusion of digital gates into applications that were once exclusively analog. Thus, an "analog moving to digital" trend emerged as the number of digital gates per acre of silicon continued to grow at a fast rate. Analog circuits will never be totally replaced, since for the most part the real world stubbornly retains its analog behavior and circuits that interface to this world must retain at least some analog circuitry. The result is that many electronic systems are mixtures of analog and digital circuitry that have come to be known as "mixed analog and digital" or simply "mixed-signal."

The single-chip microcontroller has emerged as an important component in these mixed-signal designs. Of course, micro-controllers have been around for decades, doing the lowly control tasks in cost-sensitive applications while their more powerful siblings ("real" microprocessors such as an Intel80486 or a Pentium[R]) got all of the attention, usually because of their critical role in personal computers. Meanwhile, the single-chip microcontroller improved in performance, moving from 4 bits to 8 bits and 16 bits while also improving in cost-effectiveness. Without much fanfare, these single-chip devices found their way into a wide range of designs, causing one author to refer to them as "The Ultimate ASIC."[1] These devices are often used to control or measure a physical system (e.g., antilock braking, camera control, appliance controls, industrial control systems). A generic block diagram for such a system is shown in Fig. 1, and an example of a mixed-signal single-chip microcontroller is presented on page 8.

[Figure 1 ILLUSTRATION OMITTED]

This increased use of mixed-signal electronics is showing up in a wide variety of industries and applications. Consumer electronics is an obvious area, with mixed-signal designs being used in CD players, stereo receivers, tape decks, and camera electronics. Similarly, communications devices such as modems, telephone answering machines, and multimedia boards for PCs all use mixed-signal electronics. There are many applications in industrial electronics, including process control, industrial water heater controls and other sensor-based systems. The growing area of mechatronics (the merger of mechanical and electronic technology) is primarily mixed-signal in nature. A large and growing mixed-signal area is automotive electronics, including subsystems such as the previously mentioned antilock braking systems and ignition control. Biomedical applications are another emerging area, with mixed-signal electronics being applied in pacemakers, hearing aids, and other medical devices.

Systems can be totally digital if only on-off control is required. More likely, there is some physical quantity being measured or controlled that requires a portion of the electronic system to be analog. The increased use of sensors allows engineers to create smarter control systems that monitor physical events and do a better job of controlling the system. A good example of this is antilock braking systems in automobiles. In this case, electronics (including sensor technology) is being used to make the braking effectiveness of the automobile far better than would be possible in a purely mechanical system.

Oscilloscopes

For designers of mixed-signal systems, the troubleshooting tool of choice is the oscilloscope. The versatility of the oscilloscope for viewing a wide variety of waveforms allows the design engineer to see what's happening in the device under test. Other test instruments may also be used hut the oscilloscope remains the first tool that most users turn to for debugging circuits.

Mixed-signal engineers expect many things from their oscilloscopes. The oscilloscope must have sufficient fundamental performance in terms of bandwidth and sample rate to capture and display signals accurately and reliably. At the same time, the oscilloscope must be easy to use so that the engineer can focus on the operation of the circuit and not on the operation of the oscilloscope. In the heat of the trouble-shooting battle, it is very distracting for the user to have to interrupt the debug process to deal with an uncooperative test instrument.