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

3 DC Coupled Scheme

DC Coupled systems eliminate the need for coupling capacitors. The system has the following advantages

  • Lower bill of material cost and less board space.
  • Performance degradation due to imperfections in capacitors can be eliminated. This degradation includes THD degradation at low frequency and board vibration effects.
  • Start-up pop is eliminated.
  • The ADC works down to DC which opens up the possibility to do measurements such as voltage and current for test and measurement.

DC coupled systems have the following 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.
  • 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 gains are not always possible with DC coupling.
  • 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.
  • CH1_CFG0 Register (Address = 0x3C) Bit 4, CH1_DC=1 selects DC Coupling for Channel1.

Figure 3-1 describes the signal processing for DC Coupling.

GUID-20230501-SS0I-L0WB-LBR3-KHRBJ17BCZVP-low.svgFigure 3-1 Input Signal Processing

The circuit digitizes (InxP-InxM).

For common mode signals, INxP = InxM.

InxP-InxM = 0. Common mode signals such as noise create a zero input to the ADC and are not digitized.

DC offset present in the input can be removed by the high pass filter.