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Maxim > Design
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4992
Maxim > Design support >
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> Amplifier
and Comparator Circuits > APP 4992
Maxim > Design support >
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> Analog
Switches and Multiplexers > APP
4992

Mar 08, 2011

Keywords: noise decoupling, capacitor, self
resonance star, ground filter, RC, LC, resistor, inductor,
voltage reference, LDO, LNA, analog, interface, crosstalk,
linearity, SOC, system on a chip, safety, op amps, ground
planes, power, busses, sensor conditioning
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parts

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APPLICATION NOTE 4992

Reduce the Chances of Human Error: Part 1, Power and
Ground

By:

Bill
Laumeister, Strategic Applications Engineer

Abstract: Electronic
system errors are typically analog, not digital. Analog is
prone to signal-to-noise, crosstalk, and linearity issues.
This application note discusses the need for power and ground
cleanliness and integrity. Analog interface can be a trap for
many designs, so examples show how to use existing ICs to
overcome common analog issues.

Attend
this brief webcast by Maxim on
TechOnline

Introduction
Washing our hands seems to be simple
common sense. It has often been said that many preventable
diseases still exist because of human error—failure to wash
hands and take logical safety precautions
regularly.

Now you ask, what has hand washing to so
with electronic IC design? Many times electronic system
failures are blamed on the parts when the real cause was human
error. Of these human errors, the most common is poor
understanding of analog in a digital world. All too often
brilliant digital designers have never been taught analog
techniques because engineering schools are reducing the amount
of analog taught. Maxim, however, is an analog mixed-signal
Company. In this application note we share some analog
concepts that will help overcome common "human" error and
improve analog circuits around the system.

Safety First
Losing one's life to test something is
not a good thing. Therefore, if you are not experienced in
troubleshooting with high voltages present, find a teacher
with that experience. Never, never, never remove safety
grounds (the third prong on the cord) from AC equipment.
Restated more succinctly, never defeat safety devices—it can
cost your life.

Issues with Isolating Transformers
We shall review
some alternatives to removing or defeating safety features.
Consider first an isolating transformer.
Isolating a safety ground may seem like a great idea to
minimize noise on the circuit. However, there are better
alternatives. For example, an isolation transformer can be
used to isolate the power line. Remember to always check the
insulation of the isolation transformer with an ohm
meter and check the resistance between the input and output
windings. These simple steps may save your life. More
expensive isolation transformers incorporate Faraday shields,
an electrostatic shield that helps reduce noise transfer
across the transformer. The transformer must be large enough
to carry the power of everything that you need isolate,
minimally the device under test (DUT), a power supply, an
oscilloscope, and a spectrum analyzer.

The oscilloscope
can be used as an amplifier
or an impedance converter to go from a high-impedance probe to
lower-impedance output. Many scopes have a vertical out that
can be plugged into a spectrum analyzer. If still higher input
impedance is needed, an actively powered FET
scope probe can be added.

An alternative to an
isolation transformer is an uninterruptible
power supply (UPS).
Make sure that the UPS has a clean power output. Inexpensive
UPSs have square wave or modified sine-wave outputs. The rise
times of these outputs can introduce noise in your test setup.
You need to use the battery on the UPS without having it
plugged into the wall socket. Therefore, the UPS has to be
powerful enough to power all of your test equipment. The
easiest way to verify that the UPS is suitable for your design
is to power your oscilloscope normally from a wall outlet.
With the probe shorted, increase the gain and note both the
noise and the noise floor on the spectrum analyzer. Now
connect to your UPS and see if the noise worsens. If the noise
is excessive, a more expensive UPS with a true sine-wave
output might be needed.

Now that we have observed
"safety first," we can proceed with testing.

Protecting the Power and Ground
We will now present
ideas to help save system and system-on-a-chip (SOC) designs
by making changes to the external circuit. We will list ideas
and refer to other sources for detailed
information.

