Audio Power Amplifier Measurements

Signal Conditioning: Audio Amplifiers Texas Instruments Incorporated Audio power amplifier measurements

By Richard Palmer Figure 1. Class-AB basic audio system Application Specialist, Audio Amplifiers measurement Introduction Characterization is an important step when a part is Power released to production. The data sheet is the medium Supply through which the manufacturer relates this information to the customer. It is of paramount importance that the information on the data sheet be relevant and clear. It is sometimes beneficial for the customer to re-create these characterization measurements in order to evaluate a device in his system. Analyzer Signal APA R 10 Hz to The primary goal of audio characterization measure- Generator L ment is to determine the performance of a device in the 20 kHz audible spectrum—20 Hz to 20 kHz. Although most people do not hear frequencies below 50 Hz nor above 15 kHz, the broader spectrum is an industry standard and allows more accurate comparison of devices. A method for measuring standard data sheet information is presented ideally has only the fundamental frequency (no harmonics for several key parameters: total harmonic distortion plus are present). An analyzer is then connected to the APA noise (THD+N) versus output power (PO) and frequency; output to measure the sine-wave output. The analyzer gain and phase versus frequency; and crosstalk and noise must measure from 20 Hz to 20 kHz. A regulated dc power versus frequency. supply is used to reduce the noise and distortion injected into the APA. Basic measurement system A block diagram of a class-D APA measurement system is This article provides the guidelines for measuring the shown in Figure 2. The system is the same as the class-AB parameters of Texas Instruments audio power amplifiers system, except for the addition of the LC and RC low-pass (APAs). The measurements were made using off-the-shelf filters. These filters reduce the level of the high-frequency evaluation modules (EVMs) that are compatible with the output signal of the pulse-width-modulated class-D APA TI Plug-n-Play platform. EVMs for the TPA2001D1 and prior to the analyzer inputs. The rail-to-rail square-wave TPA731 were used for all but the crosstalk measurement, signal exceeds the common-mode voltage of the analyzer, which requires a stereo device. TPA2001D2 and TPA0212 making accurate distortion measurements impossible EVMs were used for the crosstalk measurements. without some sort of filter. Figure 1 shows the block diagram of a basic measurement The RC filter is required to measure any of the filter-free system for class-AB amplifiers. These amplifiers are rela- class-D APAs, which use an improved modulation scheme tively easy to measure since they are linear—the output is that greatly reduces the quiescent current and the need a linear representation of the input. The input signal of for the expensive, bulky LC filters used with the more the APA is provided by a low-output-impedance source. contemporary class-D devices. These filters are mutually A sine wave is normally used as the input signal since it exclusive—when one is present, the other is not required. More information about the selection of the RC filter is provided later in this article. See Reference 1 Figure 2. Class-D basic audio system measurement for more information about the class-D LC filter. Interfacing with the APA inputs Power and outputs Supply The APA inputs are either differential (Diff) or single-ended (SE), and the outputs are configured for either a bridge-tied load (BTL) (otherwise known as an H-bridge circuit) or a single-ended Low-pass Low-p ass (SE) load. There are four possible configurations of Analyzer these inputs and outputs for the APA: Signal LC Filter* RC Filter APA R 10 Hz to Generator L Low-p ass Low-p ass 20 kHz Diff input and BTL output (Diff/BTL) LC Filter* RC Filter Diff input and SE output (Diff/SE) SE input and BTL output (SE/BTL) *Not required with the filter-free class-D APAs SE input and SE output (SE/SE)

Analog and Mixed-Signal Products www.ti.com/sc/analogapps July 2001 Analog Applications Journal Texas Instruments Incorporated Signal Conditioning: Audio Amplifiers

