Prototype Analog-to-digital Converter Research Articles

A Deep-Subthreshold Variation-Aware 0.2-V Open-Loop VCO-Based ADC

This article demonstrates the potential of deep-subthreshold mixed-signal circuits in delivering medium-to-high performance to supply-constrained, energy-harvesting Internet of Things (IoT) sensing applications. This effort encapsulates the design and implementation of an ultra-low-voltage (ULV) 0.2-V open-loop VCO-based analog-to-digital converter (ADC). A replica VCO facilitates variation-aware VCO analog linearization. Analog phase-domain signal processing (APSP) techniques for beat-frequency extraction, phase-interpolation, and phase-folding relax constraints on both voltage-to-frequency analog circuitry and frequency-to-digital synchronous digital hardware. High-speed multi-phase frequency-to-digital converters (FDCs) and multi-rate digital back-end enable a sampling speed of 35 MS/s. The ADC prototype is implemented in 28-nm CMOS and achieves a peak SNDR of 64.4/59.9 dB, equivalent to an ENOB of 10.4/9.7 over 80-/160-kHz bandwidth (BW). The ADC core occupies an active area of 0.12 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and consumes 15.9 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text {W}$ </tex-math></inline-formula> , resulting in a Walden and Schreier FoM of, respectively, 73.3/61.5 fJ/c-s and 161.4/159.9 dB at the corresponding BW configurations. Measurements across multiple ICs and supply voltages consolidate the value of variation-aware deep-subthreshold open-loop ADCs.

A 9-bit 500-MS/s 2-bit/cycle SAR ADC With Error-Tolerant Interpolation Technique

This article presents a 9-bit 500-MS/s 2-bit/cycle successive approximation register (SAR) analog-to-digital converter (ADC) with an error-tolerant interpolation technique. The proposed interpolation technique uses flip-flops to implement a 2-bit/cycle operation in the SAR ADC. By taking advantage of the metastable region of the flip-flop, the proposed interpolator can defer the bit decision when a decision error occurs with a high probability. Because the SAR ADC approximates the signal range step by step, the deferred decisions proceed to the next conversion cycles without any increase in quantization noise. The deferring-decision characteristic increases the error tolerance in the presence of comparator mismatches and increases the inherent linearity of the interpolation technique compared to conventional latch interpolation. A prototype ADC was designed using the 28-nm CMOS technology to verify the effectiveness of the proposed interpolation technique. The measured signal-to-noise-and-distortion ratio (SNDR) and spurious-free dynamic range (SFDR) at the Nyquist rate are 50.6 and 61.4 dB, respectively. The power consumption is 1.87 mW at a sampling frequency of 500 MS/s. The proposed ADC achieves a Walden figure of merit (FoM) of 13.5 fJ/conversion-step.

OTA-Free 1–1 MASH ADC Using Fully Passive Noise-Shaping SAR &amp; VCO ADC

We present an OTA-free 1–1 multi-stage noise-shaping (MASH) analog-to-digital converter (ADC) utilizing a fully passive noise-shaping successive approximation register (NS-SAR) as the first stage and an open-loop ring voltage-controlled oscillator (VCO) as the second stage. The key contribution of this work is to address the challenge of driving large sampling capacitors for high-resolution NS-SAR. The proposed architecture allows a low-resolution NS-SAR stage and leverages residue attenuation due to passive charge sharing in the NS-SAR to linearize the VCO. The MASH architecture suppresses quantization noise and SAR comparator noise at the ADC output, and the high pass shapes VCO thermal noise. In addition, we demonstrate a computationally inexpensive foreground inter-stage gain calibration algorithm for the proposed ADC architecture. The prototype ADC consumes 0.16 mW while achieving an SNDR/DR of 71.5/75.8 dB over a 1.1-MHz bandwidth and Walden FoM of 23.3 fJ/step, which is the lowest in 65-nm technology.

