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

4.6 MEMS Microphone: Differential AC Coupled Input

Specifications of ICS 40740

Output impedance = 355 ohms. Output DC Offset is 1.07 V.

Belden 8760 Cable: capacitance/m = 240 p, length is 5 m.

TLV320ADCX120 Settings:

  • Vref = 2.75 V
  • Zin = 2.5k
  • AC coupled
  • Analog differential input. P0 R58 (7:6) = Mode 0
  • DRE ON
GUID-20230501-SS0I-WN8R-SBBS-8H21BSTHN4LC-low.svgFigure 4-12 MEMS Microphone Connected Through Long Cable to ADC

The ADC can handle 1-VRMS signals that can appear from the mic. In addition, this system has excellent rejection of common mode signals.

Estimation of coupling capacitor C:

Equation 40. Cutoff Frequency Fc=12×3.14×C×Zin FC=20 Hz
Equation 41.   For Zin=2500  C>12×3.14×20×2500   
Equation 42.  C>3 μf  
Equation 43. Signal level on ADC  Vin=VS×ZinRO+Zin     RO=355,  Zin=2500 Ω

Effects of Long Cable:

A twisted pair cable such as Belden 8760 gives 240 p/m capacitance. For 5 m length this is 1.2 n. The output impedance of the mic forms a low pass filter with cable capacitance and this limits the frequency response.

The equation below indicates that long cables can drive signal from this MEMS mic into the ADC.

Equation 44. Fc=12×3.14×1200pf×355=373khz