Maxim > Design=20 support > App = notes=20 > A/D and=20 D/A Conversion/Sampling Circuits > APP=20 4992
Maxim > Design = support >=20 App = notes=20 > Amplifier=20 and Comparator Circuits > APP = 4992
Maxim > Design = support >=20 App = notes=20 > Analog=20 Switches and Multiplexers > APP=20 4992

Mar 08, = 2011

Keywords: noise decoupling, capacitor, = self=20 resonance star, ground filter, RC, LC, resistor, = inductor,=20 voltage reference, LDO, LNA, analog, interface, = crosstalk,=20 linearity, SOC, system on a chip, safety, op amps, = ground=20 planes, power, busses, sensor conditioning =
Related=20 parts

Download,=20 PDF format (122kB)

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APPLICATION NOTE 4992

Reduce the Chances of Human Error: Part 1, = Power and=20 Ground

By:	Bill=20 Laumeister, Strategic Applications = Engineer=20

Abstract: Electronic=20 system errors are typically analog, not digital. = Analog is=20 prone to signal-to-noise, crosstalk, and linearity = issues.=20 This application note discusses the need for power and = ground=20 cleanliness and integrity. Analog interface can be a = trap for=20 many designs, so examples show how to use existing ICs = to=20 overcome common analog issues.

Attend=20 this brief webcast by Maxim on=20 = TechOnline

Introduction

Washing our hands seems to be = simple=20 common sense. It has often been said that many = preventable=20 diseases still exist because of human = error=E2=80=94failure to wash=20 hands and take logical safety precautions=20 regularly.

Now you ask, what has hand washing = to so=20 with electronic IC design? Many times electronic = system=20 failures are blamed on the parts when the real cause = was human=20 error. Of these human errors, the most common is poor=20 understanding of analog in a digital world. All too = often=20 brilliant digital designers have never been taught = analog=20 techniques because engineering schools are reducing = the amount=20 of analog taught. Maxim, however, is an analog = mixed-signal=20 Company. In this application note we share some analog = concepts that will help overcome common "human" error = and=20 improve analog circuits around the system.

Safety First

Losing one's life to test = something is=20 not a good thing. Therefore, if you are not = experienced in=20 troubleshooting with high voltages present, find a = teacher=20 with that experience. Never, never, never remove = safety=20 grounds (the third prong on the cord) from AC = equipment.=20 Restated more succinctly, never defeat safety = devices=E2=80=94it can=20 cost your life.

Issues with Isolating Transformers

We shall = review=20 some alternatives to removing or defeating safety = features.=20 Consider first an isolating transformer.=20 Isolating a safety ground may seem like a great idea = to=20 minimize noise on the circuit. However, there are = better=20 alternatives. For example, an isolation transformer = can be=20 used to isolate the power line. Remember to always = check the=20 insulation of the isolation transformer with an ohm=20 meter and check the resistance between the input and = output=20 windings. These simple steps may save your life. More=20 expensive isolation transformers incorporate Faraday = shields,=20 an electrostatic shield that helps reduce noise transfer=20 across the transformer. The transformer must be large = enough=20 to carry the power of everything that you need = isolate,=20 minimally the device under test (DUT), a power supply, = an=20 oscilloscope, and a spectrum analyzer.

The = oscilloscope=20 can be used as an amplifier=20 or an impedance converter to go from a high-impedance = probe to=20 lower-impedance output. Many scopes have a vertical = out that=20 can be plugged into a spectrum analyzer. If still = higher input=20 impedance is needed, an actively powered FET=20 scope probe can be added.

