A method and apparatus for determining the impedance of the plurality of channels comprising a plurality of channels having a plurality of first channels and a plurality of second channels, at least one reference channel, a signal generator electrically connected to the plurality of channels, a reference signal generator connected to the reference channel, at least one amplifier connected to each of the plurality of channels and the reference channel and at least one filter connected to an output of the a least one amplifier to filter the outgoing signal from the at least one amplifier. The signal generator is configured to provide a plurality of input signals to the plurality of channels. The reference signal generator provides an input signal to the reference channel.
