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A new technique for designing precision, fully integrated, high-order filters using standard MOS technology is described. Switched capacitor integrators have been used to realize long time constants in small areas, and by interconnecting these integrators in a `leapfrog' configuration, monolithic high-order filters have been implemented with transfer functions that are very insensitive to component variations. Experimental results are presented for an NMOS fifth-order Chebyshev low-pass ladder filter with 0.1-dB passband ripple, a cutoff frequency of 3.4 kHz when clocked at 128 kHz, and a dynamic range of 83 dB. An efficient method for implementing transmission zeros is also presented, along with a complete design example, and additional experimental results for a third-order elliptic low-pass ladder filter which achieved 90-dB dynamic range, with a total power dissipation of 18 mW in a die area of 4400 mil/SUP 2/.
Date of Publication: Dec. 1978