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

1.2 DC Coupled System

DC coupled systems eliminate coupling capacitors. The DC coupled system has the following advantages:

  • Lower bill of material cost and less board space.
  • Performance degradation due to non-linear effects in capacitors can be eliminated including THD degradation at low frequency and board vibration effects.
  • Start-up pop is eliminated..
  • The ADC output is valid to DC which allows for the possibility to do measurements such as voltage and current for test and measurement.

DC coupling also has some disadvantages:

  • The DC biasing of the input pins is not determined by the ADC circuit but by the external source. Since the biasing is not always optimum, the ADC can display a reduced signal handling.
  • The Dynamic Range Extension (DRE) functionality of the ADC relies on adapting the PGA gain. This functionality can also be limited in DC coupled applications.
In DC coupling, DC differences between the INxP and INxM pins appear as an offset at the output of the ADC which can lead to saturation of the ADC output at high PGA gains. Therefore high PGA are not always possible with DC coupling.