SBOS727B November 2015  – May 2016 OPA1622

PRODUCTION DATA. 

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  6. Specifications
    1. 6.1Absolute Maximum Ratings
    2. 6.2ESD Ratings
    3. 6.3Recommended Operating Conditions
    4. 6.4Thermal Information
    5. 6.5 Electrical Characteristics:
    6. 6.6Typical Characteristics
  7. Detailed Description
    1. 7.1Overview
    2. 7.2Functional Block Diagram
    3. 7.3Feature Description
      1. 7.3.1Power Dissipation
      2. 7.3.2Thermal Shutdown
      3. 7.3.3Enable Pin
      4. 7.3.4Ground Pin
      5. 7.3.5Electrical Overstress
      6. 7.3.6Input Protection
    4. 7.4Device Functional Modes
      1. 7.4.1Shutdown Mode
      2. 7.4.2Output Transients During Power Up and Power Down
  8. Application and Implementation
    1. 8.1Application Information
      1. 8.1.1Noise Performance
        1. 8.1.1.1Noise Calculations
        2. 8.1.1.2Application Curve
        3. 8.1.1.3Basic Noise Calculations
      2. 8.1.2Total Harmonic Distortion Measurements
    2. 8.2Typical Application
      1. 8.2.1Design Requirements
      2. 8.2.2Detailed Design Procedure
      3. 8.2.3Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1Layout Guidelines
    2. 10.2Layout Example
  11. 11Device and Documentation Support
    1. 11.1Device Support
      1. 11.1.1Development Support
        1. 11.1.1.1TINA-TI (Free Software Download)
        2. 11.1.1.2TI Precision Designs
    2. 11.2Documentation Support
      1. 11.2.1Related Documentation
    3. 11.3Community Resources
    4. 11.4Trademarks
    5. 11.5Electrostatic Discharge Caution
    6. 11.6Glossary
  12. 12Mechanical, Packaging, and Orderable Information

8 Application and Implementation

NOTE

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.

8.1 Application Information

The low noise and distortion of the OPA1622 make the device well suited for a variety of applications in professional and consumer audio products. However, these same performance metrics also make the OPA1622 useful for industrial, test-and-measurement, and data-acquisition applications. The example shown here is only one possible application where the OPA1622 provides exceptional performance.

8.1.1 Noise Performance

Figure 49 shows the total circuit noise for varying source impedances with the op amp in a unity-gain configuration (no feedback resistor network, and therefore no additional noise contributions).

The OPA1622 is shown with total circuit noise calculated. The op amp contributes both a voltage noise component and a current noise component. The voltage noise is commonly modeled as a time-varying component of the offset voltage. The current noise is modeled as the time-varying component of the input bias current, and reacts with the source resistance to create a voltage component of noise. Therefore, the lowest noise op amp for a given application depends on the source impedance. For low source impedance, current noise is negligible, and voltage noise generally dominates. The low voltage and current noise of the OPA1622 op amp make the device an excellent choice for use in applications where the source impedance is less than 10 kΩ.

8.1.1.1 Noise Calculations

The equations in Figure 50 show the calculation of the total circuit noise using these parameters:

  • en = voltage noise
  • In = current noise
  • RS = source impedance
  • k = Boltzmann’s constant = 1.38 × 10–23 J/K
  • T = temperature in kelvins (K)

8.1.1.2 Application Curve

OPA1622 C302_SBOS727.png Figure 49. Noise Performance of the OPA1622 in a Unity-Gain Buffer Configuration

8.1.1.3 Basic Noise Calculations

Designing low-noise op amp circuits requires careful consideration of a variety of possible noise contributors, such as noise from the signal source, noise generated in the op amp, and noise from the feedback network resistors. The total noise of the circuit is the root sum squared combination of all noise components.

The resistive portion of the source impedance produces thermal noise proportional to the square root of the resistance. Figure 49 plots this function. The source impedance is usually fixed; consequently, select the op amp and the feedback resistors to minimize the respective contributions to the total noise.

