A 82.75-nW, 500-mV First-Order Universal Shadow Filter Employing MI-DDTAs

This paper proposes a novel first-order multifunction filter capable of simultaneously realizing non-inverting and inverting low-pass, high-pass, and all-pass filter responses using multiple-input differential difference transconductance amplifiers (MI-DDTAs). The design leverages the shadow filter technique, enabling electronic tuning of the pole frequency across all filter responses through an external voltage-controlled amplifier. This approach offers enhanced flexibility and reconfigurability, making it highly attractive for adaptive analog signal processing. The MI-DDTA core is implemented using a multiple-input bulk-driven MOS transistor architecture, optimized for ultra-low-voltage and ultra-low-power operation. Designed in Cadence Virtuoso using the 65 nm TSMC CMOS (1P9M) process, the proposed filter occupies a compact silicon area of 125 µm × 92 µm. Operating at a 500 mV supply, it achieves a dynamic range of 41.7 dB for a 140 mV peak-to-peak input signal, with total harmonic distortion limited to 1%. The circuit operates with an exceptionally low bias current of 2 nA, resulting in a total power consumption of only 10 nW—making it ideally suited for energy-constrained biomedical and IoT sensor applications where low-frequency signal processing, small form factor, and energy efficiency are critical. To validate the proposed concept, a discrete prototype was also developed using MI-DDTA configurations built with LM13700 operational transconductance amplifiers. The experimental results confirm the expected filter behavior and demonstrate the feasibility of the proposed design in practical scenarios.