Differential inputs have two pins per channel that Figure 3. Class-AB or class-D audio system measurement amplify the difference in voltage between them. They reduce noise and AP Generator Out Audio Power Amplifier AP Analyzer In distortion in the APA. CIN Diff Low-pass Low-p ass BTL outputs have two + IN+ Inputs OUT+ LC Filter* RC Filter + Channel A and R Channel A pins that provide voltages CIN L – IN– BTL OUT– Low-p ass Low-p ass – 180° out of phase. The Outputs Setup for Diff Output: LC Filter* RC Filter Setup for BTL Output: load is connected between Outputs Balanced, InputsBalanced/ac Coupled, Ω Z = 100 kΩ , these pins. This type of Z=OUT 40, IN Set Load Reference = R . Set Load Reference = RL, output is normally used L Set Internal Filter(s). C when the APA and speaker + IN SE Inputs CIN IN and OUT + Channel B R Channel B are in the same enclosure. CIN SE L It has the benefits of – GND Outputs GND – Setup for SE Output: Setup for SE Output: quadrupling the output VS GND Outputs Unbalanced, InputsBalanced/ac Coupled, power to the load and Z= 40Ω, Ω OUT ZIN = 100 k , eliminating a dc output Set Load Reference = RL. Set Load Reference = RL, coupling capacitor. V+ GND Set Internal Filter(s). The SE inputs normally Regulated have one pin per channel Power Supply and are used for line amplifiers (speakers) *Not required when measuring filter-free class-D or class-AB APAs when a differential signal is unavailable from the source. SE inputs are commonly used with amplifiers that have SE outputs, such as headphone ampli- The signal source should have balanced outputs for fiers. SE outputs have one pin that is tied directly to one APAs with differential inputs. The balanced output really end of the load; the other end of the load is tied to ground. means the outputs have the same impedance. The analyzer This output configuration is normally used when the APA should have balanced inputs, particularly when APAs with and speaker are in separate enclosures. A dc coupling BTL outputs are measured. The cable shielding (when capacitor is often required with this type of output. used) should be terminated at the end where the imped- There are class-AB devices for each configuration. Good ance is high for Diff/BTL-type connections.2 This will examples include the TPA711 with SE inputs and either occur at the APA and analyzer inputs. The single point con- BTL or SE outputs, and the TPA0212 that can be config- nection prevents return currents from flowing through the ured into any of the four combinations. Most class-AB shield between connections that are at slightly different headphone amplifiers have SE outputs and normally have voltage potentials, thus minimizing ground loops. SE inputs. There are a few exceptions, like the TPA6112A2, When APAs with SE inputs are measured, the signal which are Diff/SE. The class-D devices operate with differ- source should have unbalanced outputs to prevent any ential inputs and BTL outputs. voltage drop from occurring due to ground-current flow. An audio measurement circuit for class-AB or class-D is The analyzer should have balanced inputs to reduce the shown in Figure 3. The RC filters are included for simplicity common-mode noise. The signal and ground wires of the but are not required for class-AB measurements. The cir- twisted pair should be connected at both ends to allow cuit shows an audio precision (AP) measurement system return currents to flow. When the generator is grounded that includes an analog signal generator with sweep capa- and cable shielding is used, the latter should be connected bility and an analog analyzer optimized for audio signals. at both ends to provide an additional return current To emphasize the different input/output configurations, path, reducing the noise in the twisted-pair wire.2 See channel A (Ch A) is set up for differential inputs and BTL Reference 2 for more details concerning grounding and outputs, and channel B (Ch B) is set up for SE inputs and cable connections. outputs. The remaining combinations (Diff/SE or SE/BTL) can be set up by referencing the appropriate input or out- RC low-pass measurement filter put configuration in Figure 3. The BTL and SE settings for The RC filter is designed to reduce the square-wave out- the AP generator and analyzer are shown below the put of the filter-free class-D output so as not to interfere appropriate channel. The input ac coupling capacitors, with the measurements. It is required by the analyzer CIN, are included on the device evaluation modules. inputs, which do not have the common-mode capability to Twisted-pair wire should be used on all connections. The handle the fast-switching class-D output waveform. The twisted pair reduces the loop area between the conductors, high frequency of the square wave is not a factor in the protecting against magnetic, or inductive, coupling. Shield- audio measurements because it is well above the audible ing protects against electric, or capacitive, coupling and is frequency range, and the speaker cannot respond at such used when the system environment is noisy or to reduce a fast rate. radiation from the APA circuit to other nearby circuits. Continued on next page

