Skyler Weaver

Ring amplifiers for switched-capacitor circuits

In this paper the fundamental concept of ring amplification is introduced and explored. Ring amplifiers enable efficient amplification in scaled environments, and possess the benefits of efficient slew-based charging, rapid stabilization, compression-immunity (inherent rail-to-rail output swing), and performance that scales with process technology. A basic operational theory is established, and the core benefits of this technique are identified. Measured results from two separate ring amplifier based pipelined ADCs are presented. The first prototype IC, a simple 10.5-bit, 61.5 dB SNDR pipelined ADC which uses only ring amplifiers, is used to demonstrate the core benefits. The second fabricated IC presented is a high-resolution pipelined ADC which employs the technique of Split-CLS to perform efficient, accurate amplification aided by ring amplifiers. The 15-bit ADC is implemented in a 0.18 μm CMOS technology and achieves 76.8 dB SNDR and 95.4 dB SFDR at 20 Msps while consuming 5.1 mW, achieving a FoM of 45 fJ/conversion-step.

Stochastic Flash Analog-to-Digital Conversion

A stochastic flash analog-to-digital converter (ADC) is presented. A standard flash uses a resistor string to set individual comparator trip points. A stochastic flash ADC uses random comparator offset to set the trip points. Since the comparators are no longer sized for small offset, they can be shrunk down into digital cells. Using comparators that are implemented as digital cells produces a large variation of comparator offset. Typically, this is considered a disadvantage, but in our case, this large standard deviation of offset is used to set the input signal range. By designing an ADC that is made up entirely of digital cells, it is a natural candidate for a synthesizable ADC. Comparator trip points follow the nonlinear transfer function described by a Gaussian cumulative distribution function, and a technique is presented that reduces this nonlinearity by changing the overall transfer function of the stochastic flash ADC. A test chip is fabricated in 0.18- CMOS to demonstrate the concept.

Digitally Synthesized Stochastic Flash ADC Using Only Standard Digital Cells

An ADC is synthesized entirely from Verilog code in 90nm digital CMOS using a standard digital cell library. An analog comparator is generated by cross-coupling two 3-input NAND gates. The random comparator offsets are used as the ADC references and are Gaussian. An implicitly aligned three-section piecewise-linear inverse Gaussian CDF function on chip linearizes the output. SNDR of 35.9dB is achieved at 210MSPS.

Design of a Split-CLS Pipelined ADC With Full Signal Swing Using an Accurate But Fractional Signal Swing Opamp

Building on the technique of correlated level shifting (CLS), Split-CLS is introduced as a viable way to enable the design of high performance, high resolution A/D converters in deep submicron CMOS processes. One possible implementation of Split-CLS is presented, which achieves very high effective gain, and combines the fast, high efficiency charging of a zero-crossing based circuit (ZCBC) with the high-accuracy, low power settling of a double-cascode telescopic opamp. A dynamic zero-crossing detector (ZCD) conserves power in the ZCBC by only creating high bandwidth in the ZCD near the zero-crossing instant. Measured results are presented from a pipelined A/D converter fabricated in 0.18 m CMOS. Using the Split-CLS structure, an opamp with 300 mV output swing is used to produce a pipeline stage output swing of 1.4 V. The proof-of-concept test chip achieves 68.3 dB SNDR (11.1b ENOB) and 76.3dB SFDR while sampling at 20 MHz, and consumes 17.2 mW at 1.8 V supply.

A 6b stochastic flash analog-to-digital converter without calibration or reference ladder

A 6-bit stochastic flash ADC is presented. By connecting many comparators in parallel, a reference ladder is avoided by allowing random offset to set individual trip points. The ADC transfer function is the cumulative density function of comparator offset. A technique is proposed to improve transfer function linearity by 8.5 dB. A test chip, fabricated in 0.18 mum CMOS, achieves ENOB over 4.9 b up to 18 MS/s with 900 mV supply and comparator offset standard deviation of 140 mV Comparators are digital cells to allow automated synthesis. Total core power consumption when fs = 8 MHz is 631muW.