A 10-Bit 2.5-GS/s Two-Step ADC With Selective Time-Domain Quantization in 28-nm CMOS

In this paper, a single-channel two-step voltage-time hybrid domain analog-to-digital converter (ADC) is proposed. To achieve high sampling rate and high accuracy, 3.5-bit voltage domain MDAC and 7-bit high-speed time domain ADC (TD-ADC) are combined into a 10-bit hybrid ADC. In the first stage MDAC, a low-power push-pull amplifier is used to improve settling speed, and 1-bit redundancy is designed for calibration and dither injection. The TD-ADC with selective time domain quantization is implemented by a constant-current voltage to time converter (VTC) array and a direct positive feedback time domain comparator. The proposed VTC array can maintain high linearity with a large input swing in high-speed application. The prototype ADC was fabricated in a 28-nm CMOS process and occupied a core area of 0.074 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Under a 0.95-V power supply, the chip achieves a measured peak SNDR of 53.2 dB and SFDR of 61.7 dB respectively at conversion rate up to 2.5 GS/s. The FOM is 48.2 fJ/conversion-step.

A 20 MHz Bandwidth 79 dB SNDR SAR-Assisted Noise-Shaping Pipeline ADC With Gain and Offset Calibrations

This article presents a second-order successive approximation register (SAR)-assisted noise-shaping (NS) pipeline analog-to-digital converter (ADC) with optimized noise transfer function (NTF) zeros, background inter-stage gain, and offset calibrations. The NS, realized with an error-feedback (EF) structure, takes place in the second stage of the pipeline. A single amplifier, utilized for residue handover, realizes the EF and the optimized zeros. Copies of the EF capacitors allow their rotation role for residue saving, feedback, and sampling operations, thus omitting the extra phase for the EF operation. While further incorporating a coarse SAR ADC and partial interleaving techniques, the prototype can run at high speed with low power and strong in- band noise suppression. A split-over-time architecture with level-shift-based gain calibration is introduced to ensure the robustness of the ADC, whose hardware also facilitates the offset calibration. Besides, the split second-stage ADCs also share a single comparator, thus enabling a mismatch error insensitive split-ADC architecture with a fast convergence speed of 4000 cycles. Fabricated in 28 nm CMOS, the ADC prototype runs at a 260 MHz sampling rate and retains 20 MHz bandwidth (BW) with a signal-to-noise-and-distortion ratio (SNDR) of 79.1 dB under a small over-sampling ratio (OSR) of 6.5. It consumes 3.1 mW power at a 1 V supply and yields a Schreier figure-of-merit (FoM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> ) of 177.2 dB.

A Calibration-Free, 16-Channel, 50-MS/s, 14-Bit, Pipelined-SAR ADC with Reference/Op-Amp Sharing and Optimized Stage Resolution Distribution

This paper presents a calibration-free, 16-channel, 14-bit, 50-MS/s, pipelined successive approximation register (pipelined-SAR) analog-to-digital converter (ADC) for ultrasound imaging systems. A reference sharing scheme with reduced buffers is proposed to improve area-and-power efficiency, which is essential for multi-channel systems. Based on this, a three-stage, pipelined-SAR ADC architecture with reference/op-amp sharing and optimized stage resolution distribution is proposed. The prototype ADC is designed in a 0.18-μm process with peripheral circuits integrated, including low-voltage differential signaling (LVDS), bandgap, etc. It achieves a robust and calibration-free performance with 68.25-dB signal to noise and distortion ratio (SNDR) and 82.19-dB spurious-free dynamic range (SFDR), translating into a competitive figure of merit (FoM) of 0.47 pJ/conversion-step among other high-resolution ADCs used in ultrasound applications.

