By Kymberly Schmidt • Maxim Integrated Products Choose capacitor types to optimize PC sound quality A key challenge to designers of audio subsystems that must conform to Windows Vista requirements may be choosing coupling capacitors. These devices’ capacitance varies with the voltage across them and introduces audio distortion. to minimize the effect, start by understanding the interactions among the dielectric material, voltage rating, device size, and voltage coefficient. Then, get ready to make trade-offs. icrosoft’s (www.microsoft.com) next-genera­ The converse piezoelectric effect is the situation in reverse: tion client operating systemnow officially A change in the applied electric field causes a change of me- known as Windows Vistais enhancing desk- chanical dimension. Large K-factor capacitors, such as those top- and notebook-PC audio quality and fidel- with Class 2 dielectrics, have a discernible converse piezo- ity. Hardware manufacturers must meet strict electric effect in which applying an electrical signal causes a audio-performance requirements to license change in the capacitor’s mechanical dimension. As the ap- Mthe Windows Vista logo. Microsoft bases these requirements plied signal’s amplitude increases, the capacitor’s physical de- on audio-performance specifications, such as the THD1N (to- formation increases, causing the capacitor’s rated electrical val- tal harmonic distortion plus noise), dynamic range, and cross- ue to change. When you place the capacitor at an audio am- talk. Generally, designers think of audio amplifiers as the lim- plifier’s input to establish dc blocking between the codec and iting factor in performance specifications, such as THD1N. the amplifier (Figure 1), the capacitor’s varying electrical val- However, passive components in the signal path can introduce ue causes a nonlinear, signal-dependent change in the ampli-

THD that contributes significantly to system-level distortion. fier’s transfer function: T(jv)5K/(12j(vo/v)), where K5RF/ Passive components are critical to a successful audio design; RIN and vo51/(RINCIN). The circuit’s magnitude response, 2 they define gain, provide biasing, reject power-supply noise, |T(jv)|, equals|K|/√(11(vo/v) ). A nonlinear change of and establish dc blocking between stages. Unfortunately, por- capacitor impedance (1/jvCIN) tends to dominate at low fre- table audio devices’ space, height, and cost restrictions force quencies at which the impedance is significant in defining the the use of passive components with small footprints, low pro- gain. This phenomenon translates into audio distortion. files, and low cost. Failure to understand the nonlinearity as- This converse piezoelectric effect is by far the most signif- sociated with these small, low-cost, passive components can icant cause of increased distortion at lower audio-band fre- affect Vista compliance (Reference 1). quencies (Figure 2). The effect is maximized at the 23-dB Voltage coefficient, temperature coefficient, piezoelectric ef- bandwidth, at which the input coupling capacitor’s imped- fect, equivalent-series resistance, equivalent-series inductance, ance magnitude equals that of the audio amplifier, or f23dB51/ leakage current, dielectric absorption, and tolerance describe (2pRINCIN). Given the typical values for an audio amplifier’s how a capacitor’s behavior deviates from ideal. The terms most important to understand when you design a INPUT COUPLING CAPACITORS signal path for premium audio performance are voltage coefficient and converse piezoelectric effect, which is CIN the main contributor to voltage coefficient. OUTR HP INR HP OUTR HDA AUDIO C Piezoelectric Effect CODEC IN AMPLIFIER The piezoelectric effect is a property of certain crys- OUTL HP IN HP OUTL tals that acquire electrical charges under mechanical L loading. The effect originates from the displacement of ionic charges within the crystal structure. Without a mechanical load, the crystal structure is symmetric, and the resulting electric dipole moment is zero. When Figure 1 Input coupling capacitors establish dc blocking between the you apply a mechanical load, the charge distribution is HDA (high-definition-audio) codec and the audio amplifier. no longer symmetric, and a net polarization results.

