SBOS724A September 2015 – June 2022 OPA1688
The headphone amplifier circuit in Figure 9-1 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 9-3. 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 9-1. 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 9-1 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.
|Input signal = 1 kHz, measurement bandwidth = 22.4 kHz|
|LOAD IMPEDANCE (Ω)||MAXIMUM OUTPUT POWER BEFORE CLIPPING (mW)||THD+N AT MAXIMUM OUTPUT POWER (dB)|
Figure 9-4 shows the THD+N over frequency for a 500-mVRMS output signal into the same three load impedances previously tested.
|90-kHz measurement bandwidth|
Figure 9-5 and Figure 9-6 show the output spectrum of the OPA1688 at low (1 mW) and high (50 mW) output power levels into a 32-Ω load. The distortion harmonics in both cases are approximately 120 dB below the fundamental.
|Third harmonic is dominant at a level of –117.6 dB relative to the fundamental|
|Highest harmonic is the second harmonic at –119 dB below the fundamental|