Design and Implementation of a 0.2-V VCO-Based ADC for High-Sensitivity CDMS Instrumentation

Orem, UT

The characterization of Martian dust requires direct measurement of both particle charge and mass across a wide range of sizes and environmental conditions. Charge Detection Mass Spectrometry (CDMS) provides a viable framework for particle-resolved measurements; however, existing implementations rely on laboratory instrumentation that is not suitable for deployment on resource-constrained planetary platforms. This work presents the design of an ultra-low-voltage (ULV) analog-to-digital converter (ADC) to enable full system integration of a CDMS instrument for Mars exploration. A second-order, closed-loop VCO-based ADC architecture is proposed, adapted from prior work to operate in the deep subthreshold regime. The architecture leverages phase-domain integration and feedback to achieve improved linearity and second-order noise shaping compared to previously reported first-order ULV designs. System requirements for CDMS are established, including 12-bit resolution and sampling rates exceeding 10 kHz, and a noise model is developed to guide design trade-offs. Circuit-level challenges associated with ULV operation are analyzed for key blocks, including the VCO, phase-frequency detector (PFD), switched ring oscillator (SRO), phase quantizer, and feedback DAC. Design strategies are presented to mitigate limitations in speed, noise, and variability under reduced supply voltage constraints. This work demonstrates the feasibility of achieving high-resolution, low-power digitization for CDMS using a second-order VCO-based architecture in ULV CMOS, providing a path toward compact, energy-efficient instrumentation suitable for in-situ planetary dust analysis.