Flash analog-to-digital converter

Abstract: A flash analog-to-digital converter comprising a bank of comparators (COMPi) with a differential output, generating a thermometric code and a bank of three-input (A,B,C) logic NOR gates (NORj) for correcting errors in said thermometric code, has enhanced immunity to noise and reduced imprecisions, especially at high conversion rates upon occurence of metastability within the comparators, by providing for a passive interface constituted by a plurality of voltage dividers (Ra-Rb), each connected between the noninverted output (out_p) of a respective comparator (COMPi) and the inverted output (out_n) of the comparator of higher order (COMPi+1) of said bank; a corresponding logic NOR gate (NORj) of said bank having a first input (A) coupled to the inverted output (out_n) of said respective comparator (COMPi-1), a second input (B) coupled to the noninverted output (out_p) of said comparator (COMPi) of higher order and a third input (C) coupled to an intermediate tap of said voltage divider (Ra-Rb).

Description: FIELD OF THE INVENTION The invention relates to signal processing systems, and more in particular to an analog/digital converter capable to output a thermometric digital code. BACKGROUND OF THE INVENTION The ever expanding applications of digital signal processing in place of the traditionally analog processing, implies the conversion of analog signals into digital signals. Processing of high frequency signals requires analog/digital converters with adequately high speed characteristics. The speed and reliability requirements in terms of statistical incidence of conversion imprecisions of modern processing systems raise important technological and circuital problems for overcoming critical functioning behaviors of the converter, often required to function at the limit of their stability characteristics, and wherein metastability phenomena may significantly affect the number of errors of conversion. The phenomena of metastability in comparators used in an analog/digital converter may cause glitches on the output digital code (word). Glitches may affect a correct datum processing downstream the of analog-digital converter. Since the number of errors is exponentially correlated to the clock frequency and to the regenerative time constant of the comparators, the metastability errors may increase in number by several orders of magnitude upon increasing the clock frequency and/or decreasing the supply voltage. Many techniques have been proposed and developed to reduce the number of errors due to metastability phenomena of the comparators of an analog/digital converter, in order to ensure that the errors remain below a certain limit which, in some applications, may be fixed to a number below 10 -10 errors/cycles. The enhancement of the performance of comparators employed in a converter is an approach that ceases to be effective when operating at the technological limits of integration and of minimization of the silicon requirement and/or of power consumption. In these limit conditions, the proposed techniques consist in introducing latch arrays in cascade (pipelining) of the comparators' outputs in order to increase the resolution time of the comparators. The pipelining technique, while being capable to decisively reducing the number of errors, determines a conspicuous increment of the circuital complexity, that is, of the required silicon area and of the current absorption. Moreover, this approach implies a strict correlation between the increment of the complexity of the pipelining circuits downstream of the array of comparators and their performance in terms of their ability of reducing the number of errors. The article: "Power-efficient metastability error reduction in CMOS flash A/D converters" by C. L. Portmann, T. H. Meng, IEEE Journal of solid state circuits, Vol. 31, No. 8, August 1996, offers a review of the different known techniques of this type. Considering that in modem high speed flash analog/digital converters (flash ADC) an array of comparators for producing a thermometric code and an array of NOR gates for outputting only a "1" among all "0" to a NOR plane of a PLA are employed, as schematically shown in Fig. 1, the above mentioned article purports the use of three-input NOR cells rather than two-input NOR cells so that the third input may be used as a common mode reference input, by selecting as such a reference a value higher than the output common mode value of the comparators. However, in flash ADCs using high speed bipolar devices, the output dynamic of the comparators may be of a few thermal potentials thereby the margin of noise reduction that may be obtained by implementing a common mode circuit, according to the technique suggested in the above cited article, is practically rather reduced. Moreover, document US-5 272 461 discloses a flash analog-to-digital converter comprising a bank of comparators with differential outputs, generating a thermometric code, and a coding circuit based on differential transistor pairs for preventing coding errors produced by fractional currents caused by a comparator in a metastable state. OBJECT AND SUMMARY OF THE INVENTION In order to improve immunity to noise and imprecision of the common mode circuit, a surprisingly effective solution has now been found based on the use of a passive interface at the output of the array of comparators array, in practice realizing a logic that may be defined as pseudo-differential compared to the differential logic of the known circuits. Essentially, the flash ADC of the invention comprises an array or bank of comparators (COMPi) having a differential output and generating a thermometric code and a bank of three-input (A, B, C) logic NOR gates (NORj), and a passive interface constituted by voltage dividers (Ra-Rb). Each voltage divider is connected between the noninverted output (out_p) of a respective comparator (COMPi-1) and the inverted output (out_n) of the comparator of higher order (COMPi). The respective NOR gate (NORj) has a first input (A) coupled to the inverted output (out_n) of the respective comparator (COMPi-1), a second input (B) coupled to the noninverted output (out_p) of the comparator of higher order (COMPi) and a third input (C) coupled to a intermediate tap of the voltage divider connected between the noninverted output (out_p) of the respective comparator (COMPi-1) and the inverted output (out_n) of the comparator of higher order (COMPi). The passive interface constituted by the plurality of voltage dividers (Ra-Rb) decisively increases the immunity to common mode disturbances and noise without sensibly increasing neither the area requirement nor the power consumption. BRIEF DESCRIPTION OF THE DRAWINGS The various aspects and advantages of the invention will become even clearer through the following description of an embodiment and by referring to the attached drawings, wherein: Figure 1 shows a typical functional scheme of a flash ADC; Figure 2 is a partial scheme that illustrates the passive interface between the comparators outputs and the inputs of the NOR latches, according to the present invention; Figure 3 is an example of a three-input NOR circuit usable in a flash ADC of the invention. DESCRIPTION OF AN EMBODIMENT OF THE INVENTION Fig. 1 shows the architecture of a flash ADC in which the bank of comparators COMPi supply a thermometric code and the bank of latches NORj outputs only a "1" and all "0s" to the NOR plane of a decoding PLA which outputs an N bit word or digital datum corresponding to the sampled analog value Vin. Fig. 2 shows the detail of the flash ADC circuit of Fig. 1, modified according to the present invention. As it may be observed, between the noninverted output out_p of a comparator and the inverted output out_n of the comparator of higher order of the bank, is connected a restive voltage divider Ra-Rb, to the intermediate node of which is connected the input C of the pipelining NOR gate latch of the first comparator output. Assuming that two consecutive comparators of the bank of comparators of the converter output the same logic signal during a certain phase, the common mode input of the NOR gate coupled to the outputs of the two consecutive comparators may be considered ideally equal to the output common mode of the two comparators, in this case the two comparators are not metastable. By contrast, if two consecutive comparators, for example the comparators COMP2 and COMP3 of the bank, are considered as having both as input a signal of a level between their discriminating thresholds, should COMP3 be a metastable comparator, the input nodes of NOR3 will be at the following levels: node A at a low logic level; node B at an undetermined logic level, lower than the output common mode of the two comparators COMP2 and COMP3; node C at a higher level than the output common mode of the comparators by an amount greater than half the positive dynamic of the two comparators. Therefore, the logic level of the output of NOR3 will be high, as it should be. In an analog manner the input nodes of NOR2 will be at the following levels: node A at an undetermined logic value higher than the output common mode of the two comparators COMP2 and COMP3; node B at a low logic value; node C at an undetermined level, however, being NOR2 adjacent to NOR3, the problem may be easily solved as described in the above cited article. It is so evident how the passive interface, realized by way of voltage dividers, introduced between the comparators outputs and the inputs of the of NOR gates, increases the immunity to common mode disturbances and to noise without substantially increasing the area of integration and/or the power consumption. Fig. 3 shows a suitable circuit for each three-input logic NOR gate, usable in the flash analog-to-digital converter of the invention.

