A 30-MHz fifth-order elliptic low-pass CMOS filter with 65-dB spurious-free dynamic range

A fifth-order 30-MHz integrated low-pass filter with 65-dB spurious-free dynamic range is presented. The topology is based on an operational transconducatance amplifier (OTA)-C ladder configuration implementing an elliptic transfer function. High linearity and low noise are achieved by using polysilicon resistors and efficient highly linear transconductors based on a proposed nonlinear source degeneration technique. The linearity of a typical source degenerated structure is improved by more than 10 dB while the small-signal transconductance is reduced by less than 1 dB; the additional power needed by the auxiliary circuitry is less than 10% that of the OTA's power, and the noise level increases by no more than 1 dB. Experimental results of a prototype fabricated in a standard 0.35-mu m CMOS technology are presented and compared with previously reported solutions. The overall filter's power consumption is 85 mW with a 3.3-V power supply.