Analog Applications Journal July 2001 www.ti.com/sc/analogapps Analog and Mixed-Signal Products Signal Conditioning: Audio Amplifiers Texas Instruments Incorporated

Continued from previous page spectrum being measured. The value chosen for RFilter must then be large enough to avoid attenuating the output When the RC filter is used, it is connected as shown in voltage, yet small enough to minimize the attenuation of Figure 4. CFilter is grounded to the APA to form a path for the analyzer-input voltage through the voltage divider return current and to minimize ground loops. The input formed by RIN and RFilter. These are conflicting criteria, resistance and capacitance of the analyzer are substituted and a balance must be sought. Once RFilter is selected, for RIN and CIN. The equivalent circuit is shown in Figure 5. CFilter is calculated: The equation for this circuit is = 1   CFilter , (3) RIN 2π ××fR   Filter V  RR+  OUT = IN Filter , (1) where f is the cutoff frequency of the filter. The values V  ω  IN + used for the measurements in the following discussion were 1 j  Ω  ωO  RFilter = 100 and CFilter = 47 nF. Some current will flow through the RC filter to ground, but it is generally not a where ωO = REQCEQ, REQ = RFilter || RIN, and CEQ = consideration for these measurements. (CFilter + CIN). The filter frequency should be set above the bandwidth of the highest measurement frequency, yet THD+N versus power and frequency low enough to filter out most of the switching frequency The THD+N measurement combines the effects of white so the AP analyzer can measure it. noise, distortion, and other undesirable signals into one 20×log()f +3 dB measurement and relates it (usually as a percentage) max (2) directly to the fundamental frequency. Ideally only the funda- = 20 f 10 mental frequency is present at the output, which in practice Equation 2 provides the cutoff frequency of the RC filter is never the case. Nonlinearity of the amplifier, internal 3 dB above the highest-frequency component, fmax, of the noise sources, external noise sources, and layout and grounding issues are some of the contributors that distort the original input signal. The distortion Figure 4. RC filter for filter-free class-D shows up at the output as harmonics of the fundamental frequency. The bandwidth is usually limited RC Filter with filters in the analyzer to reduce the noise; yet this also reduces the Class-D APA AP Analyzer Input relevant harmonics of the higher- Diff RFilter frequency signals, and a tradeoff is IN+ Inputs OUT+ + + + R CFilter made. A filter cutoff frequency of and L VL RFilter VOUT Channel A BTL – – – 80 kHz is used for class-AB APAs to IN– Outputs OUT– CFilter Setup for BTL Output: allow measurement of the third har- VS GND InputsBalanced/ac Coupled, monic. The filter cutoff frequency is Ω ZIN = 100 k , set to 22 kHz for class-D APAs to Set Load Reference = RL , remove the switching waveform from V+ GND Set Internal Filter(s). the measurements. The AP measurement circuit is Regulated Power Supply shown in Figure 6 for a mono- channel, BTL output APA. The measurements for the TPA2001D1 and TPA731 output power sweep are shown in Figure 7, and an output Figure 5. Equivalent filter circuit frequency sweep is shown in Figure 8. The maximum input voltage for pro- ducing maximum output power can be found by increasing the input until AP Analyzer Input the output clips, then reducing it

until it is just below clipping. Another

Filter + method is to calculate the maximum

+ + peak-to-peak input voltage: CIN RIN CFilter

RL ××

V=LIN V VOUT 22 PROL(max)

–

– V = , (4) RFilter – IN() PP AV C Filter CIN RIN where PO(max) is the maximum- To APA GND rated RMS output power, RL is the load resistance, and AV is the voltage gain of the APA.

