PROCESS FOR THE CONVERSION OF DIGITAL CODES AND DIGITAL-TO-ANALOG CONVERTER EMPLOYING THIS PROCESS

Abstract: The process for the conversion of a digital code involves the construction of two positive unipolar voltages (Vo+, Vo-) ranging between the analog ground level (Vgnd) and an upper full scale level (Vref), and the shifting downward of the second one (Vo-) of these voltages by a quantity equal to its maximum range, so to obtain two unipolar voltages, with positive polarity (Vo+) and negative polarity (Vo-), respectively, and with an amplitude equal to the halved module of the final voltage. The algebraic difference of these two voltages (Vo+, Vo-), is then computed the sign bit determining which one of the two voltages represents time by time the minuend and the subtrahend.

Description: PROCESS FOR THE CONVERSION OF DIGITAL CODES AND DIGITAL-TO- ANALOG CONVERTER EMPLOYING THIS PROCESS DESCRIPTION The present invention concerns a procedure for the conversion of digital codes, and more in particular of signed binary codes, that is codes where the word is made of N bits, one of which constitutes the sign bit. The invention concerns also a digital-to-analog converter having high resolution and operation frequency higher than one MHz, in particular a two-phase converter where the more significant bits of the code word are converted first, and the less significant ones thereafter. This converter is preferably of the single supply type and suitable to integration in CMOS technology.' A two-phase or two-step digital-to-analog converter using a mixed technique (resistive-capacitive) is known, where the decoding of the more significant M bits is made through a voltage demultiplexing or a "potentiometric" conversion, while the remaining less significant K bits are decoded with a charge switching technique. Generally, a fully potentiometric converter employs a resistive network (or resistor string) consisting of 2 N elements (where N is the number of bits) operating as a stepping potentiometer, supplied at the ends by two reference voltages Vref+ and Vref-, whose different voltage levels are selected by a switch decoding matrix, controlled by the bits to be decoded. Such an architecture enables to obtain a very high conversion speed, but is practicable only for low resolutions. For a number of bits equal to 8 it presents in fact some drawbacks due to the complexity of the voltage divider and of the switching logic. With 8 bits it is necessary to employ a resistive string consisting of 256 definite elements, whose implementation in the integrated form turns out to be difficult since it involves the covering of a large area on the chip, and furthermore the accuracy required to the string elements results critical. A conversion circuit with a full charging switching topology employs a binary weighted capacitor matrix injecting in the virtual ground of an operational amplifier a charge quantity proportional to a reference voltage and to the digital input word. Even this conversion technique shows some drawbacks when the resolution increases since for a comparatively high number of bits (8 - 10) , it requires a great ratio of capacitive values. This involves, besides a considerable area increase, a slowing down of the slew rate of the amplifier and a decrease of the gain-band with figure, placing a, upper limit to the operation frequency in the range of some hundred kHz. The mixed architecture represents therefore a good compromise for the realization of average resolution converters (9 - 11 bit) having a conversion speed of same MHz. The advantage as for resolution arises from the fact that the first M bits are intrinsecally monotonic, while the capacitor matrix has to meet a matching and a monotonic accuracy equal to K bit only to perform a M + K bit conversion. However, as the required resolution increases, that is the number of bits forming the code, this technique is subject to the troubles mentioned above for each one of the two conversion types. The scope of the present invention is furtherly improving the resolution which can be obtained with this mixed architecture (that is to increase the total number of bits forming the binary code) limiting the drawbacks mentioned above for the two conversion types, that arise when the bit number increases. This object is achieved with the present invention, which consists of a process for the conversion of a binary digital signed code made of N bits, one of which representing the sign, characterized by the fact to involve the following operations: TITUTESHEET a. to construct two positive unipolar voltages between the analog ground level and an upper full scale level, the maximum range of each one of these voltages being equal to 1/4 of the peak-to-peak voltage to reconstruct; b. to shift downward the second one of these voltages by a quantity equal to its maximum range, so to obtain again two unipolar voltages, the first of which has always positive polarity, while the second one has always negative polarity, the amplitude of both being equal to the halved module of the final voltage; c. to make the algebraic difference of these two voltages, with the sign bit determining which one of the two voltages is time by time the minuend and the subtrahend. The invention consists also in a digital-to-analog converter having fully differential structure, including a resistive string for the decoding of the more significant M bits and a matrix of binary weighetd capacitors divided into two branches, the common plate of each branch being connected to one of the inputs of a differential amplifier, while the other plates of the capacitors can be alternatively connected either to one constant voltage pair or to a pair of voltage which depend on the configuration of the less significant K bits, characterized by the fact that said resistive string includes 2 M / 2 resistive elements of equal value, whose ends are connected to an upper reference voltage and to an analog ground voltage which form the previously said constant voltage pair, and by the fact that the