Low noise rail-to-rail pseudo-switched operational amplifier for biosensor applications

Amperometric biosensors have become increasingly popular due to the ease of processing information in the electronic domain. However, the largest obstacles to their widespread adoption lie in the difficulty of realizing large arrays of these sensors in a compact footprint, largely due to the low-noise front-end amplifier required at the input stage. Low noise current sensing often necessitates a discrete-time integrating front-end requiring large open-loop gain operational amplifiers (op-amp). Besides the large area consumption, another issue is the ability of the front-end to handle a wide common mode range required for implementation of sophisticated sensing protocols. We present a four transistor ±1 V input common-mode range amplifier operating from a ±1.25 V supply with ≥ 80 dB of open loop gain and a 100 MHz unity-gain bandwidth fabricated in a 130 nm CMOS process. The core area of the amplifier is 0.006 mm2 and consumes 75 μW, which can be further optimized. The amplifier is designed based on a pseudo-switched capacitor circuit. However, unlike conventional switched capacitor circuits, we exploit the time during reset in integrating transimpedance amplifiers to store the common-mode offset. We show by turning off the clocks during the current-sensing operation the noise can be considerably reduced (factor of 20) without adversely affecting the operation of the amplifier. We also compared the noise performance of this amplifier to a more traditional op-amp stage to show that the performance is comparable albeit at a lower area.