Don’t let analog/digital misinterpretations put your design in jeopardy

by Bill Laumeister, Principle Member of the Technical Staff, Precision Control Group, Maxim Integrated Products Inc. , TechOnline India - April 21, 2011

Sometimes designers misunderstand how a part works and make strange assumptions resulting in misusing the part. Sadly, engineering schools now focus on digital technology, almost ignoring analog design. So, inexperienced digital engineers learn analog by trial and error.

Abstract

Humans are unusual creatures, sometimes partial knowledge, ego and misplaced confidence are more dangerous in a field than no knowledge. This can lead to circuits that fail to function as expected. Some look like they should work to an inexperienced person--while the experienced engineer cringes, wondering how someone can get themselves in such a fix. Here are three cases where some simple analysis can educate designers so they can avoid similar issues in future designs.

Sometimes designers misunderstand how a part works, so they make strange assumptions that result in misusing the part. Sadly, as a general rule, today’s engineering schools concentrate on digital technology while almost totally ignoring analog design. As a result, inexperienced digital engineers have to learn analog by trial and error. Some of the resulting devices would make Rube Goldberg proud. (Who’s Rube Goldberg? A Pulitzer Prize-winning cartoonist, Rube Goldberg became famous in the early 20th century by inventing outrageously complex contraptions to perform the simplest tasks. 

Let’s consider a few horror stories from an analog engineer’s viewpoint. Common digital fallacies include not realizing the importance of clean power and grounds. Impedance DC level matching may be ignored when connecting circuit stages. Ignoring the laws of physics will eventually cause system failure.

It was beautiful on its face but . . .

The datasheet said, “Place the power supply decoupling capacitors as close to the IC’s power pin as possible.” And as one can see in the printed circuit board layout (PCB) of Figure 1, they did! 

 

                           

 

 Figure 1: A printed circuit board (PCB) layout, IC and capacitor

 

The board functioned as a mixed-signal board in a video application. Other components were around the IC of Figure 1, but these were the most important. It was a four-layer board with signal traces on the outer two
layers. An analog engineer would expect power and ground planes on the inner two planes respectively. There were high frequency analog-to-digital converters (ADC) and digital-to-analog converters (DAC) with signal-processing circuits. The component density was just moderate. There were no ball grid array (BGA) packages requiring more layers or to complicate the layout.

When testing the design, we found that the video output was noisy, really noisy. We traced most of the noise to one IC. Figure 1 shows the layout on the top side of the board. There was a huge noise signal on the power
pin. When a thin wire was placed through the “ground” side via of the decoupling capacitor and the connection followed to the other side of the board, there was a trace (not an internal ground plane) that disappeared into another via. This was definitely troublesome. By reviewing the board layout program we highlight the desired node and could view all the connections. Figure 2 is part of what was found. 

 

                       

Figure 2: Board layout viewed using the PCB design software

 

The traces looked like they were done by a digital circuit auto-router. This was the case because the board layout person was inexperienced with analog. There were no internal ground and power planes (see AN4345 “Well Grounded, Digital is Analog” for grounding tips and proper use of planes).

To the inexperienced, the circuit is correct because all grounds are connected together. That is true at DC but at operating frequencies the equivalent circuit has many parasitic elements, as shown in Figure 3.

 

           

           Figure 3: A way to visualize “Ground Bounce” in the circuit

 

Each trace and via of Figure 2 contains resistance and inductance. In Figure 3, those distributed parasitic components are collapsed into the single series inductor near the ground at the lower center of the diagram.
Picture the inductor in your mind’s eye as a mechanical coil spring. The IC is drawn here as an op amp to simplify the explanation, but it could be any circuit.

The noise of the digital and other circuits to the left and right above the “ground bounce" symbol are moving the voltage up and down when the currents in the other circuits change state. The analog signal is contaminated at several points:

1.)   The noise is applied to the op amp input through R1.
2.)   The noise is applied the ground pin of the op amp. (Yes, there is a “power supply rejection ratio” specification but that uses ground as the reference. This means the noise is directly added to the output signal.)
3.)   The noise is applied to the op amp input through R2.
4.)   The noise is applied to the op amp input through the decoupling capacitor and the resistor above R1.

Remember: a capacitor is a two-way device. The decoupling capacitor’s job is to homogenize the high frequencies on both sides of the capacitor. If there is noise on the voltage bus and the ground is a clean, low-impedance return path to the power supply, a decoupling capacitor can help reduce the noise. However, if the ground has high impedance and lots of noise, the decoupling capacitor actually adds noise to the voltage bus. The noise added at various points around the op amp above makes a real mess at the output, because the noise signals are added with phase differences. All the noise tends to add at the output as indicated by
the bouncy "plus" symbol.

