JAJSSI0 December 2023 PCM3140-Q1
ADVANCE INFORMATION
The device consists of four pairs of analog input pins (INxP and INxM) that can be configured as differential inputs or single-ended inputs for the recording channel. The device supports simultaneous recording of up to four channels using the high-performance multichannel ADC. The input source for the analog pins can be from electret condenser analog microphones, microelectrical-mechanical system (MEMS) analog microphones, or line-in (auxiliary) inputs from the system board. Additionally, if the application uses digital PDM microphones for the recording, then the INxP and INxM pins can be reconfigured in the device to support up to eight channels for the digital microphone recording. Table 7-8 shows the input source selection for the record channel.
P0_R60_D[6:5] : CH1_INSRC[1:0] | INPUT CHANNEL 1 RECORD SOURCE SELECTION |
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00 (default) | Analog differential input for channel 1 (this setting is valid only when the GPI1 and GPO1 pin functions are disabled) |
01 | Analog single-ended Input for channel 1 (this setting is valid only when the GPI1 and GPO1 pin functions are disabled) |
10 | Digital PDM input for channel 1 (configure the GPIx and GPOx pin accordingly for PDMDIN1 and PDMCLK) |
11 | Reserved (do not use this setting) |
Similarly, the input source selection setting for input channel 2, channel 3, and channel 4 can be configured using the CH2_INSRC[1:0] (P0_R65_D[6:5]), CH3_INSRC[1:0] (P0_R70_D[6:5]), and CH4_INSRC[1:0] (P0_R75_D[6:5]) register bits, respectively.
Typically, voice or audio signal inputs are capacitively coupled (AC-coupled) to the device; however, the device also supports an option for DC-coupled inputs to save board space. This configuration can be done independently for each channel by setting the CH1_DC (P0_R60_D4), CH2_DC (P0_R65_D4), CH3_DC (P0_R70_D4), and CH4_DC (P0_R75_D4) register bits. The INM pin can be directly grounded in DC-coupled mode (see Figure 7-14), but the INM pin must be grounded after the AC-coupling capacitor in AC-coupled mode (see Figure 7-15) for the single-ended input configuration. For the best dynamic range performance, the differential AC-coupled input must be used .
The device allows for flexibility in choosing the typical input impedance on INxP or INxM from 2.5 kΩ (default), 10 kΩ, and 20 kΩ based on the input source impedance. The higher input impedance results in slightly higher noise or lower dynamic range. Table 7-9 lists the configuration register settings for the input impedance for the record channel.
P0_R60_D[3:2] : CH1_IMP[1:0] | CHANNEL 1 INPUT IMPEDANCE SELECTION |
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00 (default) | Channel 1 input impedance typical value is 2.5 kΩ on INxP or INxM |
01 | Channel 1 input impedance typical value is 10 kΩ on INxP or INxM |
10 | Channel 1 input impedance typical value is 20 kΩ on INxP or INxM |
11 | Reserved (do not use this setting) |
Similarly, the input impedance selection setting for input channel 2, channel 3, and channel 4 can be configured using the CH2_IMP[1:0] (P0_R65_D[3:2]), CH3_IMP[1:0] (P0_R70_D[3:2]), and CH4_IMP[1:0] (P0_R75_D[3:2]) register bits, respectively.
The value of the coupling capacitor in AC-coupled mode must be chosen so that the high-pass filter formed by the coupling capacitor and the input impedance do not affect the signal content. Before proper recording can begin, this coupling capacitor must be charged up to the common-mode voltage at power-up. To enable quick charging, the device has modes to speed up the charging of the coupling capacitor. The default value of the quick-charge timing is set for a coupling capacitor up to 1 µF. However, if a higher-value capacitor is used in the system, then the quick-charging timing can be increased by using the INCAP_QCHG (P0_R5_D[5:4]) register bits. For best distortion performance, use the low-voltage coefficient capacitors for AC coupling. The input impedance value of 2.5 kΩ is not supported for the DC-coupled input.