SBOS724 September 2015 OPA1688
UNLESS OTHERWISE NOTED, this document contains PRODUCTION DATA.
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The OPA168x family of amplifiers is specified for operation from 4.5 V to 36 V (±2.25 V to ±18 V). Many of the specifications apply from –40°C to 85°C. Parameters that can exhibit significant variance with regard to operating voltage or temperature are presented in the Typical Characteristics.
This application example highlights only a few of the circuits where the OPA168x can be used.
The design requirements are:
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:
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:
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.
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:
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.
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.
|LOAD IMPEDANCE (Ω)||MAXIMUM OUTPUT POWER BEFORE CLIPPING (mW)||THD+N AT MAXIMUM OUTPUT POWER (dB)|
Figure 52 shows the THD+N over frequency for a 500-mVRMS output signal into the same three load impedances previously tested.
Figure 53 and Figure 54 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.