Application note 4345, "Well Grounded, Digital
Is Analog" discusses grounds and power planes. We will
expand on those concepts. Crosstalk
and signal to noise are expressed as ratios, a proportion of
good to bad. How does one improve the signal to noise of a
signal? If there is a particular circuit that contributed
considerable noise, we can do two things: first, reduce the
noise somehow, or second, increase the amplitude of the good
signal before it goes through the stage. (We will discuss the
second option in application note 4993, "Reduce the Chances of
Human Error: Part 2, Super Amps and Filters for Analog
Interface.") Figure 1 illustrates the
concept.

Figure
1. Power-supply noise or ground being added to the
signal.

We should review a few basic housekeeping
items. What is the signal level and bandwidth
coming in, and what does it need to be as it exits this stage?
What is the amplitude and bandwidth of the noise (hopefully,
what is its source)?
Is it harmonics of a switching power supply?

We first
concentrate on the power noise. We can lowpass filter the
noise inserting resistors and inductors in series with the
supply and capacitors to ground. How far (low) in frequency
can we go? We really only want pure DC from the supply. We
could use infinitely large inductors and capacitors. That's
not practical, however, because the charging time constant is
also infinitely long. Alternatively, we could switch the
device on and grow old before it gets going. In practice, the
size and cost of inductors and capacitors are the true limits.
So, we want to know the frequency characteristics of the
noise. If it is 50kHz and higher because of supply switching
and if our wanted signal is 1kHz, then filtering may be
feasible.

If we will be going into an ADC
next, the anti-aliasing
filter may help. (See application note 4993 for additional
discussion of this topic.)

The Local Ground
We should stop thinking of "ground"
as an absolute black hole that consumes everything.
Application note 4292, "Where Is Ground?"
helps us realize that ground is relative. A circuit on a space
ship has a local reference that we call ground. That circuit
functions perfectly well, even though the potential of the
space ship is not the same as the earth. The British English
word "earth" is usually translated "safety ground" in North
America. The space ship circuit does not care about such
semantic differences because the circuit only recognizes the
difference between the local ground and the power-supply
voltage.

Decoupling and Homogenizing
Now try to visualize the
role of a power-decoupling capacitor in the space ship
circuit. The capacitor (at high frequencies) makes the noise
on the power voltage match that on the local ground. It
"homogenizes" the noise. Homogenizing is tiny particle mixing,
as in milk. Homogenization forces milk under high pressure
through small holes. This breaks the milk fat particles into
very small sizes that have larger surface area and are more
resistant to separation and clumping.

In the electrical
analogy, high-frequency noise travels both ways through the
capacitor: from the power voltage to ground and from ground to
the power voltage. Because we arbitrarily call ground the
reference, it may seem that the ground is always "clean" of
noise. To measure this, we are in fact measuring the
difference between power and ground, just like the circuit on
the space ship, and this makes us happy. If we only measure
comparing with another reference, then we may be unhappy with
the noise.

Refer back to application note 4345, "Well Grounded, Digital
Is Analog," and the section titled Selecting the
Capacitor for the Application where the self-resonance of
capacitors is discussed. Links for the two, free SPICE
self-resonance programs are found at Calibrate, trim, and
adjust. Note that the capacitor is never a pure capacitor,
but has series inductance and resistance among other features.
Above the self-resonance point the capacitor acts like an
inductor, however, the decoupling action continues to be
effective for a while above the self-resonance point. The
series resistance in wires, traces, and vias in the
power-supply path acts with the inductor impedance (a small
part of an ohm just above self-resonance) as a voltage-divider
until the inductors' equivalent series resistance
rises.

Application note 883, "Improved Power Supply
Rejection for IC Linear Regulators," presents several
important concepts. Look at the power-supply rejection ratios
(PSRR)
for the linear regulators and voltage references on their
respective data sheets. Then measure the noise and its
bandwidth that the power supply (typically a switching
supply) exhibits. For a prototype, isolate the noise
sources and determine the method of intrusion. We have seen
noise radiated into a board trace act as a radio antenna, or
the magnetic field extending out of a power transformer get
into an inductor. Noise from external circuits and sources can
also contaminate the power and ground of the power supply. A
12V automotive battery and a linear regulator comprise a
relatively quiet and inexpensive tool to compare noise levels
in low-voltage circuits.

