Ultra-High-Resistance Pseudo-Resistors With Small Variations in a Wide Symmetrical Input Voltage Swing

This brief presents a new strategy and circuit configuration composed of serially-connected PMOS devices operating in the subthreshold region for implementing ultra-high-value resistors required in very low-frequency active-RC filters and
bio-amplifiers
. Depending on the application, signal bandwidth for instance in
bio-amplifiers
may vary from a few mHz up to a maximum of 10 kHz. Three different resistor structures are proposed to achieve ultra-high resistance. While ranging in the order of several
$\text{T}\Omega $
, the proposed ultra-high-resistance pseudo-resistors occupy a small on-chip silicon area, which is one of the main issues in the design of analog front-end circuits in ultra-low power implantable biomedical microsystems. In addition, these ultra-high-value resistors lead to the use of a small capacitance to create a very small cut-off frequency. Therefore, the large area to implement capacitances is also considerably reduced. The proposed resistor structures have very small variations about 7% and 12% in a wide input voltage range (−0.5 V~+0.5 V), thus significantly improving the total harmonic distortion of
bio-amplifiers
and the analog front-end of the system. Simulation results of different circuits designed in a 180nm CMOS technology, are shown to demonstrate the advantages of the proposed ultra-high-resistance pseudo-resistors.