building analog in the 2010s

Audio Precision® is a registered trademark of Audio Precision, Inc. Copyright © 2011 Audio Precision, Inc.

Bruce E. Hofer Chairman & Co-Founder

Audio Precision, Inc.

Building Analog in the 2010s

Introduction •  Based upon the typical classes now being taught in

many engineering schools, it would appear that analog circuit design is in danger of becoming a “lost art” –  Interviews of design engineer job applicants reveal a growing

weakness in the comprehension of basic analog concepts –  Too much emphasis seems to be placed on simulation, and not

enough on understanding circuit behavior at a more intuitive level

•  Even among experienced designers there is a tendency to overlook some key factors that limit performance

•  It need not be this way…all it really takes is careful attention to detail and the practical application of some fundamental concepts

Introduction, continued •  In this seminar we will examine some selected topics

that are important to the analog designer in the 2010s –  Time constraints will limit us to cover only a few key topics –  However, I will endeavor to share some of the accumulated lessons

that have been learned throughout my 41-year career in the design of high performance and state-of-the-art measurement equipment

•  Let us also recognize that professional differences of opinion exist—my own are not necessarily definitive, and they lean heavily towards the objective side due to my background (Indeed, I do not have “golden ears”) –  Civil comments and questions are welcome during seminar breaks –  I kindly ask those with a more self-serving agenda to avoid being

disruptive…go conduct your own seminar!

What are the Factors that Determine High Performance Analog Design? •  Objective (quantitative measurements)

–  Wide dynamic range (low residual noise floor) –  Very low levels of non-linearity (e.g. THD+N, IMD) –  Excellent frequency response and/or time domain response –  Stability versus time, temperature, and humidity

•  Business Related (we live in a real world) –  Manufacturing cost –  Reliability (estimated warranty expense)

•  Subjective (what sometimes really sells a product!) –  How does it look, feel, sound?

Selected Topics to be Covered Today •  Models of Non-Linearity

–  Estimation of 2H & 3H distortion

•  Passive Component Quality –  Perhaps one of the most significant factors in analog design

•  Op-Amp Selection, circa 2011

•  Noise in Analog Circuits

•  Circuit Layout Considerations

A Non-Linearity Model that Enables Estimation of 2H and 3H Distortion •  Use a Taylor Series to model non-linear behavior

–  The instantaneous value of circuit gain is modeled as having a voltage dependence: A(Vs) = Ao * (Vs + k2* Vs ^2 + k3* Vs ^3 …)

•  2HD and 3HD can be estimated with surprising accuracy using only 3 values for dynamic gain at the positive peak (Ap), negative peak (An) and zero (Ao) points of an assumed sine-wave signal: –  2HD ≈ |Ap – An| / (8*Ao) –  3HD ≈ |Ap + An – 2*Ao| / (24*Ao)

•  Higher orders exist but there is usually little additional insight to be gained in going beyond the third order

Example: Emitter Follower Distortion •  A simple emitter follower was constructed using a

MPSA18 transistor with RL = 4.02k wired to -15V and the collector to +15V. Performance with a ±5Vp (10Vpp) signal is to be estimated at +21.8C (≈295K)

•  The dynamic emitter impedance (Re) and follower gain are calculated at Vout = -5.0, 0, and +5.0 V: –  -5.0 V: Re = kT/qIe = 11.153Ω à An = 0.99723 –  0.0 V: Re = kT/qIe = 7.125Ω à Ao = 0.99823 –  +5.0 V: Re = kT/qIe = 5.234Ω à Ap = 0.99870

•  Thus the estimated distortion is -74.8 dB (0.018%) at 2H and -93.2 dB (0.0022%) at 3H

Emitter Follower Circuit

Note: Re = kT/qIe

FFT of Follower Output, Vin=10 Vpp •  Measured distortion factors are -75.1 dB and -93.4 dB

versus the estimates of -74.8 dB and -93.2 dB—note only 0.2-0.3 dB discrepancy

Component Non-Linearity Model •  Although a Taylor series model is quite successful for

estimating distortion when semiconductor non-linearity dominates, it is not as accurate in dealing with passive component non-linearity

