An Active-gm-RC Structured CMOS Analog Filter with Time Constant Auto-Tuning
Based on TSMC 0.18μm 1P6M RF CMOS process, a 4th-order Butterworth analog low-pass filter (LPF) used in the transmitting link of 2.4GHz Wireless Sensor Network (WSN) radio frequency (RF) front-end is designed in this paper. In this design, active-gm-RC structure is derived, and it can achieve the same low consumption performance as the gm-C structure while have better linearity than that of the gm-C structure. Furthermore, a RC time constant auto-tuning circuit is also designed to automatically calibrate the RC time constant variation of the filter due to the variations of process corners, so the bandwidth precision of the filter is efficiently improved. The layout size of the whole filter with the auto-tuning circuit is 740μm×880μm. With 1.8V supply voltage, post-simulation results show that as the process varies, the cut-off frequency of the filter varies very little, which means that the tuning circuit effectively calibrates the bandwidth of the filter. The out of band attenuation rate is -80dB per decade frequency, and the operating current of the core circuit of the LPF is about 0.54mA. Post-simulation results achieve the specification requirements of IEEE802.15.4/ZigBee Wireless Sensor Networks.