An alternative to an=20 isolation transformer is an uninterruptible=20 power supply (UPS).=20 Make sure that the UPS has a clean power output. = Inexpensive=20 UPSs have square wave or modified sine-wave outputs. = The rise=20 times of these outputs can introduce noise in your = test setup.=20 You need to use the battery on the UPS without having = it=20 plugged into the wall socket. Therefore, the UPS has = to be=20 powerful enough to power all of your test equipment. = The=20 easiest way to verify that the UPS is suitable for = your design=20 is to power your oscilloscope normally from a wall = outlet.=20 With the probe shorted, increase the gain and note = both the=20 noise and the noise floor on the spectrum analyzer. = Now=20 connect to your UPS and see if the noise worsens. If = the noise=20 is excessive, a more expensive UPS with a true = sine-wave=20 output might be needed.

Now that we have = observed=20 "safety first," we can proceed with testing.

Protecting the Power and Ground

We will now = present=20 ideas to help save system and system-on-a-chip (SOC) = designs=20 by making changes to the external circuit. We will = list ideas=20 and refer to other sources for detailed=20 information.

Application note 4345, "Well Grounded, = Digital=20 Is Analog" discusses grounds and power planes. We = will=20 expand on those concepts. Crosstalk=20 and signal to noise are expressed as ratios, a = proportion of=20 good to bad. How does one improve the signal to noise = of a=20 signal? If there is a particular circuit that = contributed=20 considerable noise, we can do two things: first, = reduce the=20 noise somehow, or second, increase the amplitude of = the good=20 signal before it goes through the stage. (We will = discuss the=20 second option in application note 4993, "Reduce the = Chances of=20 Human Error: Part 2, Super Amps and Filters for Analog = Interface.") Figure 1 illustrates the=20 concept.

Figure=20 1. Power-supply noise or ground being added to the=20 signal.

We should review a few basic = housekeeping=20 items. What is the signal level and bandwidth=20 coming in, and what does it need to be as it exits = this stage?=20 What is the amplitude and bandwidth of the noise = (hopefully,=20 what is its source)?=20 Is it harmonics of a switching power supply?

We = first=20 concentrate on the power noise. We can lowpass filter = the=20 noise inserting resistors and inductors in series with = the=20 supply and capacitors to ground. How far (low) in = frequency=20 can we go? We really only want pure DC from the = supply. We=20 could use infinitely large inductors and capacitors. = That's=20 not practical, however, because the charging time = constant is=20 also infinitely long. Alternatively, we could switch = the=20 device on and grow old before it gets going. In = practice, the=20 size and cost of inductors and capacitors are the true = limits.=20 So, we want to know the frequency characteristics of = the=20 noise. If it is 50kHz and higher because of supply = switching=20 and if our wanted signal is 1kHz, then filtering may = be=20 feasible.

If we will be going into an ADC=20 next, the anti-aliasing=20 filter may help. (See application note 4993 for = additional=20 discussion of this topic.)

The Local Ground

We should stop thinking of = "ground"=20 as an absolute black hole that consumes everything.=20 Application note 4292, "Where Is = Ground?"=20 helps us realize that ground is relative. A circuit on = a space=20 ship has a local reference that we call ground. That = circuit=20 functions perfectly well, even though the potential of = the=20 space ship is not the same as the earth. The British = English=20 word "earth" is usually translated "safety ground" in = North=20 America. The space ship circuit does not care about = such=20 semantic differences because the circuit only = recognizes the=20 difference between the local ground and the = power-supply=20 voltage.

Decoupling and Homogenizing

Now try to = visualize the=20 role of a power-decoupling capacitor in the space ship = circuit. The capacitor (at high frequencies) makes the = noise=20 on the power voltage match that on the local ground. = It=20 "homogenizes" the noise. Homogenizing is tiny particle = mixing,=20 as in milk. Homogenization forces milk under high = pressure=20 through small holes. This breaks the milk fat = particles into=20 very small sizes that have larger surface area and are = more=20 resistant to separation and clumping.

