Abstract:
A bubble suppression apparatus is disclosed comprising: a first set of AND gates, wherein each AND gate within the first set has an input configured to receive a binary thermometer code value and one or more adjacent binary thermometer code values; and a second set of AND gates, wherein each AND gate within the second set has an input coupled to two or more outputs of the first set of AND gates.

Description:
This application claims the benefit of the filing date for U.S. Provisional Application No. 60/135,826, filed May 21, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to parallel analog-to-digital converters (ADCs) and more particularly to techniques for reducing output errors on a flash ADC. 
     2. Description of the Related Art 
     The objective of analog-to-digital (“ADC”) converters is to receive an analog signal and produce an error free digitized version of that analog signal. A well known ADC circuit is a parallel or “flash” ADC, which receives an analog signal to be digitized and compares its voltage to a set of reference voltages. As illustrated in FIG. 1, for “n” bits of resolution in the ADC&#39;s digital output, a known voltage is applied to a ladder of 2 n  series resistors  110  to provide reference voltages at the nodes between the resistors  110 . Comparators  120 , typically implemented with high gain amplifiers, receive respective reference voltages at one input and the analog input signal at another input to produce either a low output (binary 0) if the comparator&#39;s reference voltage is greater than the analog input, or a high or “active” output (binary 1) if the analog input is higher than the comparator&#39;s reference voltage. Ideally, the resulting digital comparator  120  outputs, referred to as “thermometer code,” are a series of binary 0s for comparators which receive reference voltages greater than the analog input signal, and a series of binary is for comparators which receive a reference voltage less than the analog input signal. The transition from binary 0s to 1s indicates the amplitude of the analog input signal. 
     In a typical flash-type Analog-to-Digital Converter (ADC) shown in FIG. 1, the task of the digital back-end is to process the thermometer code output of the 2 n −1 comparators  120  and generate an equivalent n-bit digital binary output. There are numerous ways of achieving this goal. One commonly used scheme is to configure a 1-of-(2 n −1) decoder  130  followed by a Read-Only-Memory (ROM)  140  look-up table. The decoder  130  detects the 0-to-1 transition in the thermometer code output of the comparators  120  and activates the corresponding output line which enables a ROM address to place its contents on the output data-bus. 
     When high slew-rate input signals are sampled, timing skew between the clock and signal paths and differences in comparator  120  response-times can cause the effective strobe point of the comparators to be different. This can lead to an irregular thermometer code pattern where a 0 can be found below a 1 or a 1 above a 0. These unwanted irregularities are known as “bubbles” and if they are not adequately suppressed they can cause severe ROM output errors, known as “sparkle” code errors, as more than one address can become enabled at the same time. These large output errors can have significant impact on the Signal-to-Noise Ratio (SNR) performance of the ADC. 
     Several methods of preventing bubbles from causing sparkle code errors have been reported. The suppression scheme may be implemented in the 1-of-(2 n −1) decoder and/or in the ROM. Some of the well known schemes include “Democratic Decoding” (C. W. Mangelsdorf, “A 400 MHz Input Flash Converter with Error Correction”, IEEE J. Solid-State Circuits, vol. 25, pp. 184-191, February 1990); “Quasi-Gray” Code (Y. Akazawa et al., “A 400MSPS 8-bit Flash A/D Conversion LSI”, in ISSCC Dig. Tech. Papers, pp. 98-99, February 1987); “Half-Gray” Code (U.S. Pat. No. 5,633,636. Title: Half-Gray Digital Encoding Method and Circuitry”. Date: May 27, 1997); “Bit-Swap” techniques (V. E. Garuts et al., “A Dual 4-bit 2Gs/s Full Nyquist Analog-to-Digital Converter using a 70 ps Silicon Bipolar Technology with Borosenic-Poly Process and Coupling-Base Implant”, IEEE J. Solid-State Circuits, vol. 24, pp. 216-222, April 1989); and several multi-level ROM topologies. Most of these schemes, however, are quite complex and more suitable to lower sampling rates or, alternatively, require a high-speed bipolar technology which will dissipate large amounts of power. 
