Nonlinear distortions are always introduced to biomedical signals during the acquisition stage, which consequently fail the traditional linear independent component analysis (ICA) methods for further signal processing. This paper investigates the non-linearity system function in the pre-amplifier and A/D converter of the biomedical instruments. A polynomial general model structure with adjustable parameters to approximate the nonlinear relation is proposed for medical instruments. Model parameters are validated using a typical electrocardiograph (ECG) acquisition system with sinusoids of varying frequency and amplitude. Thus the inverse nonlinear transform is applied to acquired data to cancel the nonlinear distortions. The ICA method is then applied to the originally linear mixed data, non-rectified data and also rectified data and the results are favorably compared in the designed experiment using both clinical ECGs and the simulated data from cardiac simulators.