on January 21st, 2014

Dear Colleagues,

Digital engineering touches every aspect of our lives. Every device today seemingly has a computer on board – whether it be your phone/RTU/PLC/Tablet or even TV and washing machine. And these devices are based on the work of a digital design engineer and technician.

Customers are continuing to expect considerably more from their systems in terms of lower power, lower cost, reliability, wireless operation, sensitivity and speed. And naturally, more user-friendly interfaces. But engineers and technologists working on digital systems are seemingly less equipped for their job today than in past years.

A decade or so ago, computer hardware was quite ‘slow’ and thus a detailed knowledge of analog circuitry wasn’t required in order to make the overall system work. Despite this, analog circuit design was a key course for all engineering (and indeed, often computer science) programs.

However, what has happened today is that universities believe they can eliminate a detailed understanding of analog circuitry from the engineering program in favour of higher level computer courses. This approach however weakens the would-be design engineer and troubleshooting technician’s understanding of the critical underpinning concepts of good electrical engineering and thus results in a poorer design and troubleshooting ability.

A few examples (from Howard Johnson’s excellent paper) of why a strong understanding of analog design is so important is noted below.

Where is the current return path?
Today, digital schematics and related discussions don’t detail or even consider the return path of electrons (and currents).  This approach is reinforced by vendors of oscilloscopes and logic analysers who push voltage-mode probes with little mention of measuring current. This results in design flaws such as an inadequate number of ground pins and poor return current flow designs.

Weak Understanding of Magnetic Fields
In the days of the vacuum tube (who can remember these devices? I can just faintly); everything was voltage oriented with high impedance circuits (after all electrons were flowing through a vacuum) and this made electric fields the most important element in the design. This philosophy has persisted today. Unfortunately, all chip design work today is based around low impedance circuits and currents (and thus magnetic fields). Hence when considering electromagnetic compatibility (EMC) issues, one has to think of magnetic and not electric fields. Today, everyone still focuses on electric field shielding for their circuits when they should be using magnetic field shielding as a priority. This results in poor designs which cannot handle magnetic interference problems.

An Incorrect focus on Absolute Volts – not differential voltages
When one looks at a typical datasheet it talks about input voltage sensitivity in terms of absolute volts and omits to mention that one’s electronic chip actually responds to differences in voltage between an input pin and a reference pin. Often, this reference pin and its voltage is simply ignored. It is thus critical to think about different ground potentials in their systems and the problems that will arise as a result. A related problem is that of common mode voltage problems affecting chips.

What Can You Do About this?
Never forget the importance of the fundamental analog underpinnings of all electronic and computer design and ensure those that you work with do the same.

Despite what your kids may say about it being only a digital world – we all live in a real analog world and are analog beings. As are our electronic circuit designs.

Thanks to Dr Howard Johnson’s article in Proceedings of DesignCon.: Why Johnny Can’t Design a High Speed Digital System.

In terms of justifying these comments about analog design Sir Francis Bacon remarked: By far the best proof is experience.

Yours in engineering learning,


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