Improving sensitivity of direct-form switched-capacitor filters by coinciding pole-zero pairs

The aim here is to study the inclusion of mutually cancelling pole-zero pairs in the transfer functions of switched-capacitor filters to reduce their sensitivity to capacitance ratios. Combined with an optimum allocation strategy of poles and zeros for the original transfer function, the proposed technique proved effective in comparison with designs developed from classical approximations, such as Butterworth, Chebyshev and elliptic. Spice simulations carried out with a fully differential prototype filter designed in a 0.35 mum CMOS technology are shown to validate the theory. As a result of the extra circuitry to implement a coincident pole-zero pair, the capacitance spread decreased 6-fold and the frequency response deviation decreased by a factor of 2.6 in the passband, whereas the power consumption increased only 13.6%, from 30.2 to 34.3 mW.