**1. Introduction**

The Internet of things (IoT) is heavily driven by significant semiconductor and nanotechnology breakthroughs. Low-cost, reliable, and highly integrated circuits and systems have been designed, allowing for the introduction of important features such as remote access control and the operation of large amounts of data [1].

High-resolution analog-to-digital (A/D) converters (ADCs) are relevant building blocks in different IoT systems. Applications such as high-precision sensor networks, communications, imaging, and signal processing require outstanding ADC performance, including high-accuracy, low-power consumption, and, in some cases, wide-bandwidth (BW) specifications [2].

To accomplish a high resolution, delta-sigma (ΔΣ) modulators (ΔΣM) and successive approximation register (SAR) ADCs (SAR-ADCs) are frequently utilized. While in ΔΣM, larger sampling frequencies (*FS*) are used to achieve higher resolutions, in conventional SAR-ADCs, energy efficiency is often sacrificed to reach the target resolution. Furthermore, old-fashioned architectures and techniques were revisited, and hybrid structures are currently a reality, mixing these schemes with popular structures. Employing noise shaping (NS) in an SAR-ADC and using a delta modulation in a ΔΣM, resulting in a delta-deltasigma (ΔΔΣ) modulator (ΔΔΣM), are examples of this new era of hybrid ADC architectures pursuing the most outstanding and efficient ADC [3–5].

Taking into consideration that the ADC design is a complex and time-consuming task, it is desirable to reduce this effort, especially when porting between different nodes or technologies is required. Moreover, the lower nodes' technology constraints (low intrinsic

**Citation:** Correia, A.; Tavares, V.G.; Barquinha, P.; Goes, J. All-Standard-Cell-Based Analog-to-Digital Architectures Well-Suited for Internet of Things Applications. *J. Low Power Electron. Appl.* **2022**, *12*, 64. https:// doi.org/10.3390/jlpea12040064

Academic Editor: Andrea Acquaviva

Received: 24 October 2022 Accepted: 3 December 2022 Published: 7 December 2022

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gain, reduced supply voltage, leakage current, etc.) bring other challenges in porting tasks, sometimes requiring a complete redesign or the implementation of different circuit schemes.

Circuits based on digital logic can be quickly realized and modified to accomplish specifications and technological changes. Therefore, the use of digital circuits is becoming popular in analog or mixed-signal circuit design, such as in the case of ADCs. Consequently, synthesizable solutions using standard cells are used to further reduce redesign time and effort [6–8].

In this work, the most-suited ADC architectures for IoT applications are described. A hybrid ADC solution, a digital–delta (Δ) modulator (ΔM) with NS, is proposed that can be implemented using only passive and digital circuitry based on standard cells. Circuit details and some simulation results are also provided.

This paper is organized as follows. Section 2 presents the most popular ADC architectures for IoT applications, where complexity, dynamic performance, and energy efficiency are compared. In Section 3, some standard-cell-based active building blocks, comparators, and integrators, are presented with reference to their advantages and the main challenges during their integration in complex systems. Section 4 provides schematic details regarding the proposed standard-cell-based digital-ΔM employing NS and some simulation results. Lastly, the main conclusions are drawn in Section 5.
