TPA3003D2

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:

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.