•  Resistor non-linearity is usually best modeled as having a simple dependence upon the absolute value of voltage: R(Vs) = Ro * (1 ± kv* |Vs |) –  kv has units of ppm per Volt, or fractional change per Volt –  2HD ≈ 0 (due to the symmetry of assumed non-linearity) –  3HD ≈ kv* Vs / 5.9 (derived from FFT of full-wave rectified sine) –  At low frequencies this model can be extended to include thermal

modulation effects due to the resistor’s temperature coefficient

Component Non-Linearity, cont’d •  When assessing the effect of a resistor’s non-linearity,

the circuit sensitivity to the resistor value must also be taken into consideration –  Resistors in the signal path that directly affect circuit gain will

usually have a sensitivity factor that is close to 1.0 unless they are part of a series or parallel combination

–  Indeed, using a series resistor combination is typically one of the best ways to minimize the effect of voltage coefficient

•  Capacitors also have voltage coefficient effects, but these are much more difficult to analyze due to their inherent frequency dependence –  Will not be covered in this seminar

Passive Component Quality •  There is an old saying among chefs and bakers that the

quality of a meal (or dessert) depends largely upon the quality of the ingredients –  The same is true with analog circuit design

•  Resistors, capacitors, and inductors come in many different types, having different sizes, voltage ratings, power ratings, temperature coefficients, voltage coefficients, sensitivity to humidity, etc.

•  Understanding some of these differences is a good first step towards achieving higher performance in analog design


Resistors

Basic Resistor Technologies •  Composition

•  Thick Film (perhaps the most popular today)

•  Thin Film

•  Metal Film

•  Metal Foil

•  Wire-Wound

•  Other (e.g. Thermistors, Metal Oxide Varistors)

Composition Resistors •  The resistive element is a compacted mixture of

carbon and ceramic held together in a resin base –  Very popular prior to the 1970s, less popular today –  Useful in applications requiring high peak power capability or

super low series inductance

•  Unimpressive performance by today’s standards –  Tolerances from 5-20% –  Temperature coefficient is typically 150 ppm /C but can exceed

1500 ppm /C at low values –  Relatively high noise in comparison to other types

•  Not recommended for high performance analog or audio designs!

Thick Film Resistors •  The resistive element is a conductive film sputtered

onto the surface of a cylindrical or rectangular base –  Resistance is determined by film and etching pattern –  Very wide variety of sizes and power ratings available

•  Very popular in general purpose applications –  Tolerances of 0.1% to 2% (usually laser trimmed when <1%) –  Temperature coefficient is so-so, typically 100 to 250 ppm /C –  Noise can be relatively high compared to thin film and foil –  Voltage coefficient is rarely specified, and can vary considerably

from brand to brand, and also by value

•  MELF (Metal Electrode Leadless Face) versions are available for higher peak power dissipation

Thin Film Resistors •  The resistive element is a conductive film sputtered

onto the surface of a cylindrical or rectangular base; however the thickness is typically 1000 times thinner than thick film, and it has a different composition –  Resistance is determined by the etching pattern and trimming –  Wide variety of sizes and power ratings available

•  Much better, but more expensive than thick film –  Tolerances of 0.1% to 1% (usually laser trimmed when <1%) –  Temperature coefficients as low as 5 to 25 ppm /C –  Excellent (low) noise –  Usually much lower voltage coefficient than thick film

Metal Film Resistors •  The resistive element is a metallic film (commonly

nichrome) deposited or sputtered on the surface of a cylindrical base –  Very similar to thin-film, however the resistance is determined by

physically cutting (not etching) a helix pattern in the film

•  Acceptable for many analog applications –  Tolerances of 0.5% to 2% are common –  Temperature coefficient is typically 50 to 100 ppm /C –  Very low noise and voltage coefficient

•  MELF (Metal Electrode Leadless Face) versions are available for higher peak power dissipation

Metal Foil Resistors •  The resistive element is composed of a special alloy

metal foil mounted on an inert substrate –  Resistance is determined by the foil characteristics and pattern –  Trimming is usually accomplished by opening links in a carefully

designed pattern—vastly more stable than “L” cut trimming

•  Quite simply the best and highest performance of all resistor technologies –  Tolerances to 0.001% –  Temperature coefficient as low as 0.2 ppm /C (!!!) –  Ultra stable particularly when hermetically sealed –  Extremely low noise and thermal EMF –  Voltage coefficient typically <0.1ppm /Volt (!!!)

