audio Microphones disturbed by capacitance

  
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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
audio Microphones disturbed by capacitance - schematic

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



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