10-MHz 60-dB dynamic-range 4th-Order Butterworth lowpass filter

CMOS circuits for integrated continuous-time analog filters, applicable to lo-bit resolution video-rate signal processing systems, are presented. The filters utilize transconductance-C integrators as fundamental building blocks. A differential transconductance element comprising a source-coupled pair with a poly-silicon degeneration resistor and a cross-coupled high-impedance load is described. With this transconductor, tunable integrators for very-high-frequency in-tegrated filters can be realized. Good high-speed properties stem from the absence of internal nodes, which pushes non-dominant poles to the gigahertz range. These integrators are optimized for the maximum dynamic-range of greater than 64-dB, which occurs for input signal levels of 200-mV. Dual feedback loops stabilize circuit performance against temperature- and process-variations. The resonant frequency of a master VCO, comprising back-to-back integrators, tracks the frequency of a stable reference clock by virtue of an on-chip PLL. The VCO control voltage is broadcast to all matched transconductors, thus accurately controlling the natural frequencies of the filter. A separate Q-tuning loop controls the dissipation of the integrator, thus insuring a. 90-degree phase shift at the resonant frequency. Experimental results of a l0-MHz, 60-dB dynamic-range, 4th-Order Butterworth lowpass filter, realized in a O.8μm CMOS process, are given. The filter consists of a cascade of two biquadratic sections. Results show total harmonic disortion of less than -60-dB for a 130-mV input signal at 2-MHz. Third-order inter modulation distortion is below -56-dB for inputs of 8-MHz and 9-MHz. The measured filter responses agree with simulation results for the passive prototype filter to within 10%. The circuit occupies an area of 0.91mm x 0.65mm = 0.59mm2, and dissipates 40-mW from a single 5V supply