capacitor plates of one branch can be selectively connected to a first voltage pair identified by the more significant (M - 1) bits on the resistive string, while the plates of the other branch can be selectively connected to a second voltage pair, specularly symmetrical of said first voltage pair, with respect to the middle top of the resistive network. Further advantageous characteristics form the object of the remaining claims. The invention shall now be described more in detail making reference to the preferred, but not limiting, 5 realization forms shown in the enclosed drawings, where: Fig. 1 shows a general block diagram of the digital- to-analog converter of the fully differential type * employing the procedure according to the invention; Figures 2A and 2B show the structure of the resistive iθ string and the structure of the charge switching conversion circuit of the converter of Fig. 1, respectively; Figures 3A and 3B show, for example sake, the selection of the string voltage levels for two particular input words; 15 Fig. 4 shows a diagram of circuit realization of a preferred configuration of the switched capacitor structure according to the invention; Fig. 5 shows l and §2 phase signals; Fig. 6 shows the structure of one of the two portions 20 forming the sampling and hold SH block; Figures 7A to 7C show the conversion procedure referring to an example; and Fig. 8 shows a schematic variant of the converter employing a single ended circuit. 25 The conversion procedure shall now be described referring to Figures 7A - 7C for a preferred application, that is for a D/A conversion implemented with a differential topology. Let's assume, to simplify, that the voltage to be 30 reconstructed at the output end is a sinusoid Vout having peak-to-peak amplitude Vpp as shown in Fig. 7A. Assuming an input digital word made of N bits, a conventional conversion procedure would require for the decoding of a signal having such a dynamic range, 2 N conversion weights 35 or voltage levels ranging between a negative full scale Vref- and a positive full scale Vref+. The procedure according to the invention employs on the contrary only 2 N " 1 weights or voltage levels (that is one half), between an analogic ground level of the signal, marked Vgnd, and a positive full scale, hereafter called Vref. The first conversion step is the construction of two unipolar voltages ranging between Vgnd and Vref, and therefore always positive, which maximum amplitude is equal to Vpp/4, marked Vo+ and Vo- in Fig. 7B. The second step is the shifting of Vo- by an amount equal to Vpp/4, thus obtaining again two unipolar voltages, the first one of which Vo+ has always positive polarity, while the second one Vo- has always negative polarity, and which have both a plitudude equal to the halved module of the final voltage, as shown in Fig. 7C. According to the procedure of the invention, the construction of the two voltages Vo+ and Vo- is independent of the sign bit. The final output voltage is therefore obtained as the algebraic difference between these two voltages, with the sign bit determining which one of the two voltages represents time by time the minuend and the subtrahend. In the half cycle during which the output sinusoid Vout must be positive, Vo- is subtracted from Vo+, while in the other half cycle the inverse operation is made, that is [ (Vo-) - (Vo+) ] . The procedure, as better described below, is particularly advantageous when applied to a mixed two-phase conversion architecture where the more significant M bits are decoded in one of the two phases with a potentiometric arrangement, while the remaining K bits are decoded in the other phase through a switched capacity structure. In that case in fact the number of elements of the resistive string is halved, and the shifting of the waveform Vo- is simply obtained by varying the reference level at which a capacitor matrix is precharged. The selection of the final voltage polarity is simply made by inverting the connection of two sampling and hold blocks placed downstream the two voltages Vo+ and Vo-, while the difference between these two voltages is intrinsic to the circuit differential structure. The procedure according to the invention is anyhow applicable even to a single ended circuit structure. In that case the analog voltage is obtained utilizing the conversion weights ranging between Vgnd and Vref, thus obtaining the module of the final sinusoid, that is a waveform having maximum amplitude equal to Vpp/2. When the sign bit is negative, the voltage waveform is shifted by a quantity equal to Vpp/2, obtaining a two polarities final signal compared to the analogic ground level. A schematic structure of this arrangement is shown in Fig. 8 and shall be described later on. Furthermore, the procedure according to the invention is applicable also to an analog-to-digital conversion. In that case, the shifting of the input analog voltage made accordingly to its polarity is performed using a switched capacity structure. It enables even in this case to save the silicon area since it halves the number of comparison elements required to code-the signal. Therefore, with the procedure according to the invention, the number of resistive components is reduced on one hand, and on the other hand neither components nor additional phase are required for shifting. The proposed procedure enables circuit simplifications. In fact the conventional conversion procedure requires three reference generators: one for the analog ground, one for the positive full scale and one for the negative full scale. The procedure according to the invention enables to completely eliminate the negative voltage reference. The invention shall now be furtherly described in detail referring to a preferred realization form of digital-to-analog converter shown in the figures. Referring to Fig. 1, the converter according to the invention includes a first portion 10 for the conversion of the more significant M bits MSB, and a second portion 12 SUBSTITUTE SHEET for the conversion of the less significant K bits