Analog and Mixed-Signal Products www.ti.com/sc/analogapps July 2001 Analog Applications Journal Texas Instruments Incorporated Signal Conditioning: Audio Amplifiers

Figure 6. THD+N measurement circuit for a mono-channel BTL APA

Audio Power AP Generator Out RC Filter for Class-D Amplifier Measurements AP Analyzer In CIN Diff RFilter + IN+ Inputs OUT+ + C Channel A C and CL R Filter Channel A – IN BTL Filter – IN– Outputs OUT– CFilter THD+N vs. POUT : V Inputs Balanced/ac Coupled, S GND Ω, Outputs Balanced, ZIN = 100 k Ω, Z=OUT 40 Set Load Reference = RL, Set Load Reference = RL, Internal Filter = 30 kHz, Sweep 10 mW to PO(max) V+ GND Reading Meter = THD+N Ratio. with Fixed Frequency. Regulated THD+N vs. Frequency: Outputs Balanced, Power Supply Ω, Z=OUT 40 Set Load Reference = RL , Sweep 20 Hz to 20 kHz Fixed Amplitude.

When the THD+N versus output power is measured, the analog generator Figure 7. TPA2001D1 and TPA731 output power sweep sweeps the input voltage from low to high at a fixed frequency. The output 2 signal harmonics are then measured at 1 TPA731 EVM, SLOP 342 specified voltage steps, and the output Gain = –2 V/V, 10-mV to 450-mVrms input, 0.5 power is calculated for a given load- IQ = 1 mA (w/filter) impedance value that is provided to the audio analyzer. This value is then 0.2

THD (%) divided by the amplitude of the funda- 0.1 mental frequency and graphed as a 0.05 TPA2001D1 EVM, SLOP 328 percentage of the fundamental. Figure 7 Gain = 6 dB, 0.1-V to 1.05-Vrms input, IQ = 20 mA (w/filter) shows a typical THD+N-versus-power Gain = 24 dB, 10-mV to 150-mVrms input, IQ = 20 mA (w/filter) 0.02 curve. The higher distortion at low 0.01 0.02 0.05 0.1 0.2 0.5 1 Power (W ) values of POUT is due to the decrease rms in signal-to-noise ratio as the harmonics AP Setup: 30-kHz BW, gen out bal-float, analyzer in XLR-bal, PS chassis ground circuit: 8-Ω decrease in amplitude below the noise load, diff input/output,1-kHz input, 30-kHz filter (R = 100Ω , C = 47 nF), V = 3.3 V, twisted pair 3 S floor. The sudden increase at the wires, mono-BTL upper level of POUT is due to clipping of the output signal. When the THD+N versus frequency is measured, the analog generator sweeps the frequency from 20 kHz to Figure 8. TPA2001D1 and TPA731 output frequency sweep 20 Hz at a fixed voltage. The harmonics of the output are measured at specified 2 frequency steps. Each step is divided TPA2001D1 EVM, SLOP 328 by the amplitude of the fundamental 1 Gain = 24 dB, 42-mVrms input, 55-mW output, IQ = 20 mA (w/filter) Gain = 6 dB, 335-mVrms input, 55-mW output, IQ = 20 mA (w/filter) frequency and graphed as a percentage 0.5 of the fundamental. This graph provides a fundamental check when compared 0.2 to the THD+N versus power—they THD (%) 0.1 should match at one specific frequency TPA731 EVM, SLOP 342 0.05 and power. The increase at low fre- Gain = –2 V/V, 355-mVrms input, quencies is primarily due to the 1/f 250-mW output, Iq = 1 mA (w/filter) 0.02 noise. If the THD+N measured at low 20 50 100 200 500 1 k 2 k 5 k 10 k 20 k frequencies is considerably higher than Frequency (Hz) the data sheet value, the APA input ac coupling capacitance may need to be AP Setup: 30kHz BW, gen out bal-float, analyzer In XLR-bal, circuit: 8-Ω load, diff input/output, Ω reduced to limit the noise bandwidth. 30-kHz filter (R = 100 , C = 47 nF), VS = 3.3 V, twisted-pair wires all connections, PS chassis ground connected, AP Filter connected to PS ground, mono-BTL Continued on next page

