OPA1688_15 Datasheet, PDF (26/40 Pages) Texas Instruments – SoundPlus 36-V, Single-Supply, 10-MHz, Rail-to-Rail Output Operational Amplifiers

OPA1688, OPA1689
SBOS724 – SEPTEMBER 2015
www.ti.com
Typical Application (continued)
For example, a pair of headphones with a 95-dB/mW sensitivity given a 3-mW input signal produces a 100-dB
SPL. If these headphones have a nominal impedance of 32 Ω, then the voltage and current from the headphone
amplifier is as described in Equation 3 and Equation 4, respectively:
V P_IN uR_HP 3 mW u 32 : 310 mV_RMS
(3)
I
P_IN
R_HP
3 mW
32 :
9.68 mA_RMS
(4)
Headphones can present a capacitive load at high frequencies that can destabilize the headphone amplifier
circuit. Many headphone amplifiers use a resistor in series with the output to maintain stability; however this
solution also compromises audio quality. The OPA168x family is able to maintain stability into large capacitive
loads; therefore, a series output resistor is not necessary in the headphone amplifier circuit. TINA-TI™
simulations illustrate that the circuit in Figure 49 has a phase margin of approximately 50 degrees with a 400-pF
load connected directly to the amplifier output.
10.2.3 Application Curves
The headphone amplifier circuit in Figure 49 is tested with three common headphone impedances: 16 Ω, 32 Ω,
and 600 Ω. The total harmonic distortion and noise (THD+N) for increasing output voltages is given in Figure 51.
This measurement is performed with a 1-kHz input signal and a measurement bandwidth of 22.4 kHz. The
maximum output power and THD+N before clipping are given in Table 4. The maximum output power into low-
impedance headphones is limited by the output current capabilities of the amplifier. For high-impedance
headphones (600 Ω), the output voltage capabilities of the amplifier are the limiting factor. The circuit in Figure 49
is tested using ±5-V supplies that are common in many portable systems. However, using higher supply voltages
increases the output power into 600-Ω headphones.
0
-20
-40
-60
-80
-100
-120
0.001
0.01
0.1
1
Amplitude (V_RMS)
Figure 51. THD+N for Increasing Output Voltages Into Three Load Impedances
(Input Signal = 1 kHz, Measurement Bandwidth = 22.4 kHz)
10
C002
26
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