I'm using ADMP441 for a beamforming application. There 4 pairs of microphones that are connected to a Spartan 6 FPGA. Each pair has a left and a right micphone. The PCB board is powerd by a USB cable, but has power isolation ciruit that provides the microphones with 3.3 VDC. When the aqcusition is enabled the FPGA start generating SCK and WS that are used to control the microphones. I know that I have to wait for a period of time after I enable SCK and WS before I get usful data. I tried a signal source of 1 KHz and 1 inch away from the one of the microphones. When I plotted the received signal of that microphone I had a startup artifact where the 1 KHz signal appears to be carried on a very low frequency for 100 ms; then, it stabilizes and oscilates around zero, and the low frequency disappears. What could possibly cause this?
Also, I have a slight DC offset for some of the microphones; what could possiblly cause this problem?