A 12-Bit, 100 MS/s SAR ADC Based on a Bridge Capacitor Array with Redundancy and Non-Linearity Calibration in 28 nm CMOS

This paper presents a 12-bit, 100 MS/s successive approximation register (SAR) analog-to-digital converter (ADC) based on a bridge capacitor array with redundancy and non-linearity calibration. The differential non-linearity calibration method was proposed to compensate for the linearity, which is degraded by the parasitic capacitance of the bridge capacitor. To reduce the power dissipation and alleviate the settling error of the DAC capacitor array, a hybrid redundant scheme was proposed. A low-power, high-performance SAR ADC was implemented based on the proposed techniques. This SAR ADC prototype was implemented in 28 nm CMOS technology. Measurement results showed that the proposed SAR ADC could achieve a (signal-to-noise distortion ratio) SNDR of 61.46 dB and 58.82 dB at low and Nyquist input frequencies, respectively, resulting in figure of merits (FOMs) of 8.69 fJ/conversion and 11.8 fJ/conversion step, respectively. The SAR ADC core occupied an active area 0.0227 mm2.

A 56 GS/s 8 Bit Time-Interleaved ADC in 28 nm CMOS

This paper presents a real-time output 56 GS/s 8 bit time-interleaved analog-to-digital converter (ADC), where the full-speed converted data are output by 16-lane transmitters. A 64-way 8 bit asynchronous SAR array using monotonous and split switching strategy with 1 bit redundancy is utilized to achieve a high linearity and high-power efficiency. A low-power ring voltage-controlled oscillator-based injection-locked phase-locked loop combining with a phase interpolator-based time-skew adjuster is developed to generate the 8 equally spaced sampling phases. Digital gain correction, digital-detection-analog-correction offset calibration, and coarse–fine two-step time-skew calibration are combined to optimize the ADC’s performances. An edge detector and phase selector associated with a common near-end data-transmission position and far-end data-collection instant are designed to avoid reset competition and implement deterministic latency. Fabricated in a 28 nm CMOS process, the prototype ADC achieves an outstanding SNDR of 36.38 dB at 56 GS/s with a 19.9 GHz input, where 7.25 dB and 9.33 dB are optimized by offset-gain calibration and time-skew calibration, respectively. The ADC core occupies an area of 1.2 mm2 and consumes 432 mW power consumption.

A 16-Bit Calibration-Free SAR ADC With Binary-Window and Capacitor-Swapping DAC Switching Schemes

This paper presents a 16-bit successive approximation register analog-to-digital converter (ADC) achieving over-100 dB spurious-free dynamic range (SFDR). This ADC uses V_CM-based and binary-window digital-to-analog converter (DAC) switching schemes to improve the signal-to-noise and distortion ratio (SNDR). Moreover, a <i>level-2</i> capacitor swapping scheme is proposed to achieve superior DAC linearity by using two intrinsic true random number sequences. A prototype ADC is fabricated in 180 nm CMOS technology and occupies an active area of 0.53 mm². At 1 MS/s, it consumes a total power of 1.05 mW from a supply of 1.8 V. The measured differential and integral nonlinearity are &#x2212;0.65/&#x002B;0.45 and &#x2212;2.2/&#x002B;2.1 least significant bit. With an input of 1 kHz, the measured SNDR and SFDR are 83 dB and 100 dB. The effective number of bits is 13.5, which is equivalent to a Schreier figure-of-merit of 169.8 dB.

A 1.55-mW 2-GHz ERBW 7-b 800-MS/s Pipelined SAR ADC in 28-nm CMOS Using a 7T Dynamic Residue Amplifier

This letter presents a 3-stage pipelined successive-approximation-resister (SAR) analog-to-digital converter (ADC) using a seven transistor-dynamic residue amplifier (7T-DA) for kickback noise cancelation. Thanks to the pipeline structure, the input capacitance is only 16 fF, achieving an effective resolution bandwidth (ERBW) of 2 GHz. The gain sensitivity to the temperature of the dynamic amplifier is reduced by an on-chip low-dropout voltage regulator (LDO) and a pulse generator. Fabricated in the 28-nm CMOS process, the prototype ADC running at 800 MS/s achieves a peak SNDR of 42.7 dB and SFDR of 52.0 dB at Nyquist input. It also achieves SNDR of 38.9 dB and SFDR of 49.9 dB at a 2-GHz input. The total power consumption of ADC is 1.55 mW, which corresponds to the Waldon figure of merit of 17.4 fJ/conv step and Schreier FoM of 156.8 dB, respectively.