Wire-Wound Resistors •  The resistive element is wire having a low temperature

coefficient (often nichrome) wound on a substrate –  Typically appropriate only for lower resistance values –  Very high peak and average power ratings are possible

Winding Patterns

#1 / #3 - Coil

#2 - Bifilar

#4 - Ayrton-Perry

Resistor Networks •  Resistor networks are especially useful in applications

that benefit from ratio matching –  Ratio accuracies can be as good as 0.01% with thin film, and

0.001% with metal foil –  Extremely low differential temperature coefficients are possible

•  Avoid large ratios (e.g. 50:1 or higher) which can result in poorer thermal tracking performance –  Best performance is achieved if all resistors can be of equal value

•  The small size of resistor networks (typically SOIC-8 or SOIC-16) offers an advantage in challenging layout situations where large temperature gradients exist

A Bad Experience (True Story) with a Resistor Network—Beware! •  About 11 years ago a certain manufacturer decided to

switch their network substrate material from ceramic to passivated silicon without notifying its customers –  Ceramic is more expensive to process and to cut to size

•  Although the resistor DC parameters were unchanged, the AC behavior was a total disaster! –  The stray C between each resistor and the substrate was not only

higher, but NON-LINEAR –  It is believed that PIN diodes were formed between each resistor

and the semi-conducting substrate, thus causing the voltage drop in one resistor to generate distortion products in the other resistors

–  The manufacturer quickly added the “option” of specifying the original ceramic substrate when told they were being disqualified

Recommendations for Audio Circuits •  When cost is no object, the only resistor type to use in

critical signal path locations is metal foil

•  When more realistic cost targets are important, use thin film (not thick film) resistors –  Thick film resistors will likely yield higher noise in the presence of

dc biasing currents, and higher distortion overall

•  The 1206 size SMD is still a relative “sweet spot” in terms of price and availability for precision resistors –  For the same value and applied voltage, using a smaller resistor

size will likely result in significantly higher distortion –  Limit the maximum signal voltage to about 3-5 Vrms for best

performance—use series combinations for higher voltages


Capacitors

Dielectric Materials, circa 2011 •  Ceramic (a generic term embracing a wide range of

compositions and characteristics) –  Examples include “Z5U”, “X7R”, “NP0”, “Hi-K”

•  Mica (rapidly declining in popularity)

•  Glass

•  Polymer Film (another generic term) –  Examples include “polyester” (PE), “polyphenylene-

sulfide” (PPS) “polypropylene” (PP), “polystyrene” (PS), and “polytetrafluoroethylene” (PTFE) aka Teflon®

•  Electrolytic

Ceramic Capacitors •  Except for power supply decoupling, there is only one

ceramic composition that should ever be specified in high performance analog design: “NP0” (aka “COG”) –  Now available in values up to 100 nF with standard tolerances of

1-5% and voltage ratings to 1 kV (500V SMD) –  Consider paralleling multiple caps to get higher values –  Very low dissipation factor and frequency dependence –  Ultra-low temperature coefficient, typically ±30 ppm /C (!!!) –  Excellent stability, virtually immune to humidity

•  Avoid the lowest 25V rating in critical audio designs –  50V and 100V rated caps are not that much larger (perhaps 1206

versus 0603), but they will give superior distortion performance

Mica Capacitors •  30 years ago mica capacitors were highly regarded in

the analog design community –  Commonly available in values to >3 nF with tolerances to 1% –  Temperature coefficient typically 90 ppm /C –  Good stability, but mica’s brittleness can sometimes result in

unexpected and abrupt value shifts with physical stress

•  Unfortunately mica is a product of nature, and some of its better sources have now been depleted

