SLOS406A − FEBRUARY 2003 − REVISED MARCH 2003
28
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APPLICATION INFORMATION
differential input and BTL output
All 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 30. The differential input is a balanced input,
meaning the positive (+) and negative (−) pins will have the same impedance to ground. Similarly, the BTL output
equates to a balanced output.
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
Figure 30. 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 will affect 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:
D
Use a balanced source to supply the input signal.
D
Use an analyzer with balanced inputs.
D
Use twisted-pair wire for all connections.
D
Use shielding when the system environment is noisy.
D
Ensure the cables from the power supply to the APA, and from the APA to the load, can handle the large
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
°
C.
Summary of Contents for TPA3003D2
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