A BiCMOS low-distortion 8-MHz low-pass filter

An 8-MHz seventh-degree elliptic-function low-pass filter is described, demonstrating an approach to low-distortion antialias filtering for high-definition video applications. The filter's performance goals are achieved through the use of circuit design principles that capitalize on the strengths of BiCMOS technology. The integrator circuits composing the filter consist of a new wideband low-distortion transconductor circuit and a unique BiCMOS Miller-stage circuit. Integrator time constants are determined by stable RC products, enabling a simplified filter calibration scheme that is insensitive to temperature-induced variations and requires no phaselock circuits. The prototype filter IC, consisting of seven integrators assembled in an active-ladder configuration, was fabricated in a 10-V, 2-μm 2.5-GHz BiCMOS technology that also features thin-film resistors and polysilicon-plate capacitors. Measured results from the calibrated filter show passband flatness of 0.2 dB, with aberrations of less than ±1 dB over a 100°C temperature range. Stopband attenuation meets its designed goal of 60 dB. Driven by 7-V_pp, differential input signals, the filter exhibits less than -72-dBc third-order intermodulation distortion products at 1 MHz. For 5-V_pp inputs at 4 MHz, third-order intermodulation spurs remain below -65 dBc