Analog Applications Journal July 2001 www.ti.com/sc/analogapps Analog and Mixed-Signal Products Signal Conditioning: Audio Amplifiers Texas Instruments Incorporated

Figure 9. Gain and phase measurement circuit for a mono-channel BTL APA

RC Filter for Class-D AP Generator Out Audio Power Amplifier Measurements AP Analyzer In CIN RFilter + IN+ OUT+ Right + Channel A R C CIN Channel L RFilter Filter Channel A – IN– OUT– – CFilter CIN RFilter + IN+ OUT+ Left + Channel B R C Channel B CIN Channel L RFilter Filter – IN– OUT– – CFilter Setup for Diff Output: Setup for BTL Output: VS GND Outputs Balanced, Inputs: Z= 40,Ω Ch A Balanced/ac Coupled, OUT Ch B GenMon, Set Load Reference = RL, Z = 100 kΩ, Ch A and Ch B Outputs On, V+ GND IN Ch B Track Ch A, Set dBrA Ref to Generator Sweep 20 kHz to 20 Hz, Regulated Ch A and press F4, Data 1 = Analyzer Crosstalk. Power Supply Set Load Reference = RL, Set Internal Filter = 80 kHz for Class-D, 30 kHz for Class-AB.

Continued from previous page are set correctly for SE or Diff APA inputs. These measurements will vary with CBYPASS for devices that have a The high-frequency increase is due to device nonlineari- BYPASS pin, with THD increasing as CBYPASS decreases. ties. The rolloff at high frequencies is due to the attenuation of harmonics, as the band-limiting filter in the analyzer Gain and phase was set to 30 kHz. This is normally a characteristic of the The AP measurement circuit is shown in Figure 9 for a class-D measurements due to the filters used to reduce the mono-channel BTL APA. Measurements for the TPA731 switching frequency. The graph would continue in a and TPA2001D1 are shown in Figures 10 and 11. The gain straight line if there were no filter present, as is typically and phase can also be measured at a single point with an the case with class-AB amplifiers. Output filters are not oscilloscope by using Equation 5 for the gain and Equation typically used with class-AB measurements. 6 for the phase, where ∆t is the interval time between volt- The load resistance must be set properly to get the right ages and f is the frequency of the input signal: output power, and the shielded cable must be grounded   VOUT properly or the THD will increase dramatically at high AdBV()=×20 log  (5) power. If there is unusually high distortion at lower power,  VIN  check the ground connections and be sure to use twisted- pair, shielded wires. Also check that the generator outputs θ = ∆t × f × 360° (6) Both channels must be turned Figure 10. Gain-phase measurement for a TPA2001D1 on at the generator panel in the software, and Ch B must be set to track Ch A. The analyzer Ch B is +25 -120 set to GenMon, which means it +24 -130 takes its input directly from the +23 -140 generator output internal to the -150 AP and uses it as the input-phase +22 -160 Phase (Degrees) reference. The reference dBrA +21 -170 value should be set equal to the +20 -180 channel being swept, which in Gain (dB) Gain +19 this case is Ch A. It may be nec- -190 +18 essary to subtract 180° from the TPA2001D1 EVM, SLOP328 -200 measurement to get the actual +17 Magnitude w/gain = 24 dB, 128-mVrms input, 480-mW output -210 Phase (subtract 180° to get actual value) value. The APA input high-pass +16 -220 filters and the RC filter introduce +15 -230 some attenuation and phase shift 20 50 100 200 500 1 k 2 k 5 k 10 k 20 k Frequency (Hz) at the measurement endpoints, as seen in the figures. AP Setup: 30-kHz BW, gen out bal-float, analyzer in XLR-Bal, circuit: 8-Ω load, Ω diff input/output, 30-kHz filter (R = 100 , C = 47 nF), VS = 3.3 V, Mono-BTL

