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Choose Capacitor Types to Optimize PC Sound Quality

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Choose Capacitor Types to Optimize PC Sound Quality 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 systemnow officially A change in the applied electric field causes a change of me- known as Windows Vistais 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- M the 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. APRIL 12, 2007 | EDN 77 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 coefficientthe 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 RIN�40 k� �60 �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).
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