+

-

VDC

Audio DAC

Headphone

Output

R1

R2

R3

R4

ROUT

ROUT

VAC

VAC

C1

C2

OPA1622

499

499

768

470 pF

470 pF

768

Copyright © 2016, Texas Instruments Incorporated

22

OPA1622

SBOS727B – NOVEMBER 2015 – REVISED MAY 2016

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Copyright © 2015–2016, Texas Instruments Incorporated

8.2 Typical Application

The low distortion and high output-current capabilities of the OPA1622 make this device an excellent choice for

headphone-amplifier applications in portable or studio applications. These applications typically employ an audio

digital-to-analog converter (DAC) and a separate headphone amplifier circuit connected to the DAC output. High-

performance audio DACs can have an output signal that is either a varying current or voltage. Voltage output

configurations require less external circuitry, and therefore have advantages in cost, power consumption, and

solution size. However, these configurations can offer slightly lower performance than current output

configurations. Differential outputs are standard on both types of DACs. Differential outputs double the output

signal levels that can be delivered on a single, low-voltage supply, and also allow for even-harmonics common to

both outputs to be cancelled by external circuitry. A simplified representation of a voltage-output audio DAC is

shown in Figure 52. Two ac voltage sources (VAC) deliver the output signal to the complementary outputs

through their associated output impedances (ROUT). Both output signals have a dc component as well,

represented by dc voltage source VDC. The headphone amplifier circuit connected to the output of an audio DAC

must convert the differential output into a single-ended signal and be capable of producing signals of sufficient

amplitude at the headphones to achieve reasonable listening levels.

Figure 52. OPA1622 Used as a Headphone Amplifier for a Voltage-Output Audio DAC

8.2.1 Design Requirements

•

±5-V power supplies

•

150-mW output power (32-

Ω load)

•

< –115-dB THD+N at maximum output (32-

Ω load)

•

< 0.01-dB magnitude deviation (20 Hz to 20 kHz)

8.2.2 Detailed Design Procedure

Figure 52 shows a schematic of a headphone amplifier circuit for voltage output DACs. An op amp is configured

as a difference amplifier that converts the differential output voltage to single-ended. The values of the resistors

in the difference amplifier circuit are determined by the specifications of the DAC, such as output voltage and

output impedance, as well as the maximum output voltage desired at the headphone output. The op amp chosen

must be capable of delivering the necessary current to the headphones and remain stable into typical headphone

loads that can have capacitances as high as 400 pF. The following design process uses a hypothetical DAC with

common values of output voltage and impedance for the design process. The specifications of the DAC are

shown in Table 1:

Table 1. Audio DAC Specifications Used for the Design Process

PARAMETER

VALUE

Maximum differential output voltage

2 VRMS

Output impedance (ROUT)

200

Ω

Output dc offset

1.65 V