A 9-Bit 1-GS/s Hybrid-Domain Pseudo-Pipelined SAR ADC Based on Variable Gain VTC and Segmented TDC

This paper presents a 9-bit 1 GS/s successive approximation register (SAR) analog-to-digital converter (ADC). In this hybrid architecture, the pseudo-pipeline operation is realized, which increases the sampling rate effectively. The ADC adopts two key technologies: the variable gain voltage-to-time converter (VTC), which ensures the linearity is not sacrificed; the segmented time-to-digital converter (STDC), which further improves the linearity of time domain quantization. The prototype ADC is simulated in a standard 65-nm CMOS process with an active area of 0.038 mm2. The simulated SNDR and SFDR are 44.3 and 58 dB with a sampling rate of 1 GS/s. The FoMW and FoMS are 24.7 fJ/conv-step and 150.7 dB, respectively.

A CT 2–2 MASH ΔΣ ADC With Multi-Rate LMS-Based Background Calibration and Input-Insensitive Quantization-Error Extraction

This article presents a continuous-time (CT) multistage noise-shaping (MASH) delta–sigma ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \Sigma$ </tex-math></inline-formula> ) analog-to-digital converter (ADC) with enhanced tolerance to temperature and operating-frequency variations through on-chip multi-rate (MR) background calibration based on the least-mean-square (LMS) algorithm. The proposed digital calibration, which combines MR operation, post-conditioners, and pseudorandom-noise injection at the quantizer input, efficiently and accurately matches digital transfer functions to the corresponding analog ones for cancellation of quantization error. The first modulator in the proposed MASH ADC considers sufficient quantizer delay and reduces input-signal leakage into the second modulator to enable successful quantization-error transmission while minimizing distortion in the second stage, which cannot be tolerated in the case of calibrated digital noise-cancellation filter. A CT 2–2 MASH <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \Sigma $ </tex-math></inline-formula> ADC prototype fabricated in 40-nm CMOS achieves the dynamic range (DR) of 79 dB and the signal-to-noise and distortion ratio (SNDR) of 78.5 dB over an 8-MHz bandwidth with 7.3-mW analog power and 16-mW digital power, reaching Schreier figure of merit (FoMs) of 168.9 dB, and demonstrates conversion-accuracy robustness within 1-dB SNDR fluctuation for temperature variations from −40° to 125° and within 2-dB SNDR degradation for frequency variations of 20% range.

A 28-nm 10-b 2.2-GS/s 18.2-mW Relative-Prime Time-Interleaved Sub-Ranging SAR ADC With On-Chip Background Skew Calibration

This article presents a relative-prime-based time-interleaved (RP TI) sub-ranging successive-approximation register (SAR) analog-to-digital converter (ADC) with on-chip background skew calibration. The proposed calibration aligns the sampling time of every fine ADC (F-ADC) to that of a particular coarse ADC (C-ADC) that works as a reference ADC. To avoid the unwanted calibration tone from the reference ADC, the C-ADC is also time-interleaved to make all samples undergo the same kick back. By setting the numbers of the time-interleaved channels of the C-ADCs and F-ADCs in a relative-prime relationship, every C-ADC can be evenly shared by every F-ADC; thus, the timing skews between the interleaved sub-ADCs are calibrated by adjusting the sampling edges of every F-ADC to the particular C-ADC working as a reference ADC. An 18-channel TI 10-bit 2.2-GS/s SAR ADC was implemented as a prototype with 28-nm CMOS. Owing to the proposed on-chip background skew calibration, the peak tone by skew was reduced by 23 dB from -40 to -63 dB, which corresponds to the residual skew reduction from 1.6 ps to 113 fs near the Nyquist input. Thus, the prototype ADC achieved a spurious free dynamic range (SFDR) over 52.8 dB and a signal-to-noise-and-distortion ratio (SNDR) over 44.9 dB with 18.2-mW power consumption, which leads to a Walden figure-of-merit (FoM) of 57.8 fJ/conversion-step.

