1316 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 52, NO. 7, JULY 2005 A Floating-Gate Comparator With Automatic Offset Adaptation for 10-bit Data Conversion Yanyi Liu Wong, Student Member, IEEE, Marc H. Cohen, Member, IEEE, and Pamela A. Abshire, Member, IEEE Abstract—We present a novel voltage comparator that uses nonvolatile floating-gate charge storage for either offset nulling or automatic programming of a desired offset. We exploit the negative feedback mechanism of pFET hot-electron injection to achieve fully automatic offset cancellation. The adaptation guarantees an input offset less than the input-referred noise level regardless of initial device offset for a typical 8.3% observed injection mismatch. In addition, we demonstrate the ability to accurately program a desired offset. The design has been fabri- cated in a commercially available 0.35- m process. Experimental results confirm the ability to reduce the variance of the initial offset by two orders of magnitude and to accurately program a desired offset with maximum observed deviation 728 V and typical deviation 109 V. The mean offset is reduced by a factor of 416 relative to fabricated chips directly from the foundry and by a factor of 202 relative to UV-irradiated chips. Adaptation is fast, with settling time typically under 50 ms and scaling inversely with the exponential of the injection voltage. We achieve controlled injection to accurately program the input offset to voltages uni- formly distributed from 1 to 1 V. The comparator exhibits a 5 ns propagation delay and consumes 270 W. Index Terms—Adaptive systems, analog–digital conversion, analog memories, calibration, CMOS integrated circuits, com- parators, floating gate, hot carriers, offset cancellation, very large-scale integration (VLSI). I. INTRODUCTION C OMPARATORS are decision-making circuits that inter- face between analog and digital signals. Comparators are used in a wide variety of circuit applications, including analog-to-digital converters, memories, dynamic logic, and sense amplifiers. A comparator usually consists of a pre-am- plifier stage and a regenerative stage followed by a buffer. Mismatches due to process variation in the pre-amplifier and regenerative stages cause offset that directly affects resolution. A common and successful approach used to cancel offset is dynamic switching [1], which requires additional circuit Manuscript received April 12, 2004; revised October 25, 2004. The work of Y. L. Wong was supported by Johns Hopkins University Applied Physics Labo- ratory. The work of P. A. Abshire was supported by the National Science Foun- dation under CAREER Award 0238061. This paper was recommended by As- sociate Editor G. Cauwenberghs. Y. L. Wong is with the Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20742 USA, and also with the Ap- plied Physics Laboratory, The Johns Hopkins University, Baltimore, MD 21218 USA (e-mail: Yanyi.Wong@ieee.org). M. H. Cohen is with the Institute for Systems Research, University of Mary- land, College Park, MD 20742 USA and also with Gentag Inc., Potomac, MD 20854 USA (e-mail: Marc.Cohen@ieee.org). P. A. Abshire is with the Institute for Systems Research, University of Mary- land, College Park, MD 20742 USA, and also with the Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20742 USA (e-mail: Pamela.Abshire@ieee.org). Digital Object Identifier 10.1109/TCSI.2005.851389 components and multiple nonoverlapping clocks. We report an adaptation method that requires a single switch and one clock signal to either program or cancel an offset. Since offset is a property of the circuit, it is natural to store it using nonvolatile storage on a floating gate. Floating-gate circuits have been used to cancel offsets in imagers [2], to trim current sources [3]–[5], and to autozero amplifiers [6], [7]. The ability to program desired nonzero offsets in comparators is a feature that is not readily available using existing offset cancellation techniques but is intrinsic to the voltage comparator we describe here. We present the design of a comparator that automatically and accurately cancels offset, or depending on the application, can store a predetermined offset [8]. The offset may be cancelled or programmed in either a one-shot or continuous fashion to calibrate for constant or changing conditions; the offset is re- tained using nonvolatile local storage, and for certain applica- tions it is not necessary to recalibrate dynamically. The calibra- tion mechanism is self-limiting and converges to a stable value without user intervention. In Section II, we discuss methods for manipulating charge on floating-gate nodes. In Section III, we introduce a novel adaptive circuit that uses nonvolatile charge storage to achieve accurate, fast voltage comparison. We dis- cuss two calibration methods and compare their effectiveness both analytically and experimentally. We also analyze the de- sign tradeoff between resolution and speed for this circuit. We use Monte Carlo simulations in HSPICE to compare the input offset distributions of a comparator without floating-gate tran- sistors to those of a comparator with floating-gate offset cancel- lation before and after calibration. In Section IV, we present de- tailed experimental results on calibration accuracy, conversion accuracy, offset distribution before and after calibration, and the time course of calibration. We report statistical parameters for injection mismatch and the bias dependence of injection time constant as determined from empirical measurements. II. BACKGROUND A floating-gate MOSFET uses an electrically isolated mate- rial as its gate. There are no direct electrical connections to this circuit node, so charge on this gate remains trapped for a very long time. Offset correction for the comparator reported in this paper is stored in this high-retention charge form, and altered by means of injection. The impact-ionized hot-electron injection mechanism has been extensively described in the literature [7], [9], and the injection feedback mechanisms for both nFET and pFET are discussed in detail in [10]. We briefly summarize the pFET injection feedback mechanism. For pFET injection, a constant current configuration exhibits negative feedback from the drain voltage to the floating-gate voltage, which drives the floating 1057-7122/$20.00 © 2005 IEEE