Application note 883 suggests
several ways to filter power to remove noise. The first method
uses one or more cascades of external RC filters. Figure
2 shows the Spectrum Software Micro-Cap circuit simulator
with a second-order cascade lowpass filter and its frequency
response. A link to the free Microcap evaluation version can
be found at Calibrate,
trim, and adjust.

More
detailed image (PDF, 1.52MB)
Figure 2. The frequency
response of a second-order RC filter.

Application
note 883's second suggestion for filtering power to remove
noise is an LC filter. This approach proposes filters with
equal input and output termination resistors. This design
causes a large power loss to dampen the filter. The math to do
unequal terminations is beyond the scope of that application
note. Figure 3 uses a free filter design program to
produce the required unequal terminations. A link to the
Filter Free program from Nuhertz Technologies is found at Calibrate, trim, and
adjust.

More
detailed image (PDF, 1.63MB)
Figure 3. A filter
design program allowing unequal termination
resistors.

This program allows a practical filter
to be easily realized. An RC and an LC filter can be cascaded
and simulated in the Micro-Cap program.

The third
suggestion of application note 883 is to cascade two linear
regulators. The second regulator could be a very low-noise
voltage reference for light loads.

Application note
3656, "Single
Transistor Reduces LDO Noise by 46dB," uses a lowpass RC
filter to reduce noise. Application notes 3657, "Ultra-Low-Noise LDO
Achieves 6nV/√Hz Noise Performance," and 2027, "Simple Methods Reduce
Input Ripple for All Charge Pumps," both utilize LC
filters.

Scope Probe Ground
Even a 3in ground clip can be an
issue. If the ground clip has to be changed to another ground
point between measurements, the results may not be the same.
Therefore, first go look in the lab drawer for the little
plastic bag that came with the scope probe. In the bag are
strange-looking pieces of metal to make short ground
connections that are placed on the probe after the insulating
cover is removed. Tektronix® has articles on scope probes at
www.tek.com/learning/probes-tutorial/.

Here
is a trick we use at Maxim. Make a tool by pressing a sewing
needle into the end of a piece of wood (Figure
4).

Figure
4. A simple tool for inserting a ground onto a scope
probe.

Remove the ground lead and alligator clip.
Remove the plastic probe tip cover. Most probes will then have
the ground sleeve exposed. Make the ground as close as
possible to the circuit. If it is difficult to get the signal
pad, a second dissecting needle can be used to extend the
scope probe tip.

Consider an example. Look across a
decoupling capacitor to see how effective it is. Ideally the
capacitor should look like a short and there should be very
little noise in the capacitor's frequency range. Earlier we
mentioned decoupling capacitors and two SPICE programs that
can help us visualize their effectiveness. Figure 5 is
a screenshot of the Kemet® program. Here 0.1µF capacitors
self-resonate at about 15MHz; above 15MHZ they are inductors
and no longer work as decoupling capacitors.

More
detailed image (PDF, 447kB)
Figure 5. The impedance
and equivalent series resistance on a 0.1µF
capacitor.

Margining
Margining is a technique for testing
electronic parts. Some engineers compare it to the space
around the text on a page; the margin on paper means that the
text will be on the page, even if there is a tolerance in the
text-to-paper alignment. The margining technique has existed
for a long time, but lately has gained popularity in the
computer field. Some engineers found that increasing the
power-supply voltage would allow some circuits to run faster,
that is if they didn't overheat and fail. Manufacturers have
started binning or sorting their boards into speed groups,
selling the faster boards at a premium price. As things
evolved, manufacturers also learned that by changing the
power-supply voltages, the reliability of a board could be
assessed. Boards running poorly near the margins indicated
tolerance build up, which presented more opportunities for
failure. Consequently, margining has become a common
final-test procedure, that is, repeating tests at slightly
higher and lower power voltages.

