The audio signals collected by microphone are distored and all positive after AD1939.
Here is the reason what we think: The input of the audio signals exceed a certain value, and beyond the AD1939 measurement range. But, the input of the audio signals are not so big, the amplitude of the microphone's output sound pressure is not exceeding 5V.
When the test signal is sinusoidal, the frequency is 400Hz, and the collected signal graph is as follows:
Now we use the same signal, but we turn the volume down, the figure is shown below. but at this time, the signals are all positive:
Now, we want to know if we are wrong or not. If we are wrong, what is the reason of this problem. If we are right, how can we fix this problem?