A 1.2 V 10-bit 100 MS/s Successive Approximation (SA) ADC is presented. The scheme achieves high-speed and low-power operation thanks to the reference-free technique that avoids the static power dissipation of an on-chip reference generator. Moreover, the use of a common-mode based charge recovery switching method reduces the switching energy and improves the conversion linearity. A variable self-timed loop optimizes the reset time of the preamplifier to improve the conversion speed. Measurement results on a 90 nm CMOS prototype operated at 1.2 V supply show 3 mW total power consumption with a peak SNDR of 56.6 dB and a FOM of 77 fJ/conv-step.

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A 10-bit 100-MS/s Reference-Free SAR ADC in 90 nm CMOS

An 8b 1.2 GS/s single-channel Successive Approximation Register (SAR) ADC is implemented in 32 nm CMOS, achieving 39.3 dB SNDR and a Figure-of-Merit (FoM) of 34 fJ per conversion step. High-speed operation is achieved by converting each sample with two alternate comparators clocked asynchronously and a redundant capacitive DAC with constant common mode to improve the accuracy of the comparator. A low-power, clocked capacitive reference buffer is used, and fractional reference voltages are provided to reduce the number of unit capacitors in the capacitive DAC (CDAC). The ADC stacks the CDAC with the reference capacitor to reduce the area and enhance the settling speed. Background calibration of comparator offset is implemented. The ADC consumes 3.1 mW from a 1 V supply and occupies 0.0015 mm2.

A 3.1 mW 8b 1.2 GS/s Single-Channel Asynchronous SAR ADC With Alternate Comparators for Enhanced Speed in 32 nm Digital SOI CMOS

Successive-approximation analog-to-digital converters (SA-ADCs) are widely used in ultra-low-power applications. In this paper, the power consumption and the linearity of capacitive-array digital-to-analog converters (DACs) employed in SA-ADCs are analyzed. Specifically, closed-form formulas for the power consumption as well as the standard deviation of INL and DNL for three commonly-used radix-2 architectures including the effect of parasitic capacitances are presented and the structures are compared. The proposed analysis can be employed in choosing the best architecture and optimizing it in both hand calculations and computer-aided-design tools. Measurement results of previously published works as well as simulation results of a 10-bit 10 kS/s SA-ADC confirm the accuracy of the proposed equations. It will be shown that, in spite of what commonly is assumed, although the total capacitance and the power consumption of those architectures employing attenuating capacitors seem to be smaller than conventional binary-weighted structures, the linearity requirements impose much larger unit capacitance to the structure such that the entire power consumption is larger.

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Analysis of Power Consumption and Linearity in Capacitive Digital-to-Analog Converters Used in Successive Approximation ADCs

This paper describes an ultra-low power SAR ADC for medical implant devices. To achieve the nano-watt range power consumption, an ultra-low power design strategy has been utilized, imposing maximum simplicity on the ADC architecture, low transistor count and matched capacitive DAC with a switching scheme which results in full-range sampling without switch bootstrapping and extra reset voltage. Furthermore, a dual-supply voltage scheme allows the SAR logic to operate at 0.4 V, reducing the overall power consumption of the ADC by 15% without any loss in performance. The ADC was fabricated in 0.13-μm CMOS. In dual-supply mode (1.0 V for analog and 0.4 V for digital), the ADC consumes 53 nW at a sampling rate of 1 kS/s and achieves the ENOB of 9.1 bits. The leakage power constitutes 25% of the 53-nW total power.

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A 53-nW 9.1-ENOB 1-kS/s SAR ADC in 0.13- $\mu$ m CMOS for Medical Implant Devices

Although charge-redistribution successive approximation (SAR) ADCs are highly efficient, comparator noise and other effects limit the most efficient operation to below 10-b ENOB. This work introduces an oversampling, noise-shaping SAR ADC architecture that achieves 10-b ENOB with an 8-b SAR DAC array. A noise-shaping scheme shapes both comparator noise and quantization noise, thereby decoupling comparator noise from ADC performance. The loop filter is comprised of a cascade of a two-tap charge-domain FIR filter and an integrator to achieve good noise shaping even with a low-quality integrator. The prototype ADC is fabricated in 65-nm CMOS and occupies a core area of 0.03 mm2. Operating at 90 MS/s, it consumes 806 μW from a 1.2-V supply.

