A Dynamic Range Extension Technique for Pseudo-Resistive Transimpedance Amplifiers Based on Two-Step Conversion

This paper proposes a dynamic range (DR) extension technique based on a two-step conversion for high-sensitivity multi-element pseudo-resistive (MEPR) transimpedance amplifiers (TIA). In optical biomedical sensors targeted for fluorescence measurement applications, the front-end's high sensitivity and wide DR are critical for accurately recording cellular variations. In this work, the most significant bits (MSB) are extracted by a 2-bit current digital-to-analog converter (DAC) then the least significant bits (LSB) are extracted by a 10-bit successive approximation register (SAR) analog-to-digital converter (ADC). The current DACs are composed of identical pseudo-resistor (PR) elements used in the TIA feedback to make the MSB extraction robust against the process and mismatch variations. This technique preserves the linearity of the conversion while using the current DACs to improve the DR. The proposed front-end implemented in 65 nm CMOS technology achieves a DR of 72.3 dB with current detection of 1 pA up to 4.13 nA at a sampling rate of 1 kS/
$s$
. The total front-end consumes a power of 45 µW from a 2.5 V supply.