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edn070329ms4234fig1 mike input resistance and input coupling capacitor, 23-dB band- ac-coupling capacitors at amplifier inputs in PC applications. width is generally less than or equal to 100 Hz. Applying increasing positive (or negative) dc voltage to Class In Class 2 low-dielectric capacitors, the converse piezoelec- 2 dielectric materials decreases the capacitor’s value. This ar- tric effect is the major contributor to voltage coefficientthe ticle does not discuss the mechanics or physics that underlie term that describes how applied voltage affects a component’s this phenomenon. Instead, it simply presents measurements of value. These capacitors react differently depending on wheth- the effect and guides you in selecting capacitor types to opti- er you apply a changing (ac) voltage or a constant (dc) bias. mize PC sound quality. Figure 3 illustrates the typical effect of applying dc voltage Although applying increasing dc voltage tends to decrease to various 1-mF capacitors. This dc-voltage value is typical for the capacitance of a Class 2 dielectric, applying an ac voltage

14 �50 10V 12 10 �55 25V ELECTRICAL 8 VARIANCE 1-�F, 6.3V 0402 X5R 6 (%) �60 4 2 1-�F, 10V 0603 X5R �65 0 0 500 1000 1500 2000

APPLIED AC VOLTAGE (mV) THD�N �70 (dB FS) Figure 4 Increasing the amplitude of an ac signal applied to a ceramic capacitor increases the device’s capacitance, as you �75 see in these curves. The curves show electrical variance ver- sus applied ac voltage at 23-dB bandwidth of 100 Hz of a 1- �80 mF620%, 25V, X7R, 1206 ceramic capacitor and a 1-mF620%, FS�1V RMS EDN070329MS4234FIG4 MIKE 1-�F, 25V 0805 X7R 10V, X7R, 0603 ceramic capacitor at an ambient temperature �85 RL�32� of 258C.

1-�F, 25V 1206 X7R RIN�40 k� �90 10 100 1000 �60 FREQUENCY (Hz) NOTE: FS=FULL-SCALE.

Figure 2 Input-coupling-capacitor-induced total harmonic dis- �65 tortion versus frequency depends on the coupling capacitor’s 1-�F, 10V 0603 X7R dielectric material, voltage rating, and package size, as well as on the capacitance. �70

THD�N (dB FS) � EDN070329MS4234FIG2 MIKE 1- F, 200V PLASTIC 10 �75 0 25V �10 ELECTRICAL �20 VARIANCE �30 �80 FS�1V RMS (%) �40 10V RL�32� �50 1-�F, 25V 1206 X7R R �40 k� �60 IN �15 �10 � 0 5 10 15 �85 5 APPLIED DC VOLTAGE (V) 10 100 1000 FREQUENCY (Hz) Figure 3 A smaller ceramic capacitor has a lower voltage rating NOTE: FS=FULL-SCALE. than a larger unit of the same value and dielectric material. Also, a given applied dc voltage affects the smaller device’s capaci- Figure 5 The distortion introduced by 10 and 25V, 1-mF X7R tance more than it does that of the larger unit. You see this effect ceramic capacitors depends on the audio frequency as well as when you compare the electrical variance with the applied dc the capacitor size and voltage rating. You express the effect in voltage of a 1-mF620%,ED 25V,N070329M X7R,S423 12064F ceramicIG3 MI capacitorKE terms of THD1N (total harmonic distortion plus noise), although with that of a 1-mF620%, 10V, X7R, 0603 ceramic capacitor distortion (not noise) dominates this measurement and that of at an ambient temperature of 258C. Figure 6. EDN070329MS4234FIG5 MIKE

78 EDN | april 12, 2007 (within a reasonable range) tends to increase the measured ca- with a Maxim (www.maxim-ic.com) audio-amplifier input pacitance (Figure 4). If you apply a high enough ac voltage, whose typical input impedance is 40 kV. The device under the capacitance will eventually decrease in the same manner test varies from 10V-rated (0603 case) to 25V-rated (1206 as it does when you apply dc voltage. However, the high volt- case) units as a THD1N AP (Audio Precision, http://ap.com) age required to cause this effect does not represent the voltage sweep monitors the output distortion at frequencies less than swings you normally find in PC-audio circuits. Therefore, the or equal to 1 kHz. Notice the increased distortion when the preceding analysis does not include this voltage level. Figures setup uses the 10V-rated capacitor compared with that using 5 and 6 translate into audio performance the effect that fig- the 25V-rated capacitor. ures 3 and 4 illustrate. A low voltage rating (that is, high voltage coefficient) pro- A 1-mF, X7R-dielectric ceramic capacitor resides in series duces greater THD because the capacitor’s electrical value varies more during the sinusoidal cycle. To reduce THD in 20 the lower audio-frequency band, you must reduce the voltage 0 coefficient of capacitance. To reduce the voltage coefficient,

�20 you should select a capacitor with a higher voltage rating. In

�40 Class 2 dielectrics, selecting a higher voltage rating is help- AMPLITUDE ful when attempting to conform to Vista audio specifications. �60 (dB FS) Note, however, that the capacitor’s case size increases with the �80 voltage rating. A 1-mF620% ceramic capacitor with a 10V �100 rating uses an 0603 case size, whereas a 1-mF620% ceram-