•  With the ready availability and lower temperature coefficient of NP0 / COG ceramic caps, there is not much reason to specify mica capacitors anymore

Glass Capacitors •  Glass is among the most stable and inert of dielectrics

–  Typical values available up to >2 nF –  Extremely stable with almost zero aging characteristic –  Some sensitivity to frequency, perhaps a bit worse than “NP0”

ceramics and polypropylene (PP) film capacitors –  Temperature coefficient typically +140 ppm /C, not as good as

other types but glass caps can operate up to +200C –  Extremely low, almost non-existent voltage coefficient –  Highest immunity to radiation—the best choice for the survivalist

with golden-ears preparing for the post-apocalyptic world

•  Unfortunately molten glass is not so easy to form with precise dimensions –  5% tolerance typical, 1-2% available but hyper-expensive

Polymer Film Capacitors •  Films that are widely available from many sources

–  Polyester (PE), aka Mylar® –  Polyphenylene-sulfide (PPS), a relatively new film becoming more

popular as an alternative to polyester with better characteristics –  Polypropylene (PP) –  Polystyrene (PS)

•  More limited availability films –  Polycarbonate (PC), not much to favor it today, very hygroscopic

and must be hermetically sealed for acceptable stability –  Teflon® or polytetrafluoroethylene (PTFE) has excellent electrical

properties but can be difficult to use as a film with good reliability –  Kapton® (polyimide), used in very high temperature applications

Polymer Film Construction Techniques •  Metalized Film

–  The dielectric film is pre-coated with a conductive surface that is connected to one of the capacitor terminals

–  Has higher equivalent series resistance, hence higher dissipation factor (tan θ)

•  Metal Foil Film –  The dielectric film is interleaved with real metal foils that are

connected to the capacitor terminals –  Lower equivalent resistance than metalized film

•  If possible, avoid metalized film and use only the Foil Film type if a polymer film capacitor is the best choice

Polymer Film Dielectrics for Audio •  In the world of high performance analog design only

three dielectric films are recommended: –  Polystyrene (PS) offers excellent long term stability (when in a

hermetically sealed package) and very low dissipation factor; but it has a temperature coefficient of about -100 ppm /C and can be easily damaged by soldering since the film melts at +85C

–  Polypropylene (PE) is a much lower cost alternative with very low dissipation factor and a higher melting point (+105C); but it also has a much higher temperature coefficient up to -250 ppm /C

–  Teflon® (PTFE) has many superlative electrical properties and is popular among audiophiles who believe it just “sounds” better; but vendors are few and far between--can you afford it?

•  Depending upon the electrical value required, consider using “NP0” or “COG” ceramic capacitors

An Insidious Distortion Mechanism in Metalized-Film and Foil-Film Caps •  Although all polymer films have varying degrees of

voltage coefficient, and should never be used with AC potentials above about 5-10% of their voltage rating, there is yet another distortion mechanism to beware –  The metalized surfaces or foils must be electrically connected to

the external component leads –  Many of these connections are often physical in nature, and they

can exhibit diode action and/or series resistance non-linearity

•  You are at the mercy of the component manufacturer! –  This effect can not be measured using conventional LCR meters –  Ask the manufacturer if they can fabricate the capacitor using a

spot weld operation to electrically connect the foils to the leads (may not be possible)

Electrolytic Capacitors •  Polarized electrolytic capacitors are constructed from

two conducting metal terminals, one of which has an insulating layer of oxide (the cathode), both being separated from the other by a liquid “electrolyte” –  The two most popular electrode metals are aluminum and tantalum

however niobium is beginning to appear –  The electrolyte can be of many different chemistries including both

aqueous and non-aqueous (indeed, some old generation caps used sulfuric acid! )

–  Non-polarized caps are possible by using two oxidized cathodes

•  Electrolytic capacitors must be “formed” during their manufacture by applying a current limited potential across the terminals to stabilize the oxide layer

