A Small-Area Current-Mode Input ΣΔ Modulator for Under-the-Sensors ADC Arrays

Most readout systems for integrated sensor arrays rely on time-division multiplexing of analog frontends and analog-to-digital converters (ADCs) that have sample rates much faster than the one needed for acquiring the signal bandwidth of a single sensor. This complicates the system design if the sensor array is large, as switching the active sensor at high frequencies may degrade simultaneously the overall signal integrity and power integrity. Also, if the noise bandwidth is limited by a low-pass filter, the latter may require a fastsettling mechanism to be included. Additionally, in case of a large sensor impedance, buffers need to be employed in order to cope with the significant parasitics of long interconnecting. This paper proposes an architecture of a quasi-passive $\Sigma \Delta$ modulator that fits in a very small area, partially because it employs only metal-oxide-semiconductor capacitors (MOSCAPs), which are known to provide the highest capacitance density inside CMOS chips. This enables the usage of one modulator per sensor while simultaneously allowing the digitized sample to be sent over a single-wire digital signal, which conclusively eases the noise margin requirements for the sensor array. Taking a pass on employing an operational amplifier, as done in conventional $\Sigma \Delta$ modulators, reduces the design complexity while maintaining the inherent noise shaping property of the $\Sigma \Delta$ ADC. The proposed ADC is implemented and simulated in a $0.13 \mu \mathrm{~m}$ technology, fits in an area of under $20 \times 20 \mu \mathrm{~m}^{2}$ and consumes $11.8 \mu \mathrm{~W}$ at 100 MSps while providing an ENOB of 7.19 bits at an oversampling ratio of 128.