arXiv subjects

Luis Hernandez

Publications and source records attributed to Luis Hernandez.

6 recordsLinked to original sources

arXiv2026

A 399uW 114.3 dB DR Companding Readout ASIC for MEMS Microphones Employing a Multirate Time-Domain ADC

Improvements in the dynamic range and sensitivity of digital MEMS microphones are essential in applications like advanced noise canceling and voice recognition. A cost effective solution to achieve these goals is the companding ADC architecture. Companding ADCs split the dynamic range in several segments with different quantization noise levels, relaxing power constraints. A common problem of companding microphones are audible artifacts generated when the input signal crosses the boundaries between different amplitude segments. We show in this paper a companding ADC architecture that mitigates the boundary artifacts by leveraging the instantaneous and high-resolution time-domain representation of the input signal in a VCO-based ADC. The use of a multi-rate frequency-to-digital converter allows to decouple quantization noise from the VCO frequency, keeping standard audio sampling rates. Co-optimization of the driver and oscillator circuits enables our VCO-ADC to reach \textgreater 112dBc of peak SFDR without a feedback DAC, keeping a Giga-Ohm input impedance compatible with a capacitive MEMS. We show measurements of a 0.13 $\mu$m ASIC implementing a complete readout circuit for a digital MEMS microphone. This includes two analog channels and the digital signal processing and calibration blocks required to deliver a standard single-bit PDM output. This ADC reaches a dynamic range of 114.3dB with a power budget under 400 uW, a Schreier FoM_{SNDR} of 171.0 dB and a FoM_{DR} of 191.3 dB.

eess.AS↗

arXiv2026

Analytical Derivation of Quantization Error in Threshold Level Quantizers Using Bipolar PFM

Uniform quantization is a topic that has been extensively studied. However and although an analytical description of quantization noise has been proposed, most descriptions of the spectral properties of quantization error resort to statistical descriptions. In this paper, we show how the spectrum of a quantized signal can be expressed using pulse frequency modulation. We first establish the equivalence of a uniform quantizer with a system based on the bipolar pulse frequency modulation and we define afterwards the Fourier transform of the quantized signal using pulse frequency modulation properties. This model brings a more intuitive understanding of the spectral structure of quantization noise and complements prior research in the topic. The results of the paper can be directly applied to level crossing ADCs with zero-order-hold interpolators, giving an accurate estimation of their performance.

eess.SP↗

arXiv2025

Form uniqueness for graphs with weakly spherically symmetric ends

We give characterizations for the failure of form uniqueness on weakly spherically symmetric graphs. The first characterization is in terms of the graph structure, the second involves the capacity of a Cauchy boundary. We also discuss the stability of form uniqueness and give characterizations for graphs which, following the removal of a set, consist of a disjoint union of weakly spherically symmetric graphs.

math-ph↗

arXiv2024

A Second-Order Audio VCO-ADC with 103-dB-A Dynamic Range and Binary-Weighted Internal Architecture

One of the limitations of conventional VCO-ADCs is the restriction to first-order noise shaping. True-VCO architectures have been proposed to increase the noise-shaping order by cascading several VCO integrators, but without requiring analog feedback loops. A high noise shaping order allows to reduce the input VCO frequency compared to a conventional VCO-ADC with similar dynamic range, which improves power consumption. Prior-art True-VCO architectures represent state variables either with a thermometer code or with a single-bit. Thermometer encoding is a natural choice when ring oscillators are selected as loop filter integrators. However, chip area restrictions force thermometer-encoded state variables to have few levels. A reduced number of levels in the state variables limits the dynamic range of True VCO-ADCs. In this paper, we show experimentally a second-order audio VCO-based ADC which uses ring oscillators as integrators but employs Gray and binary encoding for state variables. As a consequence, the complexity and area of the True-VCO architecture is reduced, breaking the barrier that limits the dynamic range of prior designs. The implemented chip shows a dynamic range of 103~dB achieving a peak SNDR of 76.5 dB-A with a power of 250 $\mu$W occupying 0.095 $\text{mm}^2$ in 130 nm CMOS.

eess.SY↗

arXiv2023

A Different View of Sigma-Delta Modulators Under the Lens of Pulse Frequency Modulation

The fact that VCO-ADCs produce noise-shaped quantization noise suggests that a link between frequency modulation and Sigma-Delta modulation should exist. The connection between a VCO-ADC and a first-order Sigma-Delta modulator has been already explained using Pulse Frequency Modulation. In this paper, we attempt to extend the theory based on Pulse Frequency Modulation to a generic Sigma-Delta modulator. We show that this link between Sigma-Delta modulation and Pulse Frequency Modulation relies in a sampling invariance property that defines the equivalence between both entities. This novel point of view, allows to go beyond the white quantization noise model of a Sigma-Delta modulator, revealing the origin of some nonlinear phenomena. We first predict spurious tones which cannot be explained by circuit non linearity. Multi-bit and single bit modulators are shown to belong to a same generic class of systems. Finally, quantizer overload is analyzed using our model. The results are applied to Continuous-Time Sigma-Delta modulators of orders one, two and three and then extended to a generic case.

eess.SY↗

arXiv2003

Sharing secret color images using cellular automata with memory

A {k,n}-threshold scheme based on two-dimensional memory cellular automata is proposed to share images in a secret way. This method allows to encode an image into n shared images so that only qualified subsets of k or more shares can recover the secret image, but any k-1 or fewer of them gain no information about the original image. The main characteristics of this new scheme are: each shared image has the same size that the original one, and the recovered image is exactly the same than the secret image; i.e., there is no loss of resolution.

cs.CR↗