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4605
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and Comparator Circuits > APP 4605
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> Circuit
Protection > APP 4605

Mar 09, 2011

Keywords: noise, inexperienced, engineer, clean
power, ground planes, impedance DC, level matching, power
supply, decoupling capacitors, analog to digital converters,
ADC, DAC, signal processing, board layout, bounce, radio
resonance, antenna, wavelength, parts misuse abuse

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parts

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APPLICATION NOTE 4605

Avoid Design Misinterpretations that Put System
Operation in Jeopardy

By:

Bill
Laumeister, Strategic Applications Engineer

Abstract: Humans
are unusual creatures because sometimes partial knowledge,
ego, and misplaced confidence are more dangerous than no
knowledge of a subject. With electronic design,
misinterpretations and misunderstandings can lead to circuits
that fail to function as expected. To an inexperienced
designer, some circuits look like they should work. Meanwhile,
the experienced engineer cringes, wondering how someone can
get themselves in such a fix. This application note presents
three case studies that illustrate how some simple analysis
and a proper understanding of the laws of physics can educate
designers to avoid similar issues in future designs.

Click
here for an overview of the wireless components
used in a typical radio
transceiver.

Introduction
When digital designers confront analog
systems, misinterpreting device operation or design
requirements results in some fascinating analog "horror"
stories. Sometimes digital designers misunderstand how a part
works (or should work), 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.¹

This article considers a few horror
stories from an analog engineer's viewpoint. Several common
digital misunderstandings reveal some analog truths: the
importance of clean power and grounds; impedance
DC-level matching can be ignored when connecting circuit
stages; and perhaps most importantly, ignoring the laws of
physics will eventually cause system failure.

It Was Beautiful on the Surface, but...
A data sheet
says, "Place the power-supply decoupling capacitors as close
to the IC's power pin as possible." As one can see in
Figure 1, the designer did just that!

Figure
1. A printed circuit board (PCB) layout showing IC and
capacitor.

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

When testing the design, we found that the
video output was noisy, actually very 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
configuration was definitely troublesome. By reviewing the
board layout program here, we can highlight the desired node
and 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 autorouter. This was indeed the case because
the board layout designer was inexperienced with analog. There
were no internal ground and power planes. (See application
note 4345, "Well
Grounded, Digital Is Analog," for grounding tips and
proper use of planes.) To the inexperienced designer, the
circuit looks correct because all grounds are connected
together. That approach 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 operational
amplifier 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, therefore, contaminated at
several points:

The noise is applied to the op
amp input through R1.

The noise is applied to the ground pin of the op amp.
(Yes, there is a power
supply rejection ratio (PSRR) specification, but that
uses ground as the reference. This means that the noise is
directly added to the output signal.)

The noise is applied to the op amp input through R2.

The noise is applied to the op amp input through the
decoupling capacitor and the resistor above R1.

Remember that 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, then 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 in Figure 3 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 nonlinearities in the op amp.
The noise is multiplied by the various nonlinearities of
various elements, thus creating sum and difference harmonics
which splatter the noise all over the spectrum.

The
design fix was a simple explanation of how to make good ground
and power planes (You do not want to see the power traces in
the original design—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
A problem
was discovered on a radio-transceiver evaluation board. A
designer took the schematic and entered it into a printed
circuit board (PCB) autorouter designed for digital logic. The
resulting board was unusable as a radio; there was no ground
plane—and it did indeed qualify as another Rube Goldberg
design.

The critical signal paths on the board were
scattered, 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. The designer was asked about the software program
used to make such an antenna. We expected to hear about the
discovery of a "great new antenna-design software tool," but
the designer said, "that was the space left so that is 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 more
knowledgeable radio engineer in another group saved that
design.

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,
U.S.

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 did not want to pay automotive prices for
high-temperature-qualified components.

By not
understanding that engine compartments were hot, the designer
had wrongly expected consumer-grade parts with maximum temperature
ratings of 70°C to survive. An experienced radio engineer was
called to fix the design.

Minimizing Noise with Digital and Analog Grounds
The
MAX541 16-bit DAC
combines analog and digital in the same package: analog ground
and digital ground pins. The device's data sheet explains how
to connect them 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. Analog circuits cannot
ignore that noise. In the case of data converters care must be
used to connect the digital and analog grounds. This is
especially true when systems comprise many ADCs and DACs;
skill and experience are required to connect the ground planes
in a star configuration. Additionally, the analog and digital
planes need to be cross-connected at each data
converter. The objective here is to have the major
currents return to the power supply with little current
in the cross-connects. Expert engineers can 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.

Conclusion
Unfortunately, all parts can be misused and
abused. Experience is also a wonderful teacher. Most of the
time hindsight allows us to improve a design as we wonder why
we failed to see the obvious sooner.

In the end, no
part can do everything for every design and every designer.
That is a good lesson. No matter how much Rube Goldberg tried,
a single contraption of his could not incorporate every part
in every application. That is a good thing, because people
have enjoyed Rube's many cartoons for
generations.

¹See www.rubegoldberg.com/gallery.php.

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Download,
PDF format (81kB)
APP 4605: Mar 09, 2011

AN4605, AN 4605, APP4605,
Appnote4605, Appnote 4605

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