Usage of inverting or non-inverting inputs as single ended electret microphone inputs

Hi.

We are using the ADAU1761 inputs with electret mics, so fairly low level signals. The data sheet (page 4 table 1) clearly shows that the PGA inverting input resistance varies inversely with the PGA gain setting. This is unsurprising. Equally, the non-inverting input exhibits a fixed input resistance of 105K. Also unsurprising. However, we note that on page 20, figure 26, the hot side of the electret mic is connected to the inverting input of the PGA. This means that the mic is subject to a varying load resistance which is lowest at maximum PGA gain. This could cost us a dB or two due to additional loading on the mic.

Why is the use of the inverting input presented on page 20, figure 26, when the non-inverting input would present a higher - and fixed - load resistance of 105K?

Thanks in advance for your support on this!

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