A system for measuring electrical signals in a biological subject includes a variable gain amplifier with a predetermined transfer function that generates amplified signals corresponding to an input from electrical signals in the biological subject over a predetermined range of frequencies and amplification gain levels, an analog to digital converter generating digital data corresponding to the amplified signals, and a signal processing device receiving the digital data for the plurality of amplified signals. The signal processing device applies an inversion filter with another transfer function that is an inverse of the transfer function of the variable gain amplifier to remove an effect of the transfer function from the digital data, and generates an output signal corresponding to the electrical signals in the subject with reference to the filtered digital data.