Suitable compensation circuits for on-chip interference reduction in neural tripolar recordings

In this paper we present two passive compensation circuits that are suitable for reducing myoelectric interference in neural recording systems with tripolar electrode configurations. The compensation circuit is placed in series with the conventional quasi-tripolar amplifier. Once trimmed by simulation, it does not require any re-adjustment with varying signal or interference band characteristics. The simulation is based on equivalent electrode circuits that were obtained by fitting the measured electrical impedance spectrum of the platinum electrodes throughout the overlapping myoelectric / neural signal band (i.e., 20 Hz to 10 kHz). Simulation yielded optimal compensation circuit values for a 4-stages RC ladder network with an improvement of interference rejection by 37 dB at 100Hz. By testing the simulated compensation circuits with an imbalanced tripolar book electrode in a saline tank experiment, we show that 18-20 dB can be achieved over the entire frequency band of interest. The simplicity of the compensation circuit may soon make its use on-chip feasible.