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

4.3 Selection of a Microphone

This implementation uses the POM-2730L-HD-R microphone with TLV320ADC6120 ADC. The device is setup as a single-ended AC-coupled input.

Sensitivity: -30 db, 0 db = 1 V/Pascal with load resistor.

Output Impedance: 2.2 kΩ

SNR: 74 db, SPL of 94 db, or 1 Pascal

GUID-20230501-SS0I-DHZM-5NX4-RFH3TCWPCFT0-low.svgFigure 4-5 Noise in Signal Chain
A. Microphone noise = 4 µV
B. Input referred noise = 4 µV
C. Noise at input = microphone noise + input referred noise = 5.6 µV
D. Noise = 5.6 µV × 2 = 11.2 µV

For this function, DRE is disabled.

Signal Level at SPL of 94 db

Equation 15. -30 db=20×log⁡VsigPk
Equation 16.  VsigPk=31.6 mv 

Microphone Output Noise

Equation 17. 20×log⁡VsigVnoise=SNR  
Equation 18.  20×Log31.6 mvVnoise=74 db  
Equation 19. Vnoise=6.4 uvpk   
Equation 20.  Vnoise=4.4 uvrms

Dynamic Range of Mic

120 db SPL is considered the overload level for this mic.

The difference from 94 db SPL level is:

Equation 21. 120 db-94 db=26 db

94 db SPL is the reference for SNR which is 74 db.

Add 26 db to SNR to get the dynamic range.

Equation 22.  Dynamic range=74 db+26 db=100 db

Dynamic range of mic = 100 db.

Estimation of PGA Gain

The signal level at the overload point is:

Equation 23. -30 db+26 db=-4 db
Equation 24. -4 db=20×log⁡Vsig Pk    
Equation 25. Vsig Pk=0.63 V 

For a single-ended AC-coupled input the full-scale value is 1 VRMS or 1.4 Vpk.

Equation 26.   PGA Gain=1.40.63=2.2    

Estimation of SNR of Mic + ADC

Mic noise 4.4 uvrms equals V1.

Input referred noise of ADC with PGA gain of 2:

Equation 27. 4 uvrms=V2

Effective noise at input of ADC 5.65 uvrms:

Equation 28. Ve=V12+V22

Effective noise at output of PGA 11.2 uvrms:

Equation 29. 5.65 uv×PGA Gain

The following is the noise that is seen at the output of the ADC:

Equation 30. SNR of the system SNR=20×Log1Vn 
Equation 31. SNR=20×Log111.3 μv

SNR is 98.9 db. The system is able to capture sound up to the noise floor of the microphone.