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

2.4 Quick Charge Circuit

On power-up, the coupling capacitor is charged to the common mode voltage. This charge is done by connecting an internal 800-Ω resistor from pin to Vref/2. This connection is done for a duration of 2.5 ms. This time is enough to charge a 1-u capacitor to Vref/2. This process is called quick charging.

The audio output from the ADC is proper only after the coupling capacitors are charged to the steady state value.

GUID-20230501-SS0I-BF0W-RFLS-ZPCJNHMJJZ0J-low.svgFigure 2-6 Quick Charge Circuit

If the coupling capacitor is larger then 1 uf, the time duration can be set to a higher value by SHDN_CFG Register (P0 R5).

Table 2-5 SHDN_CFG Register Field Descriptions
BitFieldTypeResetDescription
7-6ReservedR0hReserved
5-4INCAP_QCHG[1:0]R/W0hThe duration of the quick-charge for the external AC-coupling capacitor is set using an internal series impedance of 800 Ω.
0d = INxP, INxM quick-charge duration of 2.5 ms (typical)
1d = INxP, INxM quick-charge duration of 12.5 ms (typical)
2d = INxP, INxM quick-charge duration of 25 ms (typical)
3d = INxP, INxM quick-charge duration of 50 ms (typical)
3-2SHDNZ_CFG[1:0]R/W1hShutdown configuration:
0d = DREG is powered down immediately after SHDNZ asserts
1d = DREG remains active to enable a clean shut down until a time-out is reached; after the time-out period, DREG is forced to power off
2d = DREG remains active until the device cleanly shuts down
3d = Reserved
1-0DREG_KA_TIME[1:0]R/W1hThese bits set how long DREG remains active after SHDNZ asserts.
0d = DREG remains active for 30 ms (typical)
1d = DREG remains active for 25 ms (typical)
2d = DREG remains active for 10 ms (typical)
3d = DREG remains active for 5 ms (typical)

The time a RC circuit takes to reach 90% of Supply Voltage is calculated as follows:

Equation 6. Tr=2.3×R×C    R=800 Ω 
Equation 7. Tr=1800×C
Equation 8.  Tr1=1800×C1    For another capacitor C1
Equation 9. C1=C × Tr1  T 
Equation 10. C1=1 μf × TR12.5 ms     For TR1=12.5 ms, C1=4.7μf

Table 2-6 lists SHDN_CFG register settings for different values of the coupling capacitor.

Table 2-6 SHDN_CFG register settings
Quick Charge TimeCoupling Capacitor
2.5 ms1 u
12.5 ms4.7 u
25 ms10 u
50 ms22 u