Texas Instruments TPA3008D2 사용자 설명서
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C
IN
Audio Power
Amplifier
Generator
Low−Pass
RC Filter
C
IN
R
GEN
R
GEN
R
IN
R
IN
V
GEN
R
OUT
R
OUT
Analyzer
R
ANA
R
ANA
C
ANA
Low−Pass
RC Filter
R
L
C
ANA
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 unbalanced
output can be used, but it may create a ground loop that affects the measurement accuracy. The analyzer must
also have balanced inputs for the system to be fully balanced, thereby cancelling out any common-mode noise in
the circuit and providing the most accurate measurement.
output can be used, but it may create a ground loop that affects the measurement accuracy. The analyzer must
also have balanced inputs for the system to be fully balanced, thereby cancelling out any common-mode noise in
the 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 large
currents (see Table 2).
currents (see Table 2).
Table 2 shows the recommended wire size for the power supply and load cables of the APA system. The real
concern is the dc or ac power loss that occurs as the current flows through the cable. These recommendations
are based on 12-inch long wire with a 20-kHz sine-wave signal at 25
concern is the dc or ac power loss that occurs as the current flows through the cable. These recommendations
are 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 LOSS
P
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-width
modulated class-D output waveform. This filter has little effect on the measurement accuracy because the cutoff
frequency is set above the audio band. The high frequency of the square wave has negligible impact on
measurement accuracy because it is well above the audible frequency range, and the speaker cone cannot
respond at such a fast rate. The RC filter is not required when an LC low-pass filter is used, such as with the
class-D APAs that employ the traditional modulation scheme (TPA032D0x, TPA005Dxx).
modulated class-D output waveform. This filter has little effect on the measurement accuracy because the cutoff
frequency is set above the audio band. The high frequency of the square wave has negligible impact on
measurement accuracy because it is well above the audible frequency range, and the speaker cone cannot
respond at such a fast rate. The RC filter is not required when an LC low-pass filter is used, such as with the
class-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
L
is the load impedance that the APA is driving for the test. The analyzer input impedance specifications should be
available and substituted for R
available and substituted for R
ANA
and C
ANA
. The filter components, R
FILT
and C
FILT
, can then be derived for the
system. The filter should be grounded to the APA near the output ground pins or at the power supply ground pin
to minimize ground loops.
to minimize ground loops.
22