SBAA583 july   2023 PCM1820 , PCM1820-Q1 , PCM1821 , PCM1821-Q1 , PCM1822 , PCM1822-Q1 , PCM3120-Q1 , PCM5120-Q1 , PCM6120-Q1 , TLV320ADC3120 , TLV320ADC5120 , TLV320ADC6120

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 AC Coupled Systems
    2. 1.2 DC Coupled System
  5. 2AC Coupling Schemes
    1. 2.1 Equivalent Circuit
    2. 2.2 Input Pin Waveforms with AC Coupling
    3. 2.3 Selection of Coupling Capacitor
    4. 2.4 Quick Charge Circuit
    5. 2.5 Selection of Capacitor Type
    6. 2.6 Single-Ended and Differential Mode
    7. 2.7 S.N.R in AC Coupled Circuits
  6. 3DC Coupled Scheme
    1. 3.1 Biasing the Pins
    2. 3.2 Electrical Characteristics
    3. 3.3 Application Circuits
      1. 3.3.1 S.N.R in DC Coupled Circuits
  7. 4Application Examples
    1. 4.1  Electret Condenser Microphone: Single Ended DC- Coupled Input
    2. 4.2  Electret Condenser Microphone: Single Ended AC Coupled Input
    3. 4.3  Selection of a Microphone
    4. 4.4  Condenser Microphone: Differential DC-Coupled Input
    5. 4.5  Condenser Microphone: Differential AC-Coupled Input
    6. 4.6  MEMS Microphone: Differential AC Coupled Input
    7. 4.7  Circuit With No Offset and Response Down to DC
    8. 4.8  Improving SNR by Summing the Output of 2 ADC Channels
    9. 4.9  Measure a High Voltage Waveform (+-50 V)
    10. 4.10 I2C Listing
  8. 5Summary
  9. 6References

4.2 Electret Condenser Microphone: Single Ended AC Coupled Input

Features: high performance

This application uses a POM-2730L-HD-R microphone with the TLV320ADC6120 ADC.

POM-2730L-HD-R Specifications:

RO = 2200 Ω.

LV320ADCX120 Settings:

  • VREF = 2.75 V
  • ZIN = 10 kΩ
  • Analog single ended
  • AC coupled
  • P0_R58[7:6] set at Mode 0
  • DRE enabled
GUID-20230501-SS0I-QLBN-DM9B-FVBZQQKJP6M8-low.svgFigure 4-3 Single-Ended AC Coupled Condenser Microphone Circuit
GUID-20230501-SS0I-9G9V-C0QJ-H5SKDLNJQM3B-low.svgFigure 4-4 Single-Ended AC Coupled Condenser Microphone Test

As shown in Figure 4-3 and Figure 4-4, RO (the output impedance of the microphone) forms a voltage divider with the bias resistor. The application biases the microphone output pin to 1.375 VDC to bias ADC circuit at center.

Equation 13. ADC bias input=3.3×ROR+RO =3.3×2200R+2200=1.375V

R = 3 kΩ

Choose C = 3.3 µF to allow a frequency response as low as 20 Hz.  

In this application, the open circuit voltage of the microphone is VMic.

Equation 14. VIN(ADC)=VMIC×ZINR+ZIN

To prevent this loading, maintain ZIN >>R.