Electrolytic Capacitors, continued •  Electrolytics offer the largest C per unit volume

–  But that is about their only advantage; electrolytics are inferior in almost every other respect compared to other capacitors

–  Typical applications are power supply decoupling and DC blocking in analog circuits where large values are required

–  Should never be used in applications where the value is critical such as frequency equalization or precision active filters

–  Value varies considerably versus frequency—difficult to model

•  Electrolytic capacitors can introduce interesting forms of low frequency distortion caused by their sensitivity to both voltage and frequency –  Non-linear modeling is almost hopeless due to process differences

from manufacturer to manufacturer, and normal batch variations

Microphonic Effect in Capacitors •  In any capacitor: d(Q) = d(C*V)

–  The above equation if often simplified as d(Q) = C*d(V) from whence the common equation I = d/dt(Q) = C*d/dt(V) is derived

•  However, “C” itself is not necessarily constant –  C is not only a function of voltage V (non-linearity), but it can also

vary with mechanical stress/vibration thus acting as a microphone –  The total derivative is really d(Q) = d(C*V) = C*d(V) + V*d(C),

thus giving I = d/dt(Q) = C*d/dt(V) + V*d/dt(C)

•  Minimize the dc potential across capacitors in series with the signal path –  The AC coupling caps in phantom powered microphone input

circuits should be as small as possible, and matched in value

Piezoelectric Effect in Some Capacitors •  Certain “junk” grades of ceramic dielectric exhibit a

piezoelectric effect in which unwanted voltages are generated by physical stresses within the cap –  “Z5U” and “Y5V” ceramics often include barium titanate

(BaTiO3) which shows a very strong piezoelectric effect –  Even when used only for power supply decoupling the designer

should verify that the circuit will not be sensitive to potentials generated within these caps

–  Never use these grades of ceramic in voltage reference filters, or as AC blocking capacitors in series with the audio signal path

•  Ceramic “NP0” & “COG” capacitors are OK –  Glass capacitors are reported to be the very best; but I have had

very limited experience with them and cannot verify this claim


Inductors

Inductors in Audio Design •  There are only a few applications requiring inductors

in high performance analog design –  RF suppression on sensitive, low noise input circuits –  Emitter degeneration compensation in discrete amplifiers –  Filters in Class-D amplifier output stages to attenuate high

frequency switching artifacts, and to suppress EMI –  By far, the most common use is in passive crossover networks

inside loudspeakers

•  Typical values range from 3.3 µH to perhaps 100 µH –  Usually too large for an air-core design except in loudspeaker

crossover applications –  Thus, many practical inductors tend to have ferrite or some other

non-linear ferromagnetic core material

Minimizing Inductor Non-Linearity •  Large value inductors should be designed with the

lowest loss core material possible –  Avoid common forms of silicon-steel used for power transformers –  High nickel content alloys (e.g. Permalloy®) are OK –  Ferrites are so-so depending upon the inductor design

•  Air core inductors should be mounted away from steel chassis parts or other ferromagnetic materials –  The flux pattern surrounding a coil should not flow through such

non-linear materials –  A thin sheet of aluminum can be an effective magnetic shield above

about 10 kHz due to “skin effect”

Air-Core Inductor Experiment


Op-Amp Selection, circa 2011

Major Categories of Op-Amps •  Op-amps are ubiquitous in analog design

–  They are a fundamental building block enabling high performance amplification, mixing, and frequency contouring of audio signals

–  They are also useful in signal analysis and generation applications

•  Op-amps are commonly divided into categories depending upon their intended application –  Precision, optimized for low DC offset and bias current –  General purpose, usually dominant-pole compensated, but many

newer designs now insert a pole-zero pair into the open loop response to get a higher GBW (gain-bandwidth product)

–  High speed, higher slew rate, not necessarily stable under unity gain situations

–  Really high speed and slew rate, typically for video signals

A More Useful Classification •  Advances in IC processes and circuit techniques now

blur these more traditional categories –  Indeed, there are a number of op-amps that feature both excellent