Analog and Mixed-Signal Products www.ti.com/sc/analogapps July 2001 Analog Applications Journal Texas Instruments Incorporated Signal Conditioning: Audio Amplifiers

Crosstalk Figure 11. Gain-phase measurement for a TPA731 Crosstalk is the measurement of the signal coupling between chan- +1 nels of a stereo device. The input +40 to one channel is swept at a con- +0.75 stant amplitude, and the outputs of +0.5 +30 both channels are then measured +0.25 +20

+0 Phase (Degrees) at specific frequencies using band- +10 pass filters to limit the noise at that -0.25 particular frequency. The value of –0.5 +0 the channel being measured is (dB) Gain –0.75 –10 compared to the channel with the –1 –20 –1.25 TPA731 EVM, SLOP 342 signal applied, and the log ratio is Magnitude, 595-mV input, 700-mW output rms –30 graphed versus frequency. –1.5 Phase (subtract 180° to get actual value) The crosstalk measurement –1.75 –40 –2 circuit is shown in Figure 12 for 20 50 100 200 500 1 k 2 k 5 k 10 k 20 k an APA with Diff inputs and BTL Frequency (Hz) outputs. This particular setup is referred to as right-to-left (R-L) AP Setup: 30-kHz BW, gen out bal-float, analyzer in XLR-Bal, circuit: 8-Ω load, Ω channel. A graph of the crosstalk diff input/output, 30-kHz filter (R = 100 , C = 47 nF), VS = 3.3 V, Mono-BTL is shown in Figure 13 for the TPA2001D2 and the TPA0212.

Figure 12. Crosstalk measurement system for an APA with Diff inputs and BTL outputs

RC Filter for Class-D AP Generator Out Audio Power Amplifier Measurements AP Analyzer In CIN RFilter + IN+ OUT+ Right + Channel A R C CIN Channel L RFilter Filter Channel A – IN– OUT– – CFilter CIN RFilter + IN+ OUT+ Left + Channel B R C Channel B CIN Channel L RFilter Filter – IN– OUT– – CFilter Setup for Diff Output: Setup for BTL Output: VS GND Outputs Balanced, Inputs Balanced/ac Coupled, Ω Z = 100 kΩ , Z=OUT 40, IN Set Load Reference = RL, Set Load Reference = RL, Ch A and Ch B Outputs On, V+ GND Set Internal Filter = 30 kHz. Ch B Track Ch A, Sweep 20 kHz to 20 Hz, Regulated Data 1 = Analyzer Crosstalk. Power Supply

It can be measured point-to-point by using Equation 7, which is correct for Figure 13. TPA2001D2 and TPA0212 crosstalk the graph in Figure 13:  VOUT  +0 =× Ch B TPA2001D2 EVM, SLOP 315 Crosstalk 20 log  (7) -20 Ω  VOUT  Gain = 6 dB, 1.05-Vrms input, 1-W output into 8- load, R to L Ch A TPA0212 EVM, SLOP 364 -40 Ω Both channels of the AP must be Gain = 6 dB, 1.48-Vrms input, 1-W output into 8- load, R to L turned on at the generator panel in the -60

Xtalk (dB) software, and Ch B set to track Ch A. -80

The input is swept over the audio fre- -100 quency range at a constant amplitude— -120 normally the maximum output power. 20 50 100 200 500 1 k 2 k 5 k 10 k 20 k The internal filter of the analyzer can Frequency (Hz) be set to 30 kHz to limit noise but is otherwise not required. The output AP Setup: 30-kHz BW, gen out bal-float, analyzer in XLR-Bal, circuit: 8-Ω load, Ω cables of the two channels should be diff input/output, 30-kHz filter (R = 100 , C = 47 nF), VS = 5 V Continued on next page 45

