9.3.5 Programmable Gain Amplifier

The PCM186x has a two-stage programmable gain amplifier (PGA). Coarse gain adjustment is done in the analog domain, and fine gain adjustment is done in the digital domain. The ±12-dB analog gain steps are designed for varying line level inputs, whereas the 20 dB and 32 dB are primarily designed for microphone inputs, and will likely need additional gain that can be done in the digital domain. The analog gain steps between –12 dB and +12 dB are in 1-dB steps. Half-dB steps between those points are done in the digital PGA. Gain steps between 12 dB and 20 dB are all done in the digital domain. (for example, 18-dB gain = 12-dB analog + 6-dB digital). The gain structure in the PCM186x is shown in Figure 28.

Figure 28. PCM186x Complete Gain Structure (PGAs and Attenuator)

The analog gain steps within the analog PGA are shown in Figure 29. Again, from –12 dB to +12 dB, the steps are 1 dB each. The digital PGA has granularity down to 0.5 dB.

Figure 29. Analog Gain Steps With Software-Controlled Devices

The PGA in the PCM186x is a hybrid analog and digital programmable gain amplifier. The devices integrate a lookup table with the optimal gain balance between analog and digital gain, allowing the gain to be set in a single register per channel. For example, set 18 dB gain, and the system allocates 12 dB to the analog PGA, and 6 dB to the digital PGA. This function is called auto gain mapping.

The PGA is a zero crossing detect type, and has the ability to set target gain, and have the device work towards it (with a timeout if there is no zero crossing). Any changes in the Analog PGA and digital PGA are designed to step towards the final level. However, any changes in the mixer PGA are immediate. Take care when changing gain levels in the digital mixer PGA. Alternatively, multiple writes can be made of small enough values that do not cause significant pops or clicks.

For example, if the current level = 0 dB, then set the target as 3.5 dB. The PGA then increases gain in 0.5-dB steps towards 3.5 dB.

The auto gain mapping function can by bypassed if required, using manual gain mapping. Manual gain mapping is useful when using digital microphones, as the PDM input signal bypasses the analog PGA and must be amplified using the digital PGA. (PGA_MODE (Page.0, 0x19). Digital PGA update is only available in the 4-channel devices because the digital gain in 2-channel devices is fixed to 0 dB when manual gain mapping is enabled.