�120 ic capacitor with a 25V rating uses a 1206 case. Regardless of the recent push for ultramobile notebook computers and �140 ever-shrinking PCB (printed-circuit-board) area, headphone- 1 2 3 4 5 6 7 8 9 10 FREQUENCY (kHz) amplifier inputs typically require large-case input-coupling ca- NOTE: pacitors to achieve Vista compliance for THD1N over the FS�FULL-SCALE. 20-Hz to 20-kHz bandwidth. Figure 6 This FFT spectrum analysis also shows the frequency dependence of coupling-capacitor-induced distortion. Full-scale Dielectric Type is 1V rms; input frequencyedn 070is 100329ms423 Hz; 4fig6and the mike device under test is You can regard a capacitor’s dielectric type as a potential a 1-mF, 25V, X7R ceramic capacitor (courtesy Audio Precision). limitation on premium-THD performance. Various dielectrics affect THD differently. Figure 7 illustrates the dc-bias depen-

20 �20 0 �20 �30 FS�1V RMS ELECTRICAL �40 X7R VOUT�0 dB FS VARIANCE �60 (%) �40 �80 RL�32� Y5V �100 RIN�40 k� �20 �10 0 10 20 �50 Y5V APPLIED DC VOLTAGE (V) AES17 X7R Figure 7 These curves illustrate the dc-bias dependency of two �60 0603-case-size capacitors with Y5V and X7R dielectrics and THD�N equal 16V ratings. (dB FS) �70

�80

80 Y5V PLASTIC 70 �90 60 EDN070329MS4234FIG7 MIKE ELECTRICAL 50 �100 VARIANCE 40 (%) 30 X7R 20 �110 10 10 100 1000 10,000 100,000 0 FREQUENCY (Hz) 0 500 1000 1500 2000

APPLIED AC VOLTAGE (mV) NOTE: FS=FULL-SCALE.

Figure 8 These curves show the variation in capacitance due to Figure 9 These curves depict the audio distortion that Y5V and applied ac voltage for 16V-rated 0603-case-size capacitors with X7R, 1-mF±20%, 16V, 0603-cased ceramic capacitors intro- Y5V and X7R dielectrics. duce.

80 EDN | april 12, 2007 EDN070329MS4234FIG9 MIKE EDN070329MS4234FIG8 MIKE Measuring capacitance variation To obtain measurements of its capacitance variation ver- sistance. In this case, the Audio Precision analog genera- sus applied dc voltage, connect the device under test tor presented a 40V source resistance.

(CDUT) in series with a 1.5-kV61% resistor (Figure A). Also note that a 100-Hz cutoff frequency for this mea-

CDUT in series with R forms a highpass filter, allowing surement highlights the effects of applied dc voltage measurement of the capacitor’s electrical value as you in- above and below the 23-dB bandwidth. A 20-Hz cutoff

crease the applied dc voltage (CDUT51/(2pRf23dB)). Super- frequency is not an appropriate choice as the analyzer impose a 100-mV-rms ac signal on the varying dc-voltage measures down to only 10 Hz. Vista-Logo Program Re- source. Note the highpass filter’s2 3-dB bandwidth at quirements V3.09 specifies 23-dB bandwidth at 20 Hz each 1V-dc increment (Figure B). Vary V dc from 0V to the into a 10-kV load. Given the 23-dB frequency and the component’s rated voltage. known series-resistor value (1.5 kV61%), you can extract

Note that the resistance of series resistor R must be the capacitance of CDUT from Table A. much greater than the audio analyzer’s finite source re-

TABLE a CDUT C V DUT V OUT Device-under-test CAPACITANCE OUT

DC-voltage 23-dB C VarianceC DUT DUT V applied bandwidth capacitance from OUT VAC 100 mV (V) (Hz) (mF) 0V dc (%) V R AC V R 0 118.9 0.89 100 mV 0 AC 1.5k 1.5k 0.1R TO 2V 1% 1.5k 1% V 1 118.9 0.89 0 1% DC � VDC � 0 TO 10V � 2 118.9 0.89 0 TO 10V �0 3 128.8 0.82 27.7 4 138.7 0.76 214.3 5 148.6 0.71 220 6 158.5 0.67 225 Figure A The model for the dc-test 7 178.3 0.60 233.3 EDFigureN070329M C TheS423 model4FIGC for the ac-test MIKE circuit consists of a variable dc-volt- circuit is a simple series combi- 8 188.2 0.56 2 age source, an ac source of 100 36.8 nation of a variable ac-voltage mV rms, the capacitor under test, 9 208 0.51 242.8 source, the capacitor under test, and a fixed resistor. 10 237.8 0.45 250 and a fixed resistor.