Analog Applications Journal July 2001 www.ti.com/sc/analogapps Analog and Mixed-Signal Products Signal Conditioning: Audio Amplifiers Texas Instruments Incorporated

Continued from previous page

twisted together to min- Figure 14. Measurement system for an APA with Diff inputs and BTL outputs imize the loop between them, and the inputs of the channel being measured should be ac RC Filter for Class-D grounded. The setup is Audio Power Measurements Amplifier reversed to graph the AP Analyzer In CIN RFilter L-R channel crosstalk. IN+ OUT+ + R CFilter Normally the two chan- CIN L RFilter Channel A IN– OUT– – nels will be different CFilter due to impedance vari- Setup for BTL Output: Inputs Balanced/ac Coupled, ations between the AP Generator Out Ω, ZIN = 100 k channels. VS GND Set Load Reference = RL, + Internal Filter = 22 Hz to 22 kHz, Channel A Output noise Reading Meter = Amplitude, – Data 1 = Analyzer Amplitude, V+ GND voltage Outputs Off (No Connect), Source = Generator Freq. The output noise volt- Sweep 20 kHz to 20 Hz Regulated age is an integrated Power Supply value that is measured over the audio frequency spectrum. The measurement circuit is shown in Figure 15. TPA2001D1 and TPA731 Diff/BTL output noise voltage Figure 14 for an APA with Diff inputs and BTL outputs. A graph depicting the Diff/BTL output noise voltage of the 500 TPA2001D1 and the TPA731 is shown 200 in Figure 15. All of the inputs of the 100 APA are ac coupled to ground. The AP 50 generator outputs are not used in this

OUT 20

measurement and should be turned off. V(µV) The analyzer is active, with the reading 10 TPA2001D1 EVM, SLOP 328 Ω meter function set to amplitude. The 5 Gain = –2 V/V, no input, RC filter (R = 1 k , C = 4700 pF), IQ = 5 mA TPA731 EVM, SLOP 342 AP bandwidth should be limited to the 2 Gain = –2 V/V, no input, no filter, IQ = 1 mA audio frequency spectrum. The data is 1 20 50 100 200 500 1 k 2 k 5 k 10 k 20 k set to measure the analyzer amplitude; Frequency (Hz) and the sweep source is the generator frequency, which is swept across the ΩΩ AP Setup: 22-Hz to 22-kHz BW, gen out bal-float, analyzer in XLR-Bal, 100-k ZIN , circuit: 8- frequency spectrum from 20 kHz to load, diff input ac-grounded, BTL ouput, VS = 3.3 V 20 Hz. The output is in Vrms and must be divided by the gain to get the input- referred noise voltage. Conclusion litnumber and replace “litnumber” with the TI Lit. # for the materials listed below. A method for measuring audio power amplifier character- istics with an audio analyer has been presented. All mea- Document Title TI Lit. # surements were taken with off-the-shelf TI APA evaluation 1. Michael D. Score, “Reducing and Eliminating modules. These values correspond closely to the data sheet the Class-D Output Filter,” Application Report . .sloa023 values, which were measured by using characterization 2. Henry W. Ott, Noise Reduction Techniques in boards that are optimized for measuring the device. This Electronic Systems (Wiley Interscience, 1976). — shows that reasonable measurements can be made quickly 3. Bob Metzler, Audio Measurement Handbook and efficiently with audio circuits by using basic test (Audio Precision, 1993). — equipment and pre-fabricated evaluation modules that save time and money spent on layout during initial evaluation of Related Web sites a device. www.ti.com/sc/opamps I want to thank the following people for their assistance: www.ti.com/sc/audio Mike Score, APA Systems Engineer and Member, Group www.ti.com/sc/apa Technical Staff; and Dave Skinner, APA Systems Engineer. Get more product information at: References www.ti.com/sc/device/device Replace device with tpa0212, tpa711, tpa731, tpa2001d1, For more information related to this article, you can down- tpa2001d2, or tpa6112a2 load an Acrobat Reader file at www-s.ti.com/sc/techlit/ 46

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