A 1V 8th-order 250MHz current-mode butterworth lowpass filter
Continuous-time filters can be used as preprocessing blocks in front of an analog-to-digital converter (A/D) in the receiving data path of a disk-drive read-channel system and as post processing blocks after a digital system. High-frequency continuous-time filters with low supply voltage are finding increased applications in high-speed Integrated Circuits (ICs) of telecommunication, video, and magnetic read-channel. This has been the driving force in the development of numerous continuous-time filter techniques exist today, and further ongoing research. Among the different topologies, current-mode functions exhibit higher frequency potential, simpler architectures, and lower supply voltage capabilities. Active RC filters have been used for years due to the high linearity. However, the uncertainty in cutoff frequency, mainly due to the integrated passive resistors and capacitors, limits the application. On the other hand, current conveyors, introduced in the early 1970's, are now emerging as an important class of circuits. A novel current-mode active-RC filter combines the advantages of current conveyor circuits and active RC filter configuration with digital frequency tuning is proposed. With digital tuning, the fabricated central frequency is measured with respect to a provided reference frequency, and a digital code is then derived which selects the appropriate capacitor elements that will return tuning to the nominal value, within some defined tolerance (e.g., ±5%). A 1V 8th order 25OMhz current-mode Butterworth lowpass filter with digital frequency tuning has been implemented in TSMC 0.25[Mu]m CMOS technology.