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

AC Coupled Systems

  • In an AC coupled system, the DC Biasing at the input of the ADC is independent of the DC level of the source. The ADC input is biased internally by the chip at an optimum DC level maintaining that ADC digitizes full scale AC input signals.
  • Coupling capacitors used at the ADC input form a high-pass filter with the input impedance of the ADC limiting the low frequency response of the system. For audio applications, the capacitors are sized such that frequencies as low as 20 Hz can be digitized.
  • A coupling capacitor must be charged to a steady state value on power-up. Until the time the capacitor reaches this value, the input audio signal is not passed properly to the ADC. Sometimes this situation is heard as a pop in the audio output.
  • A coupling capacitor needs to behave as a short circuit for all AC signals in the audio range. In practice the capacitance value and the capacitive impedance can change with signal amplitude across the capacitor. This value can result in non-linear behavior and harmonic distortion in ADC output at low frequencies.
  • Some capacitors also display microphony. When experiencing vibration, the capacitor can induce a voltage in the AC path due to the Piezo Effect.