     SUMMARY OF THE INVENTION 
     A bubble suppression apparatus is disclosed comprising: a first set of AND gates, wherein each AND gate within the first set has an input configured to receive a binary thermometer code value and one or more adjacent binary thermometer code values; and a second set of AND gates, wherein each AND gate within the second set has an input coupled to two or more outputs of the first set of AND gates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention can be obtained from the following detailed description in conjunction with the following drawings, in which: 
     FIG. 1 illustrates a prior art flash analog-to-digital converter. 
     FIG. 2 a  illustrates the concept of bubble errors which are corrected by embodiments of the invention. 
     FIG. 2 b  illustrates the concepts of 2-bubble errors and 3-bubble-errors which are corrected by embodiments of the invention. 
     FIG. 3 a  illustrates one embodiment of the invention to which error-free thermometer code is applied. 
     FIG. 3 b  illustrates one embodiment of the invention to which thermometer code with a single bubble error is applied. 
     FIG. 4 illustrates an equation used to suppress bubbles according to one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some of these specific details. In other instances, well-known structures and devices are shown in block diagram form to avoid obscuring the underlying principles of the invention. 
     Embodiments of the present invention include various steps, which will be described below. The steps may be embodied in machine-executable instructions or, alternatively, these steps may be performed by specific hardware components that contain hardwired logic for performing the steps (e.g., an integrated circuit), or by any combination of programmed computer components and custom hardware components. 
     Elements of the present invention may also be provided as a machine-readable medium for storing machine-executable instructions or other types of code/data (e.g., VHDL code). The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnet or optical cards, propagation media or other type of media/machine-readable medium suitable for storing code/data. For example, the present invention may be downloaded as a computer program which may be transferred from a remote computer (e.g., a server) to a requesting computer (e.g., a client) by way of data signals embodied in a carrier wave or other propagation medium via a communication link (e.g., a modem or network connection). 
     EMBODIMENTS OF THE INVENTION 
     As used herein, “ideal thermometer code” refers to error-free thermometer code, wherein a contiguous set of zeros is followed by a contiguous set of ones. In the illustrations and examples set forth below, “ideal” thermometer code will be defined by thermometer code with zeros above ones. It should be noted, however, that the underlying principles of the invention may be implemented with “ideal” thermometer code having ones above zeros. 
     A “bubble” refers to an error in thermometer code wherein a 0 is encountered below a 1 in the thermometer code. “Bubble depth” refers to the position of the bubble with respect to the 0-to-1 transition in the ideal thermometer code (i.e., depth=0). This concept is illustrated in FIG. 2 a . “N-Bubble-Error” refers to the size of the bubble. The most common types are “1-Bubble-Errors” (a 0 with at least a 1 above and below); “2-Bubble-Errors” (two 0s below at least a 1); and “3-Bubble-Errors” (three 0s below at least a 1). FIG. 2 b  illustrates 2-Bubble-Errors and 3-Bubble-Errors, respectively. 
     A new, look-ahead bubble-suppression apparatus and method will now be described. The apparatus and method relies on the fact that when a 2-input gate or greater is used to detect the 0-to-1 transition in the thermometer code output of the comparators, a bubble will cause two or more addresses of known distance apart to become selected. Therefore, within reason it is possible to disable all but one of the selected addresses and hence ensure correct operation of the ROM. 
     FIG. 3 a  illustrates one possible embodiment of the invention where 3-input AND/NAND gates have been used throughout (the gates are NAND gates to the extent that some inputs are inverted and are AND gates to the extent that some inputs are not inverted). More particularly, in the embodiment illustrated in FIG. 3 a , the thermometer code  305  is passed through multiple sets of AND/NAND gates  310 ,  320 , and  330  to filter out bubbles. FIG. 3 a  also shows the correct operation of this embodiment when an ideal thermometer code  305  (without any bubbles) is applied. 
