C

IN

AudioPower

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-PairWire

EvaluationModule

Twisted-PairWire

Lfilt

Lfilt

Cfilt

Cfilt

TPA3121D2

www.ti.com

SLOS537B – MAY 2008 – REVISED JANUARY 2014

DIFFERENTIAL INPUT AND BTL OUTPUT (TPA3121D2 Mono Configuration)

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°

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 36. 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 36. 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 5).

Table 5 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 (30.5-cm)-long wire with a 20-kHz sine-wave signal at 25°C.

Table 5. Recommended Minimum Wire Size for Power Cables

DC POWER LOSS

AC POWER LOSS

POUT (W)

RL(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

Copyright © 2008–2014, Texas Instruments Incorporated

Submit Documentation Feedback

23

Product Folder Links: TPA3121D2