0 VDC = 0V V = 1V �5 DC VDC = 2V V = 3V Figure B As you increase the �10 DC V = 4V bias voltage across the capacitor MAGNITUDE DC �15 V = 5V from 0 to 10V dc, the frequency (dBr) DC VDC = 6V 0 response of the voltage across the �20 VDC = 0V V = 7V resistor deteriorates—falling, at 100 DC V = 8V Hz, from approximately 4 dB below �25 VDC = 1V DC �5 V = 9V its value at 1 kHz with zero bias to V = 2V DC �30 DC VDC = 10V approximately 8 dB below with a 10 100 1000 � VDC = 3V 10 10V dc bias. FREQUENCY (Hz) VDC = 4V MAGNITUDE (dBr) �15 VDC = 5V VDC = 6V �20 VDC = 7V V = 8V �25 82 EDN | april 12, 2007 EDN070329MDCS4234FIGA,B MIKE VDC = 9V �30 VDC = 10V 10 100 1000 FREQUENCY (Hz)

EDN070329MS4234FIGA,B MIKE MORE AT EDN.COM dency of two capacitors with Y5V and X7R dielec- MORE AT EDN.COM Although X5R dielectrics outperform Y5V dielec- trics, both with a 16V rating in a 0603 case. The trics, X7R dielectrics offer the best—that is, low- dielectric material is now solely responsible for + Go to www.edn. est—THD among Class 2 dielectrics. the difference in the voltage coefficient of capaci- com/ms4234 and The space, height, and cost restrictions typical- tance. The X7R dielectric shows a 65 to 70% loss click on Feedback ly associated with portable consumer electronics at the rated voltage. The Y5V dielectric exhibits a Loop to post a com- force the use of passive components with small 70 to 80% loss over the rated-voltage range. Fig- ment on this article. footprints, low profiles, and low cost. When you ure 8 shows the variation in capacitance due to use them for audio-signal coupling, some small- applied ac voltage for capacitors with Y5V and X7R dielec- footprint, low-profile, low-cost passive components can limit trics and a 16V voltage rating in an 0603 case. low-frequency THD performance in audio circuits. Howev- Figures 7 and 8 illustrate an effect that translates into au- er, doing so compromises audio sound quality and jeopardizes dio performance in Figure 9. You quantify the effect of applied Vista compliance. Despite their slight footprint and price pre- voltage in terms of THD1N. A 1-mF, 0603-case ceramic ca- mium, large-footprint ceramic capacitors with high voltage pacitor with a 16V voltage rating is in series with a Maxim ratings and X7R dielectrics are the best choice for all passive audio-amplifier input whose typical input impedance is 40 kV. components in the signal path for a Vista-compliant audio

CDUT varies between an X7R dielectric and a Y5V dielectric as design.EDN a THD1N Audio Precision sweep monitors the output distor- tion at frequencies of 20 Hz to 20 kHz. Notice the increased R e f e r e n c e distortion at lower frequencies with the Y5V dielectric versus 1 Microsoft, Windows Vista Logo Program Device Require- the X5R dielectric. The audio amplifier’s decreasing loop gain ments, Version 3.0, www.microsoft.com/whdc/winlogo/ limits the circuit distortion at frequencies greater than 1 kHz. hwrequirements.mspx#. Also notice that the THD1N in Figure 9 begins to roll off above 6.3 kHz, because of the AES-17 (Audio Engineering So- Au t h o r ’ s b i o g r a ph y ciety) 20-kHz filtering at the analyzer’s inputs. This measure- Kymberly Schmidt is a strategic applications engineer with Max- ment standard implements steep filtering above 20 kHz, atten- im Integrated Products, where she has worked for more than three uating any third-harmonic content above an input frequency years. She holds a bachelor’s degree in electrical engineering from of 6.33 kHz. the University of California—Los Angeles and is responsible for When considering capacitors for the audio-signal path, select audio-design support and for defining products that Maxim’s multi- capacitors with X7R dielectrics for better THD performance. media-business unit will manufacture.

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