most iepe sensors work at a constant current between 2 and 20 ma. a common value is 4 ma. the higher the constant current the longer the possible cable length. cables of several hundred meters length can be used without a loss of signal quality. supplying the iepe sensor with constant current, results in a positive bias voltage, typically between 8 and 12 volts, at the output. the actual measuring signal of the sensor is added to this bias voltage. the supply or compliance voltage of the constant current source should be 24 to 30 v which is about two times the bias voltage. this ensures maximum amplitudes in positive and negative direction.