     FIG. 3 b  shows the same embodiment, but in FIG. 3 b  an irregular thermometer code  405  containing a single bubble error of depth (−2) is applied. The effect of this particular thermometer code is to activate the outputs of both gates (n) and (n−3) in the first set of gates  310 . In the absence of the second set of gates  320  and/or the third set of gates  330 , the two activated lines would select and enable two ROM addresses at the same time, thereby causing an error in the output code. 
     However, in the illustrated embodiment, the usage of first and second sets of gates  320  and  330  prevents the occurrence of this type of error by ensuring that only one ROM address can become selected at any one time (i.e., these gates remove bubbles from the ROM code transmitted from the first set of gates  310 ). In this embodiment, the output of each of the first set of gates  310  is coupled to the input of three gates at the second level of gates  320 . More particularly, the output of gate (n) is used to disable gates (n−3′) and (n−4′) (i.e., the active state or ‘1’ 0  output from gate (n) is inverted to a ‘0’ and is applied to gates (n−3′) and (n−4′)). In addition, the output of gate (n) is passed directly through to gate (n′). The end result—using the thermometer code example of FIG. 3 b —is that the bubble output from gate (n−3) is filtered out by the inverted output of gate (n) applied to gate (n−3′). It should be noted that a bubble error of depth (−3) would be filtered in a similar manner (i.e., the active state or ‘1’ from gate (n−4) would be filtered by the inverted output of gate (n) applied to gate (n−4′). 
     If a particular bubble is not filtered out by the second set of gates  320  it may still be filtered by a third set of gates  330  as illustrated in FIG. 3 b . For example, if a bubble caused the output of gate (n−5) to activate improperly, it would pass through the first set of gates  320  (e.g., if the correct active gate is the output of gate (n) as illustrated), but it would be filtered at the second set of gates  330  by the active output of gate (n′), which is inverted and applied to gate (n−5″). Accordingly, only the output of gate (n) will remain active past the progress forward to activate the ROM. 
     One particular embodiment of the invention employs the equation illustrated in FIG. 4 to suppress bubbles when selecting a value at address ‘n’ in the ROM. As used in the equation, C n  is a binary thermometer code value and C n+1  and C n−1  are thermometer code values just above and below C n , respectively (similarly, C n+2  is two thermometer code values above C n , etc.). It should be noted, however, that the equation set forth in FIG. 4 is for the purpose of illustration only and is not necessary for complying with the underlying principles of the invention. 
     It is important to note that the apparatus and method described herein may be implemented in environments other than a physical integrated circuit (“IC”). For example, the circuitry may be incorporated into a format or machine-readable medium for use within a software tool for designing a semiconductor IC. Examples of such formats and/or media include computer readable media having a VHSIC Hardware Description Language (“VHDL”) description, a Register Transfer Level (“RTL”) netlist, and/or a GDSII description with suitable information corresponding to the described apparatus and method. 
     Throughout the foregoing description, for the purpose of explanation, numerous specific details were set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention may be practiced without some of these specific details. For example, while the embodiments illustrated in FIGS. 3 a  and  3   b  apply the inverted outputs of each gate the first set of gates  310  (e.g., in position (n)) to gates which are offset by three and four gates in the second set  320  (e.g., (n−3′) and (n−4′), respectively), various alternative configurations may be implemented while still complying with the underlying principles of the invention. Similarly, although the outputs of each of the gates in the second set  320  are used as inverted inputs to gates offset by five and six gates in the second set (e.g., gates (n−5″) and (n−6″), respectively, with reference to gate (n″)), such a configuration is not necessary for complying with the underlying principles of the invention. Accordingly, the scope and spirit of the invention should be judged in terms of the claims which follow.