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Q: What can I do to reduce electret condenser microphone (ECM) noise using the AIC32x4 family?
The ADC channel path in the AIC32x4 has 93dBA SNR for 0dB channel gain. As the MicPGA analog amplifier gain is increased, the SNR remains somewhat above 90dBA until reaching 20dB channel gain for differential 10k input. This is because the rate in which the amplifier noise increases is much less than the rate the signal increases. See image below.
Typical electret condenser microphones (ECMs) have an SNR of 55dBA to 60dBA. If you would match the full-scale of the microphone to the full-scale of the ADC input and you would need around 14dB of gain MicPGA (analog) gain if the full-scale of the microphone is defined as 100mVrms (this is a function of the sensitivity of the microphone to convert SPL into voltage through its capsule/JFET). Adding 14dB gain would be more or less transparent in the AIC3254 ADC channel SNR (as shown in the graph). However, the level of the microphone, as well is its noise are being increased. When doing the loopback into the headphone output, the 60dB SNR noise floor of the microphone is pretty audible when driving 500mV into a typical 32-ohm headphone.
To test the actual noise floor subjectively, connect a line level mp3 player into the ADC input. These typically have 90+dB SNR.
The AGC feature can help with the noise floor of the microphone and with the ambient noise as well. The AGC will increase the gain if the signal level is very low, but if you configure the noise gate function, it will set the MicPGA gain back to 0dB when the noise falls below the threshold. If you have an EVM, I would recommend to use the AGC tool provided in its control software (AIC3254 CS).
Within PurePath Studio (when using miniDSP of the AIC3254), we also have a noise reduction component as well. For white noise, this should work very well to reduce the level of the microphone noise.
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