Overview - DA7212 has been used as audio codec with Mic 1 configured as pseudo-differential input with headphones connected to HP_L and HP_R of codec. All other other peripherals like AUX mic, Mic 2 and speakers are unused. Codec I2S and I2C lines are connected to STM32 MCU. We have BT controller for wireless communication. We are using A2Dp and HFP profiles for wireless audio communication.
We have observed continuous background noise even-tough mic is not connected to codec. Basically, we used audio recording app to record audio through mic 1 from codec.Recorded clip has noise from frequency(200Hz to 4kHz). To evaluate app and firmware are not sources of noise, a Loopback test is conducted and results in same noise. Loopback test is basically a test to loop back audio data from Mic 1 to HP_L and HP_R by selecting Mix_in_Las input to Mix_out. While root causing the issue, it is found that mic bias is not a stable DC. Mic bias has ripple, causing noise from mic input.
File attached contain figures to explain the cause. VDD_A has ripple, however I'm expecting PSRR of codec should have handle these ripple and provide a ripple free mic bias.