In the = electrical=20 analogy, high-frequency noise travels both ways = through the=20 capacitor: from the power voltage to ground and from = ground to=20 the power voltage. Because we arbitrarily call ground = the=20 reference, it may seem that the ground is always = "clean" of=20 noise. To measure this, we are in fact measuring the=20 difference between power and ground, just like the = circuit on=20 the space ship, and this makes us happy. If we only = measure=20 comparing with another reference, then we may be = unhappy with=20 the noise.

Refer back to application note 4345, = "Well Grounded, = Digital=20 Is Analog," and the section titled Selecting = the=20 Capacitor for the Application where the = self-resonance of=20 capacitors is discussed. Links for the two, free SPICE=20 self-resonance programs are found at Calibrate, trim, = and=20 adjust. Note that the capacitor is never a pure = capacitor,=20 but has series inductance and resistance among other = features.=20 Above the self-resonance point the capacitor acts like = an=20 inductor, however, the decoupling action continues to = be=20 effective for a while above the self-resonance point. = The=20 series resistance in wires, traces, and vias in the=20 power-supply path acts with the inductor impedance (a = small=20 part of an ohm just above self-resonance) as a = voltage-divider=20 until the inductors' equivalent series resistance=20 rises.

Application note 883, "Improved Power = Supply=20 Rejection for IC Linear Regulators," presents = several=20 important concepts. Look at the power-supply rejection = ratios=20 (PSRR)=20 for the linear regulators and voltage references on = their=20 respective data sheets. Then measure the noise and its = bandwidth that the power supply (typically a switching=20 supply) exhibits. For a prototype, isolate the = noise=20 sources and determine the method of intrusion. We have = seen=20 noise radiated into a board trace act as a radio = antenna, or=20 the magnetic field extending out of a power = transformer get=20 into an inductor. Noise from external circuits and = sources can=20 also contaminate the power and ground of the power = supply. A=20 12V automotive battery and a linear regulator comprise = a=20 relatively quiet and inexpensive tool to compare noise = levels=20 in low-voltage circuits.

Application note 883 = suggests=20 several ways to filter power to remove noise. The = first method=20 uses one or more cascades of external RC filters. = Figure=20 2 shows the Spectrum Software Micro-Cap circuit = simulator=20 with a second-order cascade lowpass filter and its = frequency=20 response. A link to the free Microcap evaluation = version can=20 be found at Calibrate,=20 trim, and adjust.

3D"Figure

= More=20 detailed image (PDF, 1.52MB)
Figure 2. The = frequency=20 response of a second-order RC = filter.

Application=20 note 883's second suggestion for filtering power to = remove=20 noise is an LC filter. This approach proposes filters = with=20 equal input and output termination resistors. This = design=20 causes a large power loss to dampen the filter. The = math to do=20 unequal terminations is beyond the scope of that = application=20 note. Figure 3 uses a free filter design = program to=20 produce the required unequal terminations. A link to = the=20 Filter Free program from Nuhertz Technologies is found = at Calibrate, trim, = and=20 adjust.

3D"Figure

= More=20 detailed image (PDF, 1.63MB)
Figure 3. A = filter=20 design program allowing unequal termination=20 resistors.

This program allows a practical = filter=20 to be easily realized. An RC and an LC filter can be = cascaded=20 and simulated in the Micro-Cap program.

The = third=20 suggestion of application note 883 is to cascade two = linear=20 regulators. The second regulator could be a very = low-noise=20 voltage reference for light loads.

Application = note=20 3656, "Single=20 Transistor Reduces LDO Noise by 46dB," uses a = lowpass RC=20 filter to reduce noise. Application notes 3657, "Ultra-Low-Noise LDO=20 Achieves 6nV/=E2=88=9AHz Noise Performance," and = 2027, "Simple Methods = Reduce=20 Input Ripple for All Charge Pumps," both utilize = LC=20 filters.