DC performance and decent slew rate and GBW characteristics, e.g. OPA1611, OPA827, LT1468 (my apologies if your favorite is not in this brief list)

•  A much more useful distinction to the analog circuit designer is input device technology: Bipolar vs. JFET –  Both can offer input voltage offset performance to below 200 µV –  However, JFET op-amps have the strong advantage of near-zero

input bias current which can be a more significant factor than input offset voltage depending upon circuit impedances

–  An interesting example of a hybrid design (using both bipolar and JFET devices) is the “Butler Amplifier” in the dual OP275

Bipolar vs. JFET Noise Performance •  Bipolar op-amps can offer the lowest noise voltage

rating (eN), typically 0.9 nV/√Hz with the AD797 –  0.9 nV/√Hz is equivalent to the noise of a 49Ω resistor! –  But super low eN usually comes with the penalty of much higher

current noise iN … typically 2.0 pA/√Hz for the AD797

•  Today’s better JFET op-amps can have eN as low as 3.8-6.0 nV/√Hz –  Comparable with the older bipolar NE5534 and NE5532 –  However JFET op-amp current noise is vanishingly small,

typically only 0.001-0.003 pA/√Hz or even lower

•  Bipolar input op-amps also have a slight advantage in terms of lower “1/f” noise below 1 kHz

Distortion Mechanisms in Op-Amps •  Input stage trans-conductance non-linearity

–  ΔIinput = Ccomp * d/dt(Vout), [part of Ccomp may be external] –  Typically 3HD and proportional to F^3 in dominant pole designs

•  Output stage or crossover non-linearity –  Caused by the non-linear output impedance variation as the output

current changes in response to the output voltage –  Some op-amps use a cancellation scheme (e.g. AD797)

•  Common mode input non-linearity –  Caused by input capacitance variation with common mode signals –  Can be avoided by using only inverting configurations –  Bipolar op-amps are usually better than JFET op-amps when a

non-inverting configuration must be used

Some Distortion Reduction “Tricks” •  Output stage non-linearity can often be significantly

reduced by forcing the output to behave more like a class-A amplifier by adding a resistor or a biasing dc current source to one of the supplies –  Watch out for increased power dissipation in the op-amp!

•  Op-amps needing an external compensation capacitor can usually benefit from either “2-pole” compensation or “feed-forward” compensation –  Instead of using the classic 22 pF between pins 5-8 of a NE5534,

use a pair of 47 pF connected in series with a 499-1k resistor connected between the two capacitors and the positive supply

–  For inverting NE5534 configurations, try connecting a capacitor having a value of about 6.8-12pF between the input and pin 8

Two-Pole Compensation

“Feed-Forward”

More “Tricks” to Minimize Distortion •  Use inverting topologies whenever possible

–  Input capacitance is usually higher and more non-linear with common mode signals in JFET op-amps

–  Most op-amps will show dramatically lower THD (particularly 2HD above 5 kHz) when operated with a gain of -1 versus +1.

•  If an op-amp must be used in a non-inverting topology (for example in a Sallen-Key active low-pass filter), arrange for both of its inputs to “feel” the same source impedance –  This usually means adding a complicated RC network in series

with the + input to match the impedance seen looking outward from the – input

–  Try it--the distortion reduction can be quite significant with JFET op-amps!

Common-Mode Distortion Reduction


Noise in Analog Circuits

Sources of Noise •  “Thermal” noise of resistors: VN = √(4*k*T*R*BW)

•  “Shot” noise of biasing currents: IN = √(2*q*Idc*BW)

•  “Op-Amp” noise, usually “eN” and “iN” in datasheets

•  “1/f” and “Popcorn” noise in op-amps –  Mechanisms still not well understood, but under control—usually

not a significant factor in most audio designs

•  “Modulation” noise in resistors –  Caused by component material imperfections usually resulting in

AM noise sidebands surrounding a pure tone –  Thick film is so-so, thin film is OK, wire and metal foil are best

