An 11uW, 0.08 mm2, 125dB-Dynamic-Range Current-Sensing Dynamic CT Zoom ADC

A Dynamic Continuous-Time Zoom-ADC current to digital converter for bio-sensing applications is presented, comprising a current DAC (IDAC), a loop filter, a comparator, and loop logic. Within the IDAC, a reference voltage is driven to resistor legs, obtaining an accurate current with high Rout. A
$1^{\mathrm {st}}$
order loop filter utilizes an inherently linear, zero-static-power, passive integrator. An isolation of the integrator from external sensor capacitance is obtained by a cascode transistor, enabling the use of the passive integrator. During the Zoom’s SAR phase, a binary search detects the approximate signal level and provides zoomed references for the Sigma-Delta phase, where fine conversion is performed. The dynamic mode tracks the comparator output data to determine when the input signal is close to the references and updates them accordingly. This paper is the first to propose a dynamic zoom ADC which utilizes a single DAC. An over-voltage detector detects integrator voltages exceeding the supply level, a result of high input currents, and prevents damage to the circuit. A 65nm implementation achieved a competitive dynamic range of 125dB, an SNR of 91dB and an FoM of 192dB. The design occupies 0.08 mm2, and a state-of-the-art power consumption of 11.4uW was measured. The dynamic mode supports up to 78Hz inputs, and an SFDR of 90.4dB was measured.