I've used several KE 4-211-2 Sennheiser microphones to assemble a device for acoustic signal analysis. They are connected to a National Instruments data acquisition device with ~1 m long cable and sampling rate is set to 50kHz.
I've added a resistor and a capacitor as described here (2nd page). However after a start of data collection instead of clear acoustic signal I get strong disturbance looking like a step function in low pass filter. It obviously distorts my frequency spectrum.
After investigation I found out that when NI device is not collecting data, capacitors charge slowly and start of collection probably results in dramatic drop of impedance on inputs. What might be a reason for this behavior and what should I do to make it work properly?
I think there is everything on this picture