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DIFFERENTIAL INPUT AND BTL OUTPUT (TPA3122D2 Mono Configuration)
C
IN
Audio Power
Amplifier
Generator
C
IN
R
GEN
R
GEN
R
IN
R
IN
V
GEN
Analyzer
R
ANA
R
ANA
C
ANA
R
L
C
ANA
Twisted-Pair Wire
Evaluation Module
Twisted-Pair Wire
L
filt
L
filt
C
filt
C
filt
TPA3122D2
SLOS527A – DECEMBER 2007 – REVISED DECEMBER 2007
Many of the class-D APAs and many class-AB APAs have differential inputs and bridge-tied load (BTL) outputs.
Differential inputs have two input pins per channel and amplify the difference in voltage between the pins.
Differential inputs reduce the common-mode noise and distortion of the input circuit. BTL is a term commonly
used in audio to describe differential outputs. BTL outputs have two output pins providing voltages that are 180
degrees out of phase. The load is connected between these pins. This has the added benefits of quadrupling the
output power to the load and eliminating a dc blocking capacitor.
A block diagram of the measurement circuit is shown in
Figure 32
. The differential input is a balanced input,
meaning the positive (+) and negative (-) pins have the same impedance to ground. Similarly, the SE output
equates to a balanced output.
Figure 32. 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.
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 4
).
Table 4
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
°
C.
Table 4. 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
18
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Copyright © 2007, Texas Instruments Incorporated
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