Application Note 4149,
"How to Add Margining
Capability to a DC-DC Converter," details a method to add
margining using current
DACs. Digital potentiometers are also useful for remote
power-voltage adjustment and margining. See the following
application notes for more information on this topic:

4564, "Add
Precise Output Adjustment to Low-Voltage Supplies"

818, "Digital
Adjustment of DC-DC Converter Output Voltage in Portable
Applications"

3484,"Upgrading
from Mechanical Potentiometers to Digital Pots"

Power Supplies
Power supplies are always critical.
Understanding switching supplies and the range of available
designs will be helpful. For more information on this topic,
see:

Application note 2031, "DC-DC Converter
Tutorial"

Parametric
selector table for 246 power circuits

A
323-page book with all 246 power circuits (see above item)
including schematics and parts lists (PDF)

Application note 4596, "Improve Two-Phase Buck
Converter Performance with a Coupled-Choke Topology"
(this reduces ripple)

Application note 2391, "Dual Voltage Tracking
Circuit for I/O, Microprocessor, and DSP Core Voltages."

Maxim's
EE-Sim® tool generates an interactive schematic that
features a highly efficient simulation engine. The EE-Sim
sofware contains 25 switched-capacitor filter programs and 25
power circuits (as of February 2011) and more continue to be
added.

We cannot leave the topic of power supplies
without a word on thermal considerations. See application
notes 4083, "Thermal
Characterization of IC Packages," and 4456, "Understand Thermal
Derating Aspects of PWM ICs to Ensure the Best System
Performance."

Noisy Switching Power Supplies
Switching power
supplies have received a bad reputation for being noisy. Some
criticism is deserved, and some is not. Many times the same IC
power supply in an experienced engineer's hands will have no
problem. Many managers mistakenly relegate power supplies to
the new engineer because they are "easy" to design in. "Just
wire the IC like the data sheet says," and it should work. We
have seen design nightmares, but then the heavy noise is
suddenly fixed when an experienced designer rotates a
component just one-eighth inch and thereby fattens a ground
trace. More often it takes further work to remove the noise,
but the solution is all rooted in the knowledge of the
currents flowing around the chip and especially in the
grounds.

The following application notes will help any
engineer who lacks exposure to analog design:

3645, "Correct
Board Layout Lowers EMI of Switchmode Converters"

2997, "Basic
Switching-Regulator-Layout Techniques"

1897, "Building
a DC-DC Power Supply that Works"

716, "Proper
Layout and Component Selection Controls EMI"

1167, "Practical aspects of
EMI protection"

735, "Layout
Considerations for Non-Isolated DC-DC Converters"

Special Challenges of Switching Supplies, Audio, and
Radio
Many times it is helpful to look at extremely
difficult examples to understand the concepts. Switching power
supplies, audio, and radios are such difficult examples. Power
supplies have issues because of the high currents and fast
switching rise and fall times. Audio has signal-to-noise
issues because human hearing has wide dynamic
range. Radios have to extract signals from the noise, many
times the wireless
part losses are more than 100dB.

Two Application notes
emphasize these issues: 3630, "Power Supply and Ground
Design for a WiFi® Transceiver," and 4079, "Minimize Noise in Audio
Channels with Smart PCB Layout."

Star Ground and Control of Return Current
We now
return to application note 4345, "Well Grounded, Digital Is
Analog." That article provides some comic relief to
demonstrate simple, but effective, star
ground and return current control.

Conclusion
Engineers would certainly be happy if we
had a cookie-cutter method to fix every noise issue. But
engineering design is not that easy. Even experienced analog
designers will plan on at least three PC
Board layouts, and even then massage the layout before
going into a pilot production run. We hope that these
techniques will help direct thinking towards ways to fix the
power and ground issues during the design process.

EE-Sim
is a registered trademark of Maxim Integrated Products,
Inc.

Kemet is a registered trademark of KRC Trade
Corporation.

Tektronix is a registered trademark and
registered service mark of Tektronix, Inc.

Wi-Fi is a registered certification mark of Wi-Fi
Alliance Corporation.

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APP 4992: Mar 08, 2011

AN4992, AN 4992, APP4992,
Appnote4992, Appnote 4992

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