A tunable, robust pseudo-resistor with enhanced linearity for scanning ion-conductance microscopy

Fast and high-resolution scanning ion-conductance microscopy (SICM) experiments require a high speed, low noise and low distortion transimpedance amplifier (TIA) as interface circuit. Recent high performance TIAs utilize capacitive feedback. However, their inability to treat DC currents renders them unfavorable for SICM applications and resistive TIAs remain the best choice here. To address this problem, this paper presents an active MOS resistor with improved robustness and linearity to make it suitable for its use as feedback resistor in TIAs for SICM experiments. The presented device features PMOS transistors operating in weak inversion as pseudo-resistors to obtain reduced noise compared to ohmic resistors of the same value. A novel bias circuit is introduced which compensates process variations of the pseudo-resistor. Monte Carlo simulation shows a standard deviation of 3.6 % of the overall resistance. The connection of multiple pseudo-resistors in series, while preserving equal overall resistance, linearizes the I-V characteristic and decreases the total harmonic distortion to 0.26 %. The presented device is incorporated into a low noise and high speed TIA achieving a simulated SNDR of 49 dB over a bandwidth of 1 MHz enabling fast high-resolution topography imaging using SICM.