SBOS259F September   2002  – June 2018 OPA2363 , OPA2364 , OPA363 , OPA364 , OPA4364

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Single-Supply Microphone Preamplifier
  4. 4 Revision History
  5. 5 Device Comparison Table
    1. 5.1 Device Comparison Table
  6. 6 Pin Configuration and Functions
    1.     Pin Functions: OPA363
    2.     Pin Functions: OPA2363
    3.     Pin Functions: OPA364
    4.     Pin Functions: OPA2364
    5.     Pin Functions: OPA4364
  7. 7 Specifications
    1. 7.1  Absolute Maximum Ratings
    2. 7.2  ESD Ratings
    3. 7.3  Recommended Operating Conditions
    4. 7.4  Thermal Information: OPA363
    5. 7.5  Thermal Information: OPA364
    6. 7.6  Thermal Information: OPA2363
    7. 7.7  Thermal Information: OPA2364
    8. 7.8  Thermal Information: OPA4364
    9. 7.9  Electrical Characteristics
    10. 7.10 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Rail-to-Rail Input
      2. 8.3.2 Operating Voltage
      3. 8.3.3 Capacitive Load
      4. 8.3.4 Input and ESD Protection
    4. 8.4 Device Functional Modes
      1. 8.4.1 Enable Function
  9. 9 Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Achieving Output Swing to the Op Amp Negative Rail
      2. 9.1.2 Directly Driving the ADS8324 and the MSP430
      3. 9.1.3 Audio Applications
      4. 9.1.4 Active Filtering
    2. 9.2 Typical Application
      1. 9.2.1 Single-Supply Electret Microphone Preamplifier
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
        3. 9.2.1.3 Application Curve
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Development Support
        1. 12.1.1.1 TINA-TI™ (Free Software Download)
        2. 12.1.1.2 DIP Adapter EVM
        3. 12.1.1.3 Universal Op Amp EVM
        4. 12.1.1.4 TI Precision Designs
        5. 12.1.1.5 WEBENCH Filter Designer
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
    3. 12.3 Related Links
    4. 12.4 Receiving Notification of Documentation Updates
    5. 12.5 Community Resource
    6. 12.6 Trademarks
    7. 12.7 Electrostatic Discharge Caution
    8. 12.8 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

9.2.1.2 Detailed Design Procedure

In this circuit, the op amp is configured as a transimpedance amplifier which converts the signal current of the microphone into an output voltage. The gain of the circuit is determined by the feedback resistor RFB, which must be calculated according to the microphone sensitivity. For this design, a microphone output current of 8 µA per Pascal (Pa) of air pressure was chosen. Using this value, the output current for a sound pressure level of 100 dBSPL, or 2 Pa air pressure, is calculated in Equation 1.

Equation 1. OPA363 OPA2363 OPA364 OPA2364 OPA4364 ai_eq1_SBOS259.gif

RFB is then calculated from this current to produce 1-VRMS output for a 100-dBSPL input signal in Equation 2.

Equation 2. OPA363 OPA2363 OPA364 OPA2364 OPA4364 ai_eq2_SBOS259.gif

The feedback capacitor (CFB) is calculated to limit the bandwidth of the amplifier to 20 kHz in Equation 3.

Equation 3. OPA363 OPA2363 OPA364 OPA2364 OPA4364 ai_eq3_SBOS259.gif

RBIAS is required to divert the microphone signal current through capacitor CIN rather than flowing from the power supply, VCC. Larger values of RBIAS allow for a smaller capacitor to be used for CIN and reduce the overall noise of the circuit. However, the maximum value for RBIAS is limited by the microphone bias current and minimum operating voltage.

The value of RBIAS is calculated in Equation 4.

Equation 4. OPA363 OPA2363 OPA364 OPA2364 OPA4364 ai_eq4_SBOS259.gif

Input capacitor CIN forms a high-pass filter in combination with resistor RBIAS. The filter corner frequency calculation is shown in Equation 5 to place the high-pass corner frequency at 20 Hz.

Equation 5. OPA363 OPA2363 OPA364 OPA2364 OPA4364 ai_eq5_SBOS259.gif

The voltage divider network at the op amp noninverting input is used to bias the op amp output to the mid-supply point (VCC / 2) to maximize the output voltage range of the circuit. This result is easily achieved by selecting the same value for both resistors in the divider. The absolute value of those resistors is limited by the acceptable power-supply current drawn by the voltage divider. Selecting 25 µA as an acceptable limit of supply current gives a value of 100 kΩ for the resistors in the divider, as Equation 6 shows.

Equation 6. OPA363 OPA2363 OPA364 OPA2364 OPA4364 ai_eq6_SBOS259.gif

Finally, to minimize the additional noise contribution from the voltage divider, a capacitor is placed at the op amp noninverting input. This capacitor forms a low-pass filter with the parallel combination of the voltage divider resistors. Selecting a filter corner frequency of 20 Hz minimizes the noise contribution of the voltage divider inside the amplifier passband; see Equation 7.

Equation 7. OPA363 OPA2363 OPA364 OPA2364 OPA4364 ai_eq7_SBOS259.gif