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Hi Joel,
A switch isn't necessary from the codec's point of view. Here's a reference for microphone pre-amp designs: https://www.ti.com/lit/ug/tidu765/tidu765.pdf?ts=1725905279928&ref_url=https%253A%252F%252Fwww.google.com%252F
Best regards,
Jeff McPherson
That circuit is using 9V power supply. Could it be 5V instead? Should this come from a linear regulator to reduce noise?
I have 12V available but it is noisy.
Hi Joel,
Chapter 7 of that app note shows an alternative using a 5V supply. Also yes, the supply should be as clean as possible to avoid that noise coupling into the microphone signal. Just be sure that your regulator can support the required current.
Best regards,
Jeff McPherson
The OPA172 Op amp in the schematic has noise of 7nV/Hz. Is this low enough? There are other op amps with low noise like 1nV/Hz
Hi Joel,
"Low enough" is heavily dependent on your application and what kind of performance you're looking for. Both of those options will result in a noise spectrum that's lower than the performance of the PCM2903C aka the noise floor of the codec will mask the noise floor of the op amp.
7nV/√Hz * √(20k-20) = 0.99uVrms
Based on the SNR of the device, the input noise floor would be around 70uVrms.
Best regards,
Jeff McPherson
What noise level should I look for regarding an LDO regulating 12V to 5V?
I found LP38798 which has good PSRR. There are parts with better spectral noise density, but they start to get expensive and might be overkill.
Hi Joel,
As I mentioned, the noise floor of the device is around 70uVrms. Any noise performance lower than this will be good as it will get masked.
I'm only an expert on the audio codec, so I recommend you create a new ticket regarding the LDO and they can help you narrow down noise performance given the threshold I've given you.
Best regards,
Jeff McPherson