Noise Estimation in Circuits •  The residual noise floor of many analog circuits can

also be estimated with surprising accuracy using only a well designed spreadsheet! –  List all noise sources including resistors, op-amps, bias currents –  Calculate the transfer function either from the input or output

depending upon the desired result –  Express all entries in the same unit (recommend nV/√Hz) –  Perform a root-mean-square (rms) summation of all sources –  Convert the final result to Volts by multiplying by √BW where BW

is the desired measurement bandwidth (e.g. 20 kHz for audio)

•  The following slide shows an example spreadsheet for a prototype AP analyzer design: noise estimates are shown in blue, actual measurements are in red

Range Vmin = 24.04 7.60 2.40 0.760 0.240 0 Range Vmax = 85.3 26.99 8.53 2.699 0.853 0.270

source resistance 173.524 173.524 2.855 2.855 2.855 2.855 input dampers 374 3.524 3.524 3.524 3.524 3.524 3.524 input current limiters 442 3.897 3.897 3.897 3.897

698 0.395 0.395 0.395 MBUF en, ie=146 uA 2.178 30.912 30.912 3.091 3.091 3.091 3.091 MBUF in 0.25 31.919 31.919 0.311 0.311 0.311 0.311 post MBUF attenuator 162 152.528 23.473 15.253 2.347 2.347 2.347 atten Rout * in 52.172 3.909 5.217 0.391 0.391 0.391

preamp en 1.10 49.377 15.615 4.938 1.561 1.561 1.561 preamp in 1.70 21.881 6.919 2.188 0.692 0.763 0.241 preamp Rg 1000 2.983 1.750 preamp Rf 402 98.755 31.229 9.876 3.123 1.844 0.583

sum stage en 2.68 170.132 53.800 17.013 5.380 1.701 0.538 sum stage in 1.60 71.822 22.712 7.182 2.271 0.718 0.227 sum stage Ri 1000 184.425 58.320 18.443 5.832 1.844 0.583 sum stage Rf 1000 184.425 58.320 18.443 5.832 1.844 0.583

inv stage en 2.68 85.066 26.900 8.507 2.690 0.851 0.269 inv stage in 1.60 50.275 15.898 5.028 1.590 0.503 0.159 inv stage Ri 1400 77.151 24.397 7.715 2.440 0.772 0.244 inv stage Rf 1400 77.151 24.397 7.715 2.440 0.772 0.244

A/D driver en 1.10 139.520 44.120 13.952 4.412 1.395 0.441 A/D driver in 1.70 76.311 24.132 7.631 2.413 0.763 0.241 A/D driver Ri 1000 92.213 29.160 9.221 2.916 0.922 0.292 A/D driver Rf 215 198.871 62.888 19.887 6.289 1.989 0.629 A/D 0dBFS 3.961 A/D noise floor -121.55 765.689 242.132 76.569 24.213 7.657 2.421 A/D headroom 3.33

Tambient, C 23.0 measurement BW 20.00 924.214 335.071 90.918 29.109 12.037 8.060

predicted uV noise = 130.0 47.1 12.79 4.10 1.694 1.134 measured noise = 129.9 47.1 12.80 4.10 1.697 1.134

Noise Measurements •  Noise measurements should be heavily averaged to

reduce random fluctuations and insure repeatability –  Noise is “noisy”, may require averaging over 30-60 seconds –  Beware that noise measurements may also include the effects of

non-noise signals such as AC mains interference

•  Resistor noise is proportional to √T (T in °K) –  The temperature of each resistor must be considered –  Temperatures on an exposed prototype board may be significantly

lower than those when the board is mounted in its package

•  Be cautious of the results from only one prototype –  Op-Amp and semiconductor noise can easily vary by 10% from

unit to unit, and from their datasheet specs

Designing for Low Noise •  Resistor noise voltage is proportional to √R

–  Use the lowest possible resistor values that are consistent with power dissipation and distortion requirements

–  Choose circuit topologies that inherently minimize the value of resistors in the signal path

–  A good design rule is to avoid resistor values >2.0 kΩ in the signal path of audio signals (exceptions include power amplifier feedback resistors where higher signal voltages are present)

