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

2.3 Selection of Coupling Capacitor

Since Vref/2 in Figure 2-2 is a DC voltage with low impedance for AC purposes, Vref/2 is considered GND as shown in Figure 2-5.

GUID-20230501-SS0I-VZNP-XXNJ-SSQNSDDZKNJJ-low.svgFigure 2-5 AC Equivalent Circuit of Input Pin

The coupling capacitor C and Zin form a high pass filter. This filter blocks DC and very low frequencies from reaching the input pin. For audio frequencies the capacitor acts as a short circuit. The cutoff frequency is calculated as follows:

Equation 2. Fc=12×3.14×C×Zin

To pass audio frequencies audible to humans, choose Fmin = 20 Hz

Equation 3. C>12 × 3.14 × Fmin × Zin
Equation 4. C>12×3.14×20×2.5 kΩ          For Fmin=20 and Zin=2.5 kΩ
Equation 5. C>3 μF

Having a smaller capacitor is possible, if input impedance is higher or Fmin is set higher.

Table 2-4 Selection of Coupling Capacitor
FminZinC
2025003.3 u
100200000.1 u