A PDM output microphone, such as the ADMP421 or ADMP521, cannot directly interface to an I2S port. Theses microphones should be connected to the PDM input on codecs such as the ADAU1361 and ADAU1761. The ADMP441 MEMS microphone has an I2S output that can be connected directly to a processor or codec's I2S input port.
We have a few Circuits from the Lab that explain connections between digital microphones and other devices:
CN0078: Digital MEMS Microphone Simple Interface to a SigmaDSP Audio Codec (ADMP421, ADMP521, ADAU1361, ADAU1761)
CN0208: High Performance MEMS Digital Microphone's Simple Interface to SigmaDSP Audio Processor (ADMP441, ADAU1446)
CN0266: High Performance Digital MEMS Microphone Standard Digital Audio Interface to Blackfin DSP (ADMP441, ADSP-BF527 Blackfin)