–  Series resistor combinations may be good for lower distortion because they reduce the voltage drop across any given resistor; but they do not result in lower noise

•  Use only thin-film or metal foil resistors when they must pass significant dc bias currents

Designing for Low Noise, continued •  Watch out for unwanted leakage currents

–  Input overload protection diodes should be low leakage types (consider using a diode-connected transistors or JFETs instead of discrete diodes)

–  The leakage current of a reverse biased diode can be incredibly sensitive to light depending upon the opacity of its package

–  Surface leakage on a circuit board can usually be eliminated with the addition of guard traces or guard rings around critical nodes

–  Leakage in AC coupling caps is usually inversely proportional to its voltage rating, but directly proportional to its C value

•  Choose your op-amps carefully –  Total noise performance in a circuit is often a delicate tradeoff

between voltage (eN) and current (iN) noise parameters –  The lowest eN op-amps may not be the best due to high iN


Circuit Layout Considerations

Circuit Layout is Important •  Poor circuit layout can cause many electrical problems

–  Excessive stray capacitance around op-amp inputs will exacerbate feedback instability

–  Excessive stray capacitance can adversely affect circuit response –  Excessive stray capacitance can degrade crosstalk –  Excessively long traces can add unwanted inductance –  Routing of power supply busses near sensitive circuit nodes

(especially on outer layers) will increase the likelihood of surface leakage problems caused by dust and air pollution accumulation

•  Poor circuit layout can also degrade performance due to thermal considerations –  Inferior cooling due to poor location of taller components –  Gradient effects on matched resistors and transistor pairs

Lenz’s Law and Magnetic Pickup •  The presence of a time varying current in a loop will

cause the induction of potentials in other nearby loops –  Mutual inductance –  The classic example is the induction of unwanted “hum” or AC

mains interference caused by a nearby power transformer –  Testing for circuit susceptibility can easily be accomplished in a

controlled manner using an external coil fixture and oscillator

•  Mutual inductance between power supply busses and signal path loops can cause unexpected distortion –  The current waveform in most power supplies is typically very

non-linear with very high harmonic content –  When coupling occurs into feedback paths, it can become the

dominant distortion mechanism above 5 kHz!

Minimizing Mutual Inductance •  Simple layout guidelines that can greatly reduce the

unwanted effects of mutual inductance –  Arrange for the positive and negative supplies to run parallel to

each other so that loop area is minimized –  Consider differential decoupling (i.e. a large capacitor between

the positive and negative rail) for any circuit that can cause significant non-linear currents in the power supplies—the cap needs to be close to the offending generator to minimize loop area

•  Pay particular attention to loop areas of signal paths to minimize the pickup of stray magnetic fields from unintended sources –  Signal path loops can sometimes be difficult to identify in more

complicated circuits such as active filters

Layout of Output Termination

Layout of Preamp with Gain Switching

Other Suggestions for Good Layout •  Try to arrange the circuit in closely clustered “islands” during the initial layout –  Leave space for low loop area signal routing –  Leave space for the inevitable additions and “circuit growth” that

always seem to occur during evaluation –  Consider ground plane cutouts if necessary to minimize stray C

•  Exploit multi-layer circuit board design –  Sometimes it is easier to arrange differential traces to be on top of

each other in different layers (watch out for different stray C) –  Use power busses, not power planes, for analog supplies…the

current paths within power planes are generally unpredictable –  Carefully locate and route power supply busses for minimum

mutual inductance with critical signal paths

In Conclusion… •  Today we have discussed some selected topics that

influence the performance of analog circuits

•  Hopefully you have gained a better awareness for some of the imperfections and tradeoffs the analog design engineer faces in the 2010s

•  Many of these issues can be overcome through careful attention to detail and clever circuit design; others will continue to challenge engineers well into the future

•  Analog design engineering can be a very rewarding career--let us never allow it to become a “lost art”

If you want a copy of today’s slides…

Please send your email address to

[email protected] or [email protected]


Bruce E. Hofer Chairman & Co-Founder

Audio Precision, Inc.

Building Analog in the 2010s

building analog in the 2010s

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