OPA1622 Datasheet, PDF (23/36 Pages) Texas Instruments – OPA1622 SoundPlus™ High-Fidelity, Bipolar-Input, Audio Operational Amplifier

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OPA1622
SBOS727A – NOVEMBER 2015 – REVISED NOVEMBER 2015
The gain of the difference amplifier in Figure 52 is determined by the resistor values, and includes the output
impedance of the DAC. For R₂ = R₄ and R₁ = R₃, the output voltage of the headphone amplifier circuit is shown
in Equation 5:
V_OUT
V_{DAC R1}
R₂
R _OUT
(5)
The output voltage required for headphones depends on the headphone impedance, as well as the headphone
efficiency (η), a measure of the sound pressure level (SPL, measured in dB) for a certain input power level
(typically given at 1 mW). The headphone SPL at other power levels is calculated using Equation 6:
SPL(dB)
K
§
10 log _¨©
1
P_IN
mW
·
¸¹
where
• η = efficiency
• P_IN = input power to the headphones
(6)
Figure 53 shows the input power required to produce certain SPLs for different headphone efficiencies. Typically,
over-the-ear style headphones have lower efficiencies than in-ear types with 95 dB/mW being a common value.
150
140
130
120
110
100
90-dB/mW
90
95-dB/mW
100-dB/mW
80
105-dB/mW
70
110-dB/mW
60
0.01
0.1
1
10
Input Power (mW)
115-dB/mW
100
1000
C001
Figure 53. Sound Pressure Level vs Input Power for Headphones of Various Efficiencies
In-ear headphones may have efficiencies of 115 dB/mW or greater, and therefore have much lower power
requirements. The output power goal for this design is 150 mW; sufficient power to produce extremely loud
sound pressure levels in a wide range of headphones. A 32-Ω headphone impedance is used for this
requirement because 32 Ω is a very common value in headphones for portable applications. Equation 7 shows
the voltage required for 32-Ω headphones:
V_O P u R 150 mW u 32
9_RMS
(7)
A tradeoff exists when selecting resistor values for this design. First, high resistor values contribute additional
noise to the circuit, degrading the audio performance. However, extremely low resistor values draw excessive
current from the DAC, increasing distortion. A value of 499 Ω is used for resistors R₁ and R₃ as a reasonable
compromise between these two considerations. Resistor R₂ and R₄ can then be calculated as shown in
Equation 8:
V_OUT
V_{DAC R1}
R₂
R_OUT
o 1.095
R₂
499
o R₂ 765.4 o
(8)
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