A new Tunable Pseudo-Resistor for Extremely/Ultra Low Frequency applications

This paper presents a new architecture for a pseudo-resistor (PR) implemented with MOS transistors, working in subthreshold region, tunable from Mega Ω to Giga Ω range. The proposed PR is programmed by controlling the VGS of the MOS transistors with a differential pair. An ultra-low frequency bandpass filter which uses the proposed PR with adjustable highpass cut-off frequency for a tuning range of 0.28 Hz to 900 Hz was designed. The simulated total harmonic distortion is for an input voltage of 224 mVpp. Simulations were performed using HSPICE Analog Design Environment and ON-SEMI 0.5 μm technology parameters which validates the feasibility of the proposed circuit.

I. Introduction

The realization of fully integrated filters for extremely low frequencies (ELF) and ultra low frequencies (ULF) can be considered a great challenge in the analog design, since a cut-off frequency in the ELF (3 Hz - 30 Hz) and ULF (300 Hz - 3 kHz) requires large time constants, which is related to the total capacitance and resistance seen in a node. However, resistor values than can be implemented in integrated circuits can not exceed the Mega range, and capacitors are limited to a few tens of pF, because that values are feasible only if they consume a large silicon area. On the other hand, those values present low absolute accuracy due to manufacturing process [1] variations, which in turn causes a loss of precision on the filter's cut-off frequency after fabrication. In the same fashion, many applications require that the filter's bandwidth can be adjusted, needing programmability capacities in some of the used components [2].