My company want to use the ADMP441 microphone in our product, but first we want to know everything about it. I checked its frequency response and found something strange. The lower part of the characteristic looks same as drawing from datasheet, but the upper part is completely different, it is going up even 8dB, when it should go down. (plot attached to post)
Microphone is mounted on small board with recommended pattern, ground plane under and around IC and have 100n X7R capacitor close to the supply pad. Board is connected to microcontroller board with 12cm ribbon cable with gnd between clk and data lines. Sampling frequency is 48k.
-I tried five other microphones, four on two types of PCB and one glued on its back and soldered by wires - with the same results.
-I tried to improve power supply, by adding more decoupling capacitors with different values, 10uH choke and finally use independent source -two AA batteries. Without significant changes.
-it is not math - I downloaded the raw data from microphone, and signal amplitude is growing with frequency.
-added 100 ohm resistor on clock line - nothing changed
I have no more ideas what's wrong.