I'm trying to connect the ADA8691 amplifier with ADMP411 microphone, but I can't find the input resistance of the amplifier. Thanks,
Because the AD8691 is a CMOS amplifier, it should have very high input impedance. I do not think there will be any problems with interfacing this to the ADMP411. I have communicated with the engineer of this part to try and see whether I can provide you a typical value of its input impedance. I'll get back to you on this.
Just to follow up with this.. I heard back from the engineer of this part. The AD8691 is one of the older parts so unfortunately, it does not have Rin specs. However, its Ibias spec of 1pA should clue you in that it has very high input impedance. It is safe to assume a value in the high Mohms or Gohms range.
I hope this helps!