OPA1622 Datasheet, PDF (22/36 Pages) Texas Instruments – OPA1622 SoundPlus™ High-Fidelity, Bipolar-Input, Audio Operational Amplifier

OPA1622
SBOS727A – NOVEMBER 2015 – REVISED NOVEMBER 2015
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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 may 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 may 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 (V_AC) deliver the output signal to the complementary outputs
through their associated output impedances (R_OUT). Both output signals have a dc component as well,
represented by dc voltage source V_DC. 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.
C1 470 pF
VDC
ROUT
VAC
VAC
ROUT
Audio DAC
R1
499
R3
499
R4
768
R2
768
-
+ _OPA1622
Headphone
Output
C2
470 pF
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 may 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
Maximum differential output voltage
Output impedance (R_OUT)
Output dc offset
VALUE
2 V_RMS
200 Ω
1.65 V
22
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