Scope Probe Ground

Even a 3in ground clip can = be an=20 issue. If the ground clip has to be changed to another = ground=20 point between measurements, the results may not be the = same.=20 Therefore, first go look in the lab drawer for the = little=20 plastic bag that came with the scope probe. In the bag = are=20 strange-looking pieces of metal to make short ground=20 connections that are placed on the probe after the = insulating=20 cover is removed. Tektronix=C2=AE has articles on = scope probes at=20 www.tek.com/learning/probes-tutorial/.

Here=20 is a trick we use at Maxim. Make a tool by pressing a = sewing=20 needle into the end of a piece of wood (Figure=20 4).

Figure=20 4. A simple tool for inserting a ground onto a scope=20 probe.

Remove the ground lead and alligator = clip.=20 Remove the plastic probe tip cover. Most probes will = then have=20 the ground sleeve exposed. Make the ground as close as = possible to the circuit. If it is difficult to get the = signal=20 pad, a second dissecting needle can be used to extend = the=20 scope probe tip.

Consider an example. Look = across a=20 decoupling capacitor to see how effective it is. = Ideally the=20 capacitor should look like a short and there should be = very=20 little noise in the capacitor's frequency range. = Earlier we=20 mentioned decoupling capacitors and two SPICE programs = that=20 can help us visualize their effectiveness. Figure = 5 is=20 a screenshot of the Kemet=C2=AE program. Here = 0.1=C2=B5F capacitors=20 self-resonate at about 15MHz; above 15MHZ they are = inductors=20 and no longer work as decoupling capacitors.

3D"Figure

= More=20 detailed image (PDF, 447kB)
Figure 5. The = impedance=20 and equivalent series resistance on a 0.1=C2=B5F=20 capacitor.

Margining

Margining is a technique for testing = electronic parts. Some engineers compare it to the = space=20 around the text on a page; the margin on paper means = that the=20 text will be on the page, even if there is a tolerance = in the=20 text-to-paper alignment. The margining technique has = existed=20 for a long time, but lately has gained popularity in = the=20 computer field. Some engineers found that increasing = the=20 power-supply voltage would allow some circuits to run = faster,=20 that is if they didn't overheat and fail. = Manufacturers have=20 started binning or sorting their boards into speed = groups,=20 selling the faster boards at a premium price. As = things=20 evolved, manufacturers also learned that by changing = the=20 power-supply voltages, the reliability of a board = could be=20 assessed. Boards running poorly near the margins = indicated=20 tolerance build up, which presented more opportunities = for=20 failure. Consequently, margining has become a common=20 final-test procedure, that is, repeating tests at = slightly=20 higher and lower power voltages.

Application = Note 4149,=20 "How to Add = Margining=20 Capability to a DC-DC Converter," details a method = to add=20 margining using current=20 DACs. Digital potentiometers are also useful for = remote=20 power-voltage adjustment and margining. See the = following=20 application notes for more information on this topic:=20

Power Supplies

Power supplies are always = critical.=20 Understanding switching supplies and the range of = available=20 designs will be helpful. For more information on this = topic,=20 see:=20

Application note 2031, "DC-DC = Converter=20 Tutorial"=20
Parametric=20 selector table for 246 power circuits=20
A=20 323-page book with all 246 power circuits (see above = item)=20 including schematics and parts lists (PDF)=20
Application note 4596, "Improve = Two-Phase Buck=20 Converter Performance with a Coupled-Choke = Topology"=20 (this reduces ripple)=20
Application note 2391, "Dual Voltage = Tracking=20 Circuit for I/O, Microprocessor, and DSP Core = Voltages."=20

Maxim's=20 EE-Sim=C2=AE tool generates an interactive = schematic that=20 features a highly efficient simulation engine. The = EE-Sim=20 sofware contains 25 switched-capacitor filter programs = and 25=20 power circuits (as of February 2011) and more continue = to be=20 added.

We cannot leave the topic of power = supplies=20 without a word on thermal considerations. See = application=20 notes 4083, "Thermal=20 Characterization of IC Packages," and 4456, "Understand = Thermal=20 Derating Aspects of PWM ICs to Ensure the Best System=20 Performance."