A 90-MS/s 11-MHz-Bandwidth 62-dB SNDR Noise-Shaping SAR ADC

This paper presents a reconfigurable, low offset, low noise and high speed dynamic clocked-comparator for medium to high resolution Analog to Digital Converters (ADCs). The proposed comparator reduces the input referred noise by half and shows a better output driving capability when compared with the previous work. The offset, noise and power consumption can be controlled by a clock delay which allows simple reconfiguration. Moreover, the proposed offset calibration technique improves the offset voltage from 11.6mV to 533μV at 1 sigma. A prototype of the comparator is implemented in 90nm 1P8M CMOS with experimental results showing 320μV input referred noise at 1.5GHz with 1.2V supply.

A reconfigurable low-noise dynamic comparator with offset calibration in 90nm CMOS

An 8b 1GS/s ADC is presented that interleaves two 2b/cycle SARs. To enhance speed and save power, the prototype utilizes segmentation switching and custom-designed DAC array with high density in a low parasitic layout structure. It operates at 1GS/s from 1V supply without interleaving calibration and consumes 3.8mW of power, exhibiting a FoM of 24fJ/conversion step. The ADC occupies an active area of 0.013mm2 in 65nm CMOS including on-chip offset calibration.

A 3.8mW 8b 1GS/s 2b/cycle interleaving SAR ADC with compact DAC structure

This paper presents the linearity analysis of a successive approximation registers (SAR) analog-to-digital converters (ADC) with split DAC structure based on two switching methods: conventional charge-redistribution and Vcm-based switching. The static linearity performance, namely the integral nonlinearity and differential nonlinearity, as well as the parasitic effects of the split DAC, are analyzed hereunder. In addition, a code-randomized calibration technique is proposed to correct the conversion nonlinearity in the conventional SAR ADC, which is verified by behavioral simulations, as well as measured results. Performances of both switching methods are demonstrated in 90 nm CMOS. Measurement results of power, speed, and linearity clearly show the benefits of using Vcm-based switching.

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Split-SAR ADCs: Improved Linearity With Power and Speed Optimization

This paper presents a 2 $\times $ time-interleaved 7-b 2.4-GS/s 1-then-2 b/cycle SAR ADC in 28-nm CMOS. The process-voltage-temperature sensitivity of a multi-bit SAR architecture has been improved by the proposed 1-then-2 b/cycle scheme with background offset calibration. With the pre-charge reduction scheme, the traditional large switching energy and time consuming pre-charge operation have been removed, which simultaneously enables a simple control logic without the need of a ${V}_{\mathrm{ cm}}$ voltage. Besides, a background offset calibration is implemented on chip which does not involve any extra phase or calibration input signal. Its operation is well embedded within the 1-then-2 b/cycle architecture, thus leading to a very minimal modification of the ADC core. With an improved fringing DAC structure and a high-speed dynamic logic circuit, a single-channel ADC can work at 1.2 GS/s under a 0.9-V supply. Using two-way time interleaving, the prototype samples at 2.4 GHz and consumes 5-mW power including the on-chip background offset calibration. It exhibits a 40.05-dB SNDR at Nyquist, leading to a Walden FoM of 25.3 fJ/conversion step. Measurement results show that the SNDR of the ADC can be kept above 38 dB at 2 GS/s under a wide range of temperature, supply, and input common-mode variation.

Yan Zhu

Papers

A 10-bit 100-MS/s Reference-Free SAR ADC in 90 nm CMOS

A reconfigurable low-noise dynamic comparator with offset calibration in 90nm CMOS

A 3.8mW 8b 1GS/s 2b/cycle interleaving SAR ADC with compact DAC structure

Split-SAR ADCs: Improved Linearity With Power and Speed Optimization

A Two-Way Interleaved 7-b 2.4-GS/s 1-Then-2 b/Cycle SAR ADC With Background Offset Calibration