Low-Noise and Low-Sensitivity Coupled Fourth-Order Low-Pass Filters

The output thermal noise and sensitivity of the low-pass fourth-order Butterworth filter are reduced by the effect of coupling. The structure as two coupled second-order biquads is compared to the two cascaded second-order biquads structure, where both realize the same lowpass transfer function. The manufacturing technology used to realize the coupled circuit is the same as that of the cascaded, non-coupled circuit. Physical circuits are designed as active-RC filters using multiple-amplifier biquads. The optimization by equating maximum voltage levels at the opamp outputs is presented. First, Pspice simulation investigates an optimum design of the coupled circuit with regard to minimum sensitivity and compares it to the cascaded realization. The sensitivity analysis is performed using Monte Carlo runs. Second, the measurements of the two circuits are performed on the PCB realizations in SMD technology using HP 4195A Network Analyzer in the laboratory setup. The measurement results proof the stability of the coupled filters, although they are using positive feedback. Output thermal noise of the two filter structures is then compared. Substantially reduced values of the output thermal noise is measured for the coupled case.