A 1-GΩ CMOS transimpedance amplifier (TIA) suitable for processing sub-nA-level currents in electrochemical biosensor signal-acquisition circuits is presented. Use of a twostage active transconductor provides resistive feedback in place of a large-area linear resistor. The TIA feedback loop is engineered to suppress output offset caused by DC input leakage currents of ±0.9 nA. A mechanism to tune the low-frequency cutoff of the TIA from 0.7 Hz to 500 Hz is implemented which permits operation under variable environmental conditions. Simulated and experimental results from a custom TIA fabricated in a 3.3-V 0.35-µm CMOS process are presented.v

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