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CIN
Audio Power
Amplifier
Generator
Low−Pass
RC Filter
CIN
RGEN
RGEN RIN
RIN
VGEN ROUT
ROUT
Analyzer
RANA
RANA CANA
Low−Pass
RC Filter
RL
CANA
Twisted-Pair Wire
Evaluation Module
Twisted-Pair Wire

CLASS-D RC LOW-PASS FILTER

TPA3008D2
SLOS435A – MAY 2004 – REVISED JULY 2004
Figure 22. Differential Input, BTL Output Measurement Circuit
The generator should have balanced outputs, and the signal should be balanced for best results. An unbalancedoutput can be used, but it may create a ground loop that affects the measurement accuracy. The analyzer mustalso have balanced inputs for the system to be fully balanced, thereby cancelling out any common-mode noise inthe circuit and providing the most accurate measurement.
The following general rules should be followed when connecting to APAs with differential inputs and BTL outputs:
•Use a balanced source to supply the input signal.
•Use an analyzer with balanced inputs.
•Use twisted-pair wire for all connections.
•Use shielding when the system environment is noisy.
•Ensure that the cables from the power supply to the APA, and from the APA to the load, can handle the largecurrents (see Table 2 ).
Table 2 shows the recommended wire size for the power supply and load cables of the APA system. The realconcern is the dc or ac power loss that occurs as the current flows through the cable. These recommendationsare based on 12-inch long wire with a 20-kHz sine-wave signal at 25°C.
Table 2. Recommended Minimum Wire Size for Power Cables
DC POWER LOSS AC POWER LOSSP
OUT
(W) R
L
(Ω) AWG Size
(MW) (MW)
10 4 18 22 16 40 18 42
2 4 18 22 3.2 8 3.7 8.5
1 8 22 28 2 8 2.1 8.1
< 0.75 8 22 28 1.5 6.1 1.6 6.2
An RC filter is used to reduce the square-wave output when the analyzer inputs cannot process the pulse-widthmodulated class-D output waveform. This filter has little effect on the measurement accuracy because the cutofffrequency is set above the audio band. The high frequency of the square wave has negligible impact onmeasurement accuracy because it is well above the audible frequency range, and the speaker cone cannotrespond at such a fast rate. The RC filter is not required when an LC low-pass filter is used, such as with theclass-D APAs that employ the traditional modulation scheme (TPA032D0x, TPA005Dxx).
The component values of the RC filter are selected using the equivalent output circuit as shown in Figure 23 . R
Lis the load impedance that the APA is driving for the test. The analyzer input impedance specifications should beavailable and substituted for R
ANA
and C
ANA
. The filter components, R
FILT
and C
FILT
, can then be derived for thesystem. The filter should be grounded to the APA near the output ground pins or at the power supply ground pinto minimize ground loops.
22