A Fully Dynamic Low-Power Wideband Time-Interleaved Noise-Shaping SAR ADC

This article proposes a fully dynamic, low-power, wideband, time-interleaved (TI), noise-shaping (NS) successive-approximation-register (SAR) analog-to-digital converter (ADC) based on the cascade of integrators with feedforward (CIFF) architecture. Both the interleaving operation and the loop filter are implemented by using the fully passive switched-capacitor circuits. The feedforward summation is realized by a multi-path dynamic comparator. The overall noise transfer function (NTF) is highly robust against process, voltage, and temperature (PVT) variations since the NTF is determined by device ratios. This allows the NTF zeros to be close to 1, with a good NS effect. Compared with a recently published TI NS SAR based on the error-feedback (EF) architecture, this work does not need a static amplifier, thus leading to the fully dynamic operation. Meanwhile, it eliminates the dependence of NTF on the PVT-sensitive amplifier gain. Implemented in a 40-nm CMOS process, the prototype ADC achieves an SNDR of 69.1 dB, a bandwidth of 50 MHz, and a power consumption of 8.5 mW under a 1.1-V supply, leading to a Walden FoM of 36.3 fJ/conv.-step.

A 13-bit 312.5-MS/s Pipelined SAR ADC With Open-Loop Integrator-Based Residue Amplifier and Gain-Stabilized Integration Time Generation

This article describes a gain-stabilized integration time generation technique suitable for the pipelined successive approximation register (SAR) analog-to-digital converter (ADC) utilizing an open-loop integrator-based residue amplifier (RA). The gain variation of RA under different process, voltage, and temperature (PVT) conditions is alleviated by adaptively adjusting the length of the generated integration time. A 13-bit 312.5-MS/s two-stage pipelined SAR ADC employing the presented technique is fabricated in a 28-nm process. At a sampling rate of 312.5 MS/s, the prototype ADC achieves a 65.1-dB SNDR and a Walden FoM of 13.95 fJ/conversion-step with an over Nyquist input. Less than 1-dB SNDR variation are obtained for supply voltage varying within ±60 mV and the temperature varying from -40 °C to 120 °C, respectively.

An 8-Bit 800 MS/s Loop-Unrolled SAR ADC With Common-Mode Adaptive Background Offset Calibration in 28 nm FDSOI

This paper presents a low-power single-channel 8-bit loop-unrolled (LU) successive approximation register (SAR) analog-to-digital-converter (ADC) with a novel common-mode adaptive background comparator offset calibration scheme. LU-SAR ADCs use multiple comparators to reduce the SAR loop delay. Offset mismatch between the comparators severely degrades the effective resolution. This paper addresses the common-mode voltage variation in the LU-SAR architecture due to comparator kickback and the problems related to the common-mode dependency of the comparator offset. The proposed offset calibration scheme ensures that the comparators are calibrated at the same input common-mode voltage at which they each operate during the SAR conversion to prevent the common-mode dependent offset mismatch between them. Moreover, the proposed ADC design exploits the common-mode variation immunity of the proposed calibration scheme to optimize the figure-of-merit (FoM). The prototype ADC manufactured in 28nm FDSOI CMOS achieves 42.57dB signal-to-noise-and-distortion ratio and 22.8fJ/conv.-step FoM at 800MS/s with near Nyquist frequency input, and occupies an area of 0.0037mm2.

A 1 GS/s 10bit SAR ADC with background calibration in 28 nm CMOS

This paper presents a two-channel 10-bit successive approximation register (SAR) analog-to-digital converter (ADC) in 28 nm CMOS. Multi-bit/cycle SAR ADC with redundancy is proposed. Novel background calibration for multi-bit/cycle SAR ADC is applied in the prototype ADC monitoring and calibrating the offset between DACs. The ADC achieves both high speed and low power by combining several features, namely digital calibration, redundancy, foreground calibrated dynamic comparator and leakage-immune dynamic logic. According to the simulation, the proposed calibration technique significantly improves linearity exhibiting 77.69 dB SFDR and 60.95 dB SNDR with Nyquist-frequency input at 1 GS/s sampling rate.

