I'm developing digital avionics intercom system using AD1938 CODEC and ADAU1446 DSP.
Analog audio signals are digitized by AD1938 and processed by ADAU1446 and converted to analog by AD1938.
The AD1938 CODEC is used with stand alone (No SPI control) mode and audio interface is I2S which has 48kHz sampling rate.
The DSP also has 48kHz sampling rate and schematic is as below.
Whenever the temperature of DSP package becomes more than 75 Celsius degrees, clicking noise comes out to headset.
I already check the analog part of my intercom system and the clicking noise didn't come out due to analog circuit.
The clicking noise has following symptoms.
- At the high temperature, the noise comes out.
- As the temperature increases, the noise comes out more frequently.
- If there is no audio input (ex, 1kHz sine wave), no noise comes out.
I have modified and checked following items, but the clicking noise still comes out.
- I added pull-up and pull-down at the clocks between the CODEC and the DSP to prevent distortion of clock due to high temperature.
- I modified external filter connected with LF pin of the DSP and checked the noise.
- Master clock status of the DSP and CODEC. (The DSP and CODEC are using same master clock.)
So, I wonder that what point I can check more? Please let me know.