OPA1688 Datasheet, PDF (25/41 Pages) Texas Instruments – OPA168x SoundPlus 36-V, Single-Supply, 10-MHz, Rail-to-Rail Output Operational Amplifiers

www.ti.com
OPA1688, OPA1689
SBOS724 – SEPTEMBER 2015
Typical Application (continued)
10.2.2 Detailed Design Procedure
The OPA168x family offers an excellent combination of specifications for headphone amplifier circuits (such as
low noise, low distortion, capacitive load stability, and relatively high output current). Furthermore, the low-power
supply current and small package options make the OPA1688 an exceptionally good choice for headphone
amplifiers in portable devices. A common headphone amplifier circuit for audio digital-to-analog converters
(DACs) with differential voltage outputs is illustrated in Figure 49. This circuit converts the differential voltage
output of the DAC to a single-ended, ground-referenced signal and provides the additional current necessary for
low-impedance headphones. For R₂ = R₄ and R₁ = R₃, the output voltage of the circuit is given by Equation 1:
V_OUT
= 2 u V_{AC R1}
R₂
+ R _OUT
where
• R_OUT is the output impedance of the DAC and
• 2 × V_AC is the unloaded differential output voltage
(1)
The output voltage required for headphones depends on the headphone impedance as well as the headphone
efficiency. Both values can be provided by the headphone manufacturer, with headphone efficiency usually given
as a sound pressure level (SPL) produced with 1 mW of input power and denoted by the Greek letter η. The SPL
at other input power levels can be calculated from the efficiency specification using Equation 2:
SPL (dB)
K
10
log^§¨©
1
P_IN
mW
·
¸¹
(2)
Note that at extremely high power levels, the accuracy of this calculation decreases as a result of secondary
effects in the headphone drivers. Figure 50 allows the SPL produced by a pair of headphones of a known
sensitivity to be estimated for a given input power.
10000
1000
100
90-dB SPL
100-dB SPL
110-dB SPL
120-dB SPL
95-dB SPL
105-dB SPL
115-dB SPL
10
1
0.1
0.01
90
95
100
105
110
115
Headphone Efficiency (dB/mW)
C001
Figure 50. SPLs Produced for Various Headphone Efficiencies and Input Power Levels
Copyright © 2015, Texas Instruments Incorporated
Product Folder Links: OPA1688 OPA1689
Submit Documentation Feedback
25