The output is also plagued by the effect of small non-linearities in the op amp. The noise is multiplied by the various non-linearities of various elements, thus creating sum and difference harmonics which splatters the
noise all over the spectrum. The fix was a simple explanation of how to make good ground and power planes
(and you don’t want to see the power traces. They were equally problematic). Once the inexperienced engineer and layout person understood the concepts, the next layout had no noise problems.

 

An analog radio with an impossible layout.

In another example, let’s examine a problem discovered on a radio-transceiver design evaluation board. A designer took the schematic and entered it into a PCB auto router designed for digital logic. The resulting board was unusable as a radio, there was no ground plane and it did qualify as a Rube Goldberg.

The critical signal paths on the board were scattered and stitched through vias (inductors), and power was not decoupled properly. Then the antenna on the board was a strange shape. It is hard enough to design a
straight-line antenna. When the designer was asked about the software program used to make such an antenna, instead of discovering a great new antenna design software tool, it was a surprise to hear the designer state “Well that was the space we had left so that’s where we put the antenna.”

Although the designer was a good microprocessor engineer, he had no idea that antenna size is controlled by the signal wavelength. He also had no concept of a ground plane being the other half of the antenna. A knowledgeable radio engineer was available in another group within the company. With his guidance the design was saved.

Does this resonate with you?

Musical instruments and radios utilize resonance to operate. Figure 4 shows pipes of a pipe organ. Each is pipe is tuned to a specific musical note. When we tune a radio from one station (frequency) to another resonance helps us select one station and reject all others.

 

                                                           

                            Figure 4: A pipe organ located in a church in Berne, Indiana, USA

 

Radio antennas sizes are tuned to resonate at specific frequencies, but therein lies the problem. While working on an automotive project, a designer wanted to have long-distance communication by placing an antenna inside the engine compartment of an automobile. This was an aftermarket product that needed to transmit and receive under the metal hood of the engine compartment of any car.

The designer thought that the engine compartment of the cars would form a resonate cavity at one specific frequency and amplify the signal. Unfortunately, resonate cavities require exacting design and since each
car type has different size engine compartment, resonance was hard to achieve. Additionally, the designer didn’t want to pay automotive prices for high-temperature-qualified components.

By not understanding that engine compartments were hot, the designer had expected that consumer-grade parts with maximum temperature ratings of  70°C would survive. By examining the requirements, an experienced radio engineer was called in to straighten out the design.

Parts that combine analog and digital in the same package, such as the MAX541 16-bit DAC, typically have a pin called analog ground and another called digital ground. In the MAX541’s data sheet, pages nine and ten explain how to connect them together and use star points for the ground planes.

The terms for ground can be misleading; instead of analog and digital, “clean and dirty” might be better descriptions. As explained in Application Note 4345, “Well Grounded, Digital Is Analog”, digital circuits can ignore some noise because of the threshold effect, while analog circuits cannot. In the case of data converters care must be used to connect the digital and analog grounds. Especially when systems are made of many ADCs and DACs, skill and experience are required to connect the ground planes in a star configuration.

At the same time, the analog and digital planes need to be cross-connected at each data converter. The objective is for the major currents to return to the power supply with little current in the cross connects. Expert
engineers may use resistors, ferrite beads or inductors for cross connects to direct currents as a function of frequency. Experience allows one to minimize the number of board layouts, but the only way to be sure is to empirically and iteratively optimize the circuit layout and reduce the noise issues.

Unfortunately, all parts can be misused and abused. Experience is a wonderful teacher. When we learn things by the school of hard knocks we do remember.

Hopefully by sharing those experiences we can spare other engineers the pain of finding out the hard way. Most of the time hindsight allows us to improve on a design as we scratch our heads and wonder why we didn’t
see the obvious sooner. No part can do everything for every designer. No matter how much Rube Goldberg
tried, a single contraption couldn’t incorporate every part in every application. That’s a good thing because people have enjoyed Rube’s many cartoons for generations.


About the author:

Bill Laumeister is an engineer with the Precision Control Group, Strategic Applications at Maxim Integrated Products, and works with companies using DACs, Digital Potentiometers and Voltage References. He has thirty-eight years experience and holds several patents in the video field. He is the inventor of a video communications method called VEIL (Video Encoded Invisible Light), which is being considered by the U.S. Congress in the "Digital Transition Content Security Act" as a possible patch for the "analog hole".)

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