A Conversion Mode Reconfigurable SAR ADC for Multistandard Systems

A single-channel reconfigurable successive approximation register (SAR) analog-to-digital converter (ADC) is presented, which features its speed expanding with conversion mode. The reconfigurable capacitor digital to analog converter (CDAC) is proposed to achieve multiple conversion modes without wasting capacitance. In 1-b/cycle conversion mode, the proposed ADC can achieve the sampling rate of 60-Ms/s and 9-b resolution. Based on the 2-b/cycle conversion mode, the proposed ADC can double the sampling rate and be reconfigured as a 120-MS/s, 8-b converter. Besides, the detection skip algorithm and charge sharing technology are involved to remove the precharge operation time and reduce the switching energy consumption. Moreover, the control logic is optimized to minimize the chip area and matches the reconfigurable characteristics well. A prototype ADC is fabricated in the 180-nm standard CMOS process, which achieves the 54.1-/46.7-dB signal-to-noise-plus-distortion ratio (SNDR) at 60-/120-MHz sampling frequency with the power consumption of 1.9/3.5 mW. The prototype ADC achieves a peak figure of merit (FoM) of 77-fJ/Conv.step at 2-b/cycle conversion mode. The ADC core occupies an active area of only 0.12 mm2.

A 96dB Dynamic Range 2kHz Bandwidth 2nd Order Delta-Sigma Modulator Using Modified Feed-Forward Architecture With Delayed Feedback

This brief presents a second-order discrete-time (DT) modified feed-forward (FF) delta-sigma modulator. To reduce the attenuation of the quantizer's input signal due to switched-capacitor (SC) passive summing, the proposed modulator eliminates the internal FF path and reduces the number of input signals of the adder. A 4-bit asynchronous successive-approximation-register (SAR) analog-to-digital converter (ADC) incorporated with a passive adder is used to reduce power consumption and area. To allow the conversion delay of the SAR ADC, a delayed feedback is adopted. The prototype ADC is fabricated in a 0.11 μm CMOS process using four metal layers with an active die area of 0.165mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . It achieves a dynamic range (DR) of 96.3 dB and a peak signal-to-noise and distortion ratio (SNDR) of 93.9 dB in a 2 kHz signal bandwidth while consuming 62.43 μW from a 1.8V/1.65V power supply, corresponding to a Schreier figure-of-merit (FOM) of 171dB.

A 6-Bit 1.5-GS/s SAR ADC With Smart Speculative Two-Tap Embedded DFE in 130-nm CMOS for Wireline Receiver Applications

Implementing wireline receivers with a front-end analog-to-digital converter (ADC) allows for complex, flexible, and robust signal processing algorithms in the digital domain, as well as easy implementation of advanced modulation schemes beyond binary PAM2. However, the power consumption of the ADC and ensuing digital equalization is a key issue for such receivers in high-speed applications. Embedding analog equalization inside the ADC architecture allows for both a lower ADC resolution and a reduced-complexity digital equalizer, resulting in a more power-efficient receiver. This article presents a 6-bit 1.5-GS/s time-interleaved successive approximation register (SAR) ADC with low-overhead two-tap embedded decision-feedback equalizer (DFE). A smart speculative DFE is proposed to reduce additional conversion cycles required for the equalization realization in the ADC. Moreover, DFE functions are efficiently embedded in the capacitive digital-to-analog converter (DAC) references. The prototype ADC with two-tap DFE is implemented in a 130-nm CMOS process and achieves a 5.24-bit peak effective number of bits and 0.59-pJ/conversion-step figure-of-merit (FOM) at a 1.5-GS/s sampling rate while consuming 34.1 mW and occupying a core area of 0.32 mm2. The effectiveness of the embedded DFE in timing margin improvement is verified for 1.5-Gb/s operation over high-loss FR4 channels at a bit error rate (BER) of 10−9.