A 61.5dB SNDR pipelined ADC using simple highly-scalable ring amplifiers

A ring amplifier based pipelined ADC is presented that uses simple cells constructed from small inverters and capacitors to perform amplification. The basic ring amplifier structure is characterized and demonstrated to be highly scalable, power efficient, and compression-immune (inherent rail-to-rail output swing). The prototype 10.5-bit ADC, fabricated in 0.18μm CMOS technology, achieves 61.5dB SNDR at a 30MHz sampling rate and consumes 2.6mW, resulting in a FoM of 90fJ/conversion-step.

A 1.4V signal swing hybrid CLS-opamp/ZCBC pipelined ADC using a 300mV output swing opamp

Scaling in CMOS technologies has made the application of traditional opamp topologies increasingly difficult. In the face of decreasing voltage headroom and intrinsic device gain, designers have employed techniques such as gain-boosting, correlated double sampling , and correlated level-shifting (CLS) [1] to maximize output swing for a given gain specification. Zero-crossing based circuits (ZCBC) remove the opamp altogether and use a comparator and current sources [2], which are more amenable to scaling and have proven capable of high efficiency, as in [3]. However, the open loop nature of ZCBC creates challenges for designs that must reliably track over process, voltage, and temperature. In this paper, we describe a hybrid CLS-opamp/ZCBC pipelined ADC that introduces techniques to improve accuracy, robustness, and power efficiency in scaled technologies. It incorporates CLS and a low power, small output swing double-cascoded telescopic opamp to achieve very high effective gain. A dynamically biased zero-crossing detector (ZCD) is introduced that increases the power efficiency of ZCBC designs.

Domino-Logic-Based ADC for Digital Synthesis

A low-power synthesizable analog-to-digital converter (ADC) is presented. By cascading many digital-like domino-logic cells whose propagation delay is influenced by an analog input voltage, a digital value is obtained at the end of the allowed ripple period by determining the number of cells that the ripple passed through. The sample-and-hold is simply a bootstrapped switch into a small sampling capacitor. As each domino-logic cell passes the ripple, charge is kicked back onto the input capacitor, which creates a significant second harmonic. Distortion caused by even harmonics is canceled by implementing a pseudodifferential structure. A test chip is fabricated in 0.18-μm CMOS. The test chip achieves over 5.4-bit effective number of bits up to 50 MS/s with a 1.3-V supply. With a sampling frequency of 50 MS/s and a 24-MHz input, a 34.2-dB signal-to-noise-plus distortion ratio is achieved while consuming 433 μW and occupying only 0.094 mm2.

10.5 A 5.9pJ/b 10Gb/s serial link with unequalized MM-CDR in 14nm tri-gate CMOS

High-speed serial links integrated in advanced CMOS are ubiquitous in modern microprocessor systems. These commodity links have fixed performance specs and therefore realize the benefit of technology scaling in area and power reduction at high data rates. To realize significant scaling benefits, these designs must overcome the challenges associated with implementing analog functions in scaled logic-optimized processes while maintaining link robustness over a wide range of channel characteristics and third party components.This work describes a 2.5-to-10 Gb/s serial link implemented in 14nm tri-gate CMOS using logic-pitch transistors exclusively. The half-rate embedded-clock transceiver architecture consists of a 3-tap current-mode (CM) TX, an RX with a CTLE, a 4-tap integrating DFE, and a phase-interpolator-based CDR. It is 60% smaller and consumes 11% less energy per bit than reported links at comparable data rates and channel losses. It also introduces a baud-rate CDR algorithm that uses the real-time extracted channel response at the DFE to optimize the sampling point, relaxes the headroom/swing tradeoff at the TX driver by using dynamic signal boosting, eliminates the process cost for a precision resistor by using tunable serpentine resistors, and includes a low-power and low-area RX squelch circuit with a digital peak detector.

ENOB calculation for ADCs with input-correlated quantization error using a sine-wave test

The equation for calculating ENOB from SNDR of a sine-wave test is only accurate when noise is uncorrelated to the input. In this paper, the equation for calculating ENOB from SNDR is derived for an ideal and a uniform stochastic ADC. The result of these derivations shows that calculating ENOB from SNDR using the conventional equation causes a better-than-actual result in the case a uniform stochastic ADC.