A 1 GHz equiripple low-pass filter with a high-speed automatic tuning scheme

A continuous-time fourth-order equiripple linear phase G_m-C filter with an automatic tuning circuit is presented. A high speed OTA based on the inverter structure is realized. The combined common-mode feedforward and common-mode feedback circuit ensures the input and output common-mode stability. The gain performance could be maintained by combining an equivalent negative resistor circuit at the output nodes. Transconductance tuning can be achieved by adjusting the bulk voltage by using the deep-NWELL technology. The modified automatic tuning circuit relaxes the speed requirement of the tuning blocks. Through the use of the operational transconductance amplifier as a building block with the automatic tuning scheme, the filter -3 dB cutoff frequency is 1 GHz with the group delay less than 4% variation up to 1.5 fc frequency. The -43 dB of IM3 at filter cutoff frequency is obtained with -4 dbm two-tone signals. Implemented in 0.18-μm CMOS process, the chip occupies 1 mm² and consumes 175 mW at a 1.5-V supply voltage.