I'm just wondering what's the purpose of the inductor in this microphone biasing schematic?

enter image description here

Could it be that I have very low input signal from the microphone because of missing L1? Bias generator derives it's power from 3.3V LDO. The microphone I use with this is:

Sensitivity:-30dB+/-2dB Frequency Range: 50Hz-20KHz Output Impedance: ≤2.2 kΩ SNR: >58dB Standard Operation Voltage: 4.5V Operation Voltage: 1.0V-10V.DC

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    \$\begingroup\$ I don't think that L1 affects the output signal of the microphone, it appears to be just some HF-filter thing... \$\endgroup\$ – aschipfl May 20 '19 at 15:47

The value of 15 nanohenry is about the inductance of 15 millimeters of wire.

The 15 pF (suspiciously the same numbers as the inductor) is more than casual stray parasitic, but in the range.

The resonance is in the hundreds of megahertz; with no dampening shown, the usefulness of 15nH and 15pF is doubtful.

I'd expect these to be residual values, and if PCB solder pads exist, the L and C are provided as optional EMI filtering.

The microphone cable may have Positive and Negative pins. Are you using those?

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  • \$\begingroup\$ The microphone is connected through 3.5mm jack and it's connected correctly GND -> GND, MIC -> MIC+. I double checked the recommended schematic in CSR8675 datasheet and it's seems this schematic is correct. They just say that C13 and C14 can be adjusted for bass response, but 100n should be suitable for most microphones. It seems it works correctly, but only when you have the microphone very close to your mouth. If it's 30-40cm away - you have to shout... I checked the software settings as well and increased the gain as much as I can.. \$\endgroup\$ – user1258202 May 21 '19 at 8:32

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