Figure 50 shows both inverting and noninverting op amp circuit configurations with gain. In circuit configurations with gain, the feedback network resistors also contribute noise.

The current noise of the op amp reacts with the feedback resistors to create additional noise components. Choose feedback resistor values that make these noise sources negligible. The equations for total noise are shown for both configurations.

OPA1622 ai_noise_calc_bos450.gif

NOINDENT:

For the OPA1622 at 1 kHz, en = 2.8 nV/√Hz and in = 800 fA/√Hz.
Figure 50. Noise Calculation in Gain Configurations

8.1.2 Total Harmonic Distortion Measurements

The distortion produced by OPA1622 is below the measurement limit of many commercially-available distortion analyzers. However, a special test circuit, as shown in Figure 51, can be used to extend the measurement capabilities.

OPA1622 d307_disttest.gif Figure 51. Distortion Test Circuit

Consider op amp distortion an internal error source that is referred to the input. Figure 51 shows a circuit that causes the op amp distortion to be 101 times (approximately 40 dB) greater than that normally produced by the op amp. The addition of R3 to the otherwise standard noninverting amplifier configuration alters the feedback factor or noise gain of the circuit. The closed-loop gain is unchanged, but the feedback available for error correction is reduced by a factor of 101, thus extending the resolution by 101. Note that the input signal and load applied to the op amp are the same as with conventional feedback without R3. Keep the value of R3 small to minimize its effect on the distortion measurements.

Verify the validity of this technique by duplicating measurements at high gain or high frequency where the distortion is within the measurement capability of the test equipment.

8.2 Typical Application

The low distortion and high output-current capabilities of the OPA1622 make this device an excellent choice for headphone-amplifier applications in portable or studio applications. These applications typically employ an audio digital-to-analog converter (DAC) and a separate headphone amplifier circuit connected to the DAC output. High-performance audio DACs can have an output signal that is either a varying current or voltage. Voltage output configurations require less external circuitry, and therefore have advantages in cost, power consumption, and solution size. However, these configurations can offer slightly lower performance than current output configurations. Differential outputs are standard on both types of DACs. Differential outputs double the output signal levels that can be delivered on a single, low-voltage supply, and also allow for even-harmonics common to both outputs to be cancelled by external circuitry. A simplified representation of a voltage-output audio DAC is shown in Figure 52. Two ac voltage sources (VAC) deliver the output signal to the complementary outputs through their associated output impedances (ROUT). Both output signals have a dc component as well, represented by dc voltage source VDC. The headphone amplifier circuit connected to the output of an audio DAC must convert the differential output into a single-ended signal and be capable of producing signals of sufficient amplitude at the headphones to achieve reasonable listening levels.

OPA1622 ai_frontpage_sbos727.gif Figure 52. OPA1622 Used as a Headphone Amplifier for a Voltage-Output Audio DAC

8.2.1 Design Requirements

  • ±5-V power supplies
  • 150-mW output power (32-Ω load)
  • < –115-dB THD+N at maximum output (32-Ω load)
  • < 0.01-dB magnitude deviation (20 Hz to 20 kHz)

8.2.2 Detailed Design Procedure

Figure 52 shows a schematic of a headphone amplifier circuit for voltage output DACs. An op amp is configured as a difference amplifier that converts the differential output voltage to single-ended. The values of the resistors in the difference amplifier circuit are determined by the specifications of the DAC, such as output voltage and output impedance, as well as the maximum output voltage desired at the headphone output. The op amp chosen must be capable of delivering the necessary current to the headphones and remain stable into typical headphone loads that can have capacitances as high as 400 pF. The following design process uses a hypothetical DAC with common values of output voltage and impedance for the design process. The specifications of the DAC are shown in Table 1:

Table 1. Audio DAC Specifications Used for the Design Process

PARAMETER VALUE
Maximum differential output voltage2 VRMS
Output impedance (ROUT)200 Ω
Output dc offset1.65 V

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:

Equation 5. OPA1622 ai_typapp_eq1.gif

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:

Equation 6. OPA1622 ai_typapp_eq2.gif

where

  • η = efficiency
  • PIN = input power to the headphones

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.

