A 1.8V CMOS, 80-200MHz continuous-time 4th order 0.05° equiripple linear phase filter with automatic tuning system

A 1.8V CMOS 80-200MHz continuous time 4th-order 0.05° equiripple linear phase filter with an automatic tuning system is presented. An operational transconductance amplifier (OTA) based on transistors operating in triode region is used to achieve a wide linear input range (1.8V_ppd peak-to-peak differential input with a single 1.8V power supply) and a wide transconductance tuning range. A novel and high performance system which integrates common-mode feedback (CMFB), common-mode feedforward (CMFF), and adaptative-bias into one circuit is introduced. The filter is based on Gm-C biquads and the post-layout simulation results in a 0.18μm process have shown a THD of less than -40dB with a 1.8V_ppd input and a 80-200MHz cutoff frequency tunable range. The group delay ripple is less than 3% for frequencies up to 1.4 times of the cutoff frequency and over the whole frequency tuning range. A simple automatic tuning system which adjusts the transconductance of the OTA is used to compensate the process parameter variations.