Noisy Switching Power Supplies

Switching power = supplies have received a bad reputation for being = noisy. Some=20 criticism is deserved, and some is not. Many times the = same IC=20 power supply in an experienced engineer's hands will = have no=20 problem. Many managers mistakenly relegate power = supplies to=20 the new engineer because they are "easy" to design in. = "Just=20 wire the IC like the data sheet says," and it should = work. We=20 have seen design nightmares, but then the heavy noise = is=20 suddenly fixed when an experienced designer rotates a=20 component just one-eighth inch and thereby fattens a = ground=20 trace. More often it takes further work to remove the = noise,=20 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=20 engineer who lacks exposure to analog design:=20

3645, "Correct=20 Board Layout Lowers EMI of Switchmode = Converters"=20
2997, "Basic=20 Switching-Regulator-Layout Techniques"=20
1897, "Building=20 a DC-DC Power Supply that Works"=20
716, "Proper=20 Layout and Component Selection Controls EMI"=20
1167, "Practical = aspects of=20 EMI protection"=20
735, "Layout=20 Considerations for Non-Isolated DC-DC = Converters"=20

Special Challenges of Switching Supplies, Audio, = and=20 Radio

Many times it is helpful to look at = extremely=20 difficult examples to understand the concepts. = Switching power=20 supplies, audio, and radios are such difficult = examples. Power=20 supplies have issues because of the high currents and = fast=20 switching rise and fall times. Audio has = signal-to-noise=20 issues because human hearing has wide dynamic=20 range. Radios have to extract signals from the = noise, many=20 times the wireless=20 part losses are more than 100dB.

Two = Application notes=20 emphasize these issues: 3630, "Power Supply = and Ground=20 Design for a WiFi=C2=AE Transceiver," and 4079, = "Minimize Noise = in Audio=20 Channels with Smart PCB Layout."

Star Ground and Control of Return Current

We = now=20 return to application note 4345, "Well Grounded, = Digital Is=20 Analog." That article provides some comic relief to=20 demonstrate simple, but effective, star=20 ground and return current control.

Conclusion

Engineers would certainly be happy = if we=20 had a cookie-cutter method to fix every noise issue. = But=20 engineering design is not that easy. Even experienced = analog=20 designers will plan on at least three PC=20 Board layouts, and even then massage the layout = before=20 going into a pilot production run. We hope that these=20 techniques will help direct thinking towards ways to = fix the=20 power and ground issues during the design process. =

EE-Sim=20 is a registered trademark of Maxim Integrated = Products,=20 Inc.

Kemet is a registered trademark of KRC = Trade=20 Corporation.

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

Wi-Fi is a registered certification = mark of Wi-Fi=20 Alliance Corporation.

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MAX8655<= /TD>	Highly = Integrated, 25A,=20 Wide-Input, Internal MOSFET, Step-Down=20 Regulator	Free=20 samples
MAX8686<= /TD>	Single/Multiphase,=20 Step-Down, DC-DC Converter Delivers Up to = 25A Per=20 Phase	Free=20 samples
MAX8860<= /TD>	Low-Dropout, = 300mA=20 Linear Regulator in =C2=B5MAX	Free=20 samples
MAX8863<= /TD>	Low-Dropout, = 120mA=20 Linear Regulators	Free=20 samples
MAX8864<= /TD>	Low-Dropout, = 120mA=20 Linear Regulators	Free=20 samples
MAX8867<= /TD>	Low-Noise, = Low-Dropout,=20 150mA Linear Regulators in SOT23	Free=20 samples
MAX8875<= /TD>	150mA, = Low-Dropout=20 Linear Regulator with Power OK Output	Free=20 samples=20

Automatic updates
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Download,=20 PDF format (122kB)
APP 4992: Mar = 08, 2011=20
AN4992, AN 4992, = APP4992,=20 Appnote4992, Appnote 4992=20 =