A 12-pA Resolution Sigma Delta ADC Topology for Chemiresistive Sensor-Based Applications

Chemiresistive sensor technologies can detect chemical trace in the air down to the low Part Per Billion (ppb)/Part Per Trillion (ppt) level. Achieving such low detection levels requires low-noise and high-accuracy analog front-end interfaces. One such interface uses a Sigma Delta $(\Sigma\triangle)$ Analog to Digital Converter (ADC) topology that leads to lower noise at low frequency and better effective resolution. The slow charging dynamics of chemicals in the environment is well-suited for $\Sigma\triangle$ converters. While $\Sigma\triangle$ converters are a well-studied topic, the potential resolution of chemical sensing using chemiresistive sensors has not yet been demonstrated in a fully integrated mixed-signal interface. In this paper, we develop a $\Sigma\triangle$ architecture combined with a 10-pA resolution Digitally Controlled Current Sources (DCCS) and an integrated Proportional Integrator Derivative (PID) controller feedback loop. The PID drives back the current sources at 250 kHz to maintain a constant voltage across the sensor, acting as a variable resistor Implemented on a 180 nm CMOS process with an 8-bits current source resolution at 250 kHz and coupled with nanofiber chemiresistive sensors, it translates into lppb level detection. Compared to the state-of-the-art chemical sensing interface, our circuit shows a 5$\times$improvement in detection level and a $ 10\times$ improvement in power efficiency, given the same detection level.