OPA1622 ai_C321_SBOS727.png Figure 53. Sound Pressure Level vs Input Power for Headphones of Various Efficiencies

In-ear headphones can 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:

Equation 7. OPA1622 ai_typapp_eq3.gif

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:

Equation 8. OPA1622 ai_typapp_eq4.gif

In order to accommodate higher impedance headphones, increase the gain of the circuit to produce greater output voltages. However, increasing the gain also increases the noise of the circuit and limits the dynamic range of the circuit into lower impedance headphones. For this reason, some designers choose to select the headphone amplifier gain by using a switch.

Capacitors C1 and C2 limit the bandwidth of the circuit to prevent the unnecessary amplification of interfering signals. The maximum value of these capacitors is determined by the limitations on frequency response magnitude deviation detailed in the Design Requirements section. C1 and C2 combine with resistors R2 and R4 to form a pole, as shown in Equation 9:

Equation 9. OPA1622 ai_typapp_eq5.gif

Calculate the minimum pole frequency allowable to meet the magnitude deviation requirements using Equation 10:

Equation 10. OPA1622 ai_typapp_eq7.gif

where

  • G represents the gain in decimal for a –0.01-dB deviation at 20 kHz.

Use Equation 11 to calculate the upper limit for the value of C1 and C2 in order to meet the goal for minimal magnitude deviation at 20 kHz.

Equation 11. OPA1622 ai_typapp_eq8.gif

8.2.3 Application Curves

The circuit described in Figure 52 is constructed using 0.1% tolerance thin-film resistors (0603 package) and surface-mount film capacitors. The performance of the circuit is measured using a high-performance audio analyzer and is displayed in Figure 54 through Figure 59. The maximum output power for three common headphone impedances is shown in Table 2

Table 2. Maximum Output Power and THD+N Before Clipping for Common Headphone Impedances

LOAD IMPEDANCE
(Ω)
MAXIMUM OUTPUT POWER BEFORE CLIPPING
(mW)
THD+N AT MAXIMUM OUTPUT POWER
(dB)
16 95–114.2
32 150 –118.7
60011.3–119.4

The maximum output power delivered to low impedance headphone loads (16 Ω and 32 Ω) is limited by the output current capabilities of the amplifier. For the 600-Ω case, the maximum power delivered is limited by the output voltage capability of the amplifier and depends greatly on the power-supply voltages used. Figure 55 shows the maximum output voltage achievable for each load before the onset of clipping (±5-V supplies), indicated by a sharp increase in distortion.

As more current is delivered by the output transistors of an amplifier, additional distortion is produced. At low frequencies, this distortion is corrected by the feedback loop of the amplifier. However, as the loop gain of the amplifier begins to decline at high frequencies, the overall distortion begins to climb. The unique output stage design of the OPA1622 greatly reduces the additional distortion at high frequency when delivering large currents, as shown in Figure 56. High-ordered harmonics (above the 2nd and 3rd) are also kept to a minimal level at high output powers, as shown in Figure 57 through Figure 59.

OPA1622 ai_C311_SBOS727.png
Figure 54. Headphone Amplifier Transfer Function
OPA1622 ai_C313_SBOS727.png
90-kHz measurement bandwidth, 1-VRMS output
Figure 56. THD+N vs Frequency for Headphone Loads
OPA1622 ai_C315_SBOS727.png
1 kHz, 32-Ω load, 50 mW
Figure 58. Output Spectrum
OPA1622 ai_C312_SBOS727.png
22.4-kHz measurement bandwidth
Figure 55. THD+N vs Output Voltage for Headphone Loads
OPA1622 ai_C314_SBOS727.png
1 kHz, 32-Ω load, 10 mW
Figure 57. Output Spectrum
OPA1622 ai_C316_SBOS727.png
1 kHz, 32-Ω load, 150 mW
Figure 59. Output Spectrum