A power-scalable SAR ADC for sensor applications is presented. The ADC features a reconfigurable 5-to-10-bit DAC whose power scales exponentially with resolution. At low resolutions where noise and linearity requirements are reduced, supply voltage scaling is leveraged to further reduce the energy-per-conversion. The ADC operates up to 2 MS/s at 1 V and 5 kS/s at 0.4 V, and its power scales linearly with sample rate down to leakage levels of 53 nW at 1 V and 4 nW at 0.4 V. Leakage power-gating during a SLEEP mode in between conversions reduces total power by up to 14% at sample rates below 1 kS/s. Prototyped in a low-power 65 nm CMOS process, the ADC in 10-bit mode achieves an INL and DNL of 0.57 LSB and 0.58 LSB respectively at 0.6 V, and the Nyquist SNDR and SFDR are 55 dB and 69 dB respectively at 0.55 V and 20 kS/s. The ADC achieves an optimal FOM of 22.4 fJ/conversion-step at 0.55 V in 10-bit mode. The combined techniques of DAC resolution and voltage scaling maximize efficiency at low resolutions, resulting in an FOM that increases by only 7x over the 5-bit scaling range, improving upon a 32x degradation that would otherwise arise from truncation of bits from an ADC of fixed resolution and voltage.

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A Resolution-Reconfigurable 5-to-10-Bit 0.4-to-1 V Power Scalable SAR ADC for Sensor Applications

A single-channel, asynchronous successive-approximation (SA) ADC with improved feedback delay is fabricated in 40 nm CMOS. Compared with a conventional SAR structure that employs a single quantizer controlled by a digital feedback logic loop, the proposed SAR-ADC employs multiple quantizers for each conversion bit, clocked by an asynchronous ripple clock that is generated after each quantization. Hence, the sampling rate of the 6-bit ADC is limited only by the six delays of the Capacitive-DAC settling and each comparator's quantization delay, as the digital logic delay is eliminated. Measurement results of the 40 nm-CMOS SAR-ADC achieves a peak SNDR of 32.9 dB and 30.5 dB, at 1 GS/s and 1.25 GS/s, consuming 5.28 mW and 6.08 mW, leading to a FoM of 148 fJ/conv-step and 178 fJ/conv-step, respectively, in a core area less than 170 um by 85 um.

A Single-Channel, 1.25-GS/s, 6-bit, 6.08-mW Asynchronous Successive-Approximation ADC With Improved Feedback Delay in 40-nm CMOS

The regenerative comparator circuit which lies at the heart of A/D conversion, slicer circuits, and memory sensing, is unstable, time-varying, nonlinear, and with multiple equilibria. That does not mean, as this paper shows, that it cannot be understood with simple equivalent circuits that reveal its dynamics completely, and enable it to be designed to specifications on static and dynamic offset and noise. The analysis is applied to the StrongArm latch. I. INTRODUCTION Flip-flops used as regenerative amplifiers are found every- where in electronic circuits. It is well-understood that they have two stable states, and that given sufficient time, the circuit will regenerate an input voltage unbalance to reach one of these states. The latched comparator must be symmetric by its very nature, since its binary states are symmetrical. Practical unbal- ances in the circuit arising from transistor and load mismatch lead to uncertainty in the regenerated binary output when a small analog input is applied. This problem of offset is well- known to circuit designers, and since the observed offsets can be much larger than in simple linear amplifiers, a certain mystery attends to "dynamic offsets" that appear only in a latched comparator. This paper clears much of the mystery by visualizing regen- eration. By using one trajectory as a frame of reference, circuit unbalances can in most cases be modelled by a sequence of linear, time-varying equivalent circuits that capture, piecewise, the variation of the circuit with time. Thus, simple expressions are obtained for all offsets, and a straightforward method emerges to null them to a desired accuracy at very small cost in power or chip area.

Understanding the Regenerative Comparator Circuit

We make the case that in most comparators, offset and noise are determined by a dynamic preamplifier always embedded ahead of a regenerative latch. An analysis of this amplifier follows, from which simple expressions are obtained for input-referred offset and noise bandwidth. Practical circuit methods to compensate offset and lower noise become evident. We compare the StrongArm comparator with the double tail topology, and identify requirements for correct operation.

Analysis and Design of Regenerative Comparators for Low Offset and Noise

A successive approximation analog-to-digital converter (ADC) architecture is presented that programs its comparator threshold at runtime to approximate the input signal via binary search. While targeting medium resolutions and speed, the threshold configuring (TC) ADC achieves low power consumption and small area occupation by using a fully dynamic configurable comparator and an asynchronous controller, with no need for a highly linear feedback D/A converter. The TC-ADC embeds its own references, and relies on a minimal amount of passive components or calibration loops. A 6-bit prototype implementation in 90-nm digital CMOS technology achieves 32-dB SNDR at 50 MS/s and consumes 240 μW from 1-V analog and 0.7-V digital supplies. This results in 150 fJ/conversion-step in a core area occupation of only 0.0055 mm .

C. Nani

Papers

A 6-Bit 50-MS/s Threshold Configuring SAR ADC in 90-nm Digital CMOS