Claims: A flash analog-to-digital converter comprising a bank of comparators (COMPi) with a differential output, generating a thermometric code and a bank of three-input (A, B, C) logic NOR gates (NORj), characterized in that it comprises a passive interface constituted by a plurality of voltage dividers (Ra-Rb), each connected between the noninverted output (out_p) of a respective comparator (COMPi) and the inverted output (out_n) of the comparator of higher order (COMPi+1) of said bank; a corresponding logic NOR gate (NORj) of said bank having a first input (A) coupled to the inverted output (out_n) of said respective comparator (COMPi-1), a second input (B) coupled to the noninverted output (out_p) of said comparator (COMPi) of higher order and a third input (C) coupled to an intermediate tap of said voltage divider (Ra-Rb). The converter according to claim 1, characterized in that it is realized in BiCMOS technology. The converter according to claim 1, characterized in that it is a flash converter, using a decading PLA device, the inputs of a NOR plane of which are coupled to the outputs of said bank of logic NOR gates, generating an output datum of N bits corresponding to said thermometric code.

Inventors: BOLLATI GIACOMINO, BRUCCOLERI MELCHIORRE, DEMICHELI MARCO, OTTINI DANIELE, BOLLATI, GIACOMINO, BRUCCOLERI, MELCHIORRE, DEMICHELI, MARCO, OTTINI, DANIELE
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: H03M1
IPC codes: H03M1