LSB of a code word formed by M+K bits. Portion 10 involves a RSW arrangement, consisting of a resistive string (called also ladder) and of a switch matrix, and of a selector SEL controlling the turning on and off of switches accordingly to phases which depends on the values of the bits to be decoded. The RSW arrangement is supplied by a reference voltage Vref and by an analog ground voltage Vgnd, for instance 3.5 and 2.5 V respectively in case of 5V supply and of an output dynamic specification equal to 2V. According to the input configuration of the (M-l) MSB, the RSW arrangement produces at the output a voltage level pair identifying the ends of the interval in which the output voltage shall be included. More particularly, due to the differential structure of the converter, voltage pairs Vi+1 and Vi, Vj+1 and Vj shall be supplied at the output, specularly symmetric compared to the middle voltage level VM between Vref and Vgnd. The second portion 12 for the conversion of the less significant K bits LSB, which takes place in the second clock half cycle, includes a matrix or array of capacitors AC, supplied by RSW output voltages and controlled at the input by LSB bits and by two phase signals Φl and Φ2 shown in Fig. 5. The outputs of the capacitor matrix are connected to the inputs of an operational amplifier AP. Outputs Vo+ and Vo- of the operational amplifier AP are connected to a sample and hold circuit SH under the control of the Φ2 phase and the converted differential analog signals Vout+ and Vout- are available at the outputs of this last block. Analog voltages at the outputs of the differential amplifier AP are available only during the useful phase Φ2 and it is therefore necessary to put a circuit in cascade to perform the reading operation of the voltages during the Φ2 phase and to the maintaining of them during the subsequent phase (phase Φl of the subsequent conversion cycle) . As it shall be explained more in detail later on, two sample and hold blocks are actually implemented, and the sign bit b s of the word to be decoded, controls the connection between the operational amplifier and the circuit SH. Fig. 2A, showing the RSW structure more in detail, shows also some of the resistances forming the ladder according to the invention, made up with the resistive elements Rl, R2,...Rx-2, Rx-1, Rx. All the ladder resistances have a value equal to R which represents the minimum resistive element and are 2 H " 1 in number, that is one half of the resistive strings of the known technique. As a consequence, the ladder shall be called hereafter half-string. The ends of the half-string are connected to the Vref voltage and to the Vgnd analog ground to which the sign of the output analog voltage is referred, respectively. With the known arrangements, outlined in Fig. 2A with SW switches, the more significant bits MSB select two voltage pairs Vi+1, Vi and Vj+1, Vj on the half-string. Through a MX circuit, including switches essentially of the known type and controlled by the b ss bit having the more significant weight immediately subsequent to the sign bit, the two voltage pairs Vi+1, Vi and Vj+1, Vj selected. on the string by the selector SEL on the basis of the more significant bits (and therefore independent on the sign bit b s ) are made available to the capacitor matrix being part of the AC structure. The MX circuit thus establishes the correct connection with the voltages of the two pairs, which are now indicated by VI +, V2+ and VI- and V2- as it shall result more clear later on. Fig. 2B shows more in detail the structure of the capacitor matrix AC including two portions or branches, indicated by CP and CM, whose outputs supply the not inverting input and the inverting input of the differential amplifier AP, respectively. Outputs Vo+ and Vo- of the T differential amplifier AP are connected to the corresponding inputs through reaction capacitors having a capacity of 2 K C, where K is the number of LSB bits. Each one of the CP and CM branches includes a series of capacitors with a value equal to C, C, 2C, 4C, ... 2 K " 1 C, connected in parallel, having the common top plate connected to the corresponding input of the operational amplifier AP. The bottom plate of the capacitors is selectively connected, through electronic switches controlled by signals enabled during the phase Φ2, to one of the four voltages belonging to the two pairs V1+, V2+ and VI-, V2-. More particularly, the voltages of the pair V1+, V2+ are made available to the CP branch of the capacity matrix, while those of the pair VI-, V2- to the CM branch of the capacity matrix, following the methods which shall be detailed later on. As it can be seen in Fig. 5 showing a simplified diagram of phases Φl and Φ2 within the conversion time T, Φl is at high logical level in the first half of the clock cycle during which the MSB conversion takes places through the selection operated by the SEL block and by the MX block of the different voltage levels available on the half- string. The Φ2 phase is on the contrary high in the second half of the clock time during which the LSB conversion, described hereafter, occurs. As it can be seen in Fig. 2B, in the upper branch CP the switches driven by the control phase Φl connect the relative capacitor to the Vref voltage, that is to the positive full scale voltage, while the switches driven by control phases B j Φ2 and b j Φ2 during the time in which Φ2 is high connect the relative capacitor either to V1+ or V2+ voltage, depending on the logic status of the j- th bit belonging to the less significant K bits set. A capacitor of minimum value equal to C is always connected during phase Φ2 to the lowest voltage between those selected (that is always to V1+) in order that the minimum voltage outgoing the amplifier AP is equal to that is equal to V1+, whichever is the less significant digital word present at the converter input. Similarly, in the lower branch CM, the switches driven by the control phase Φl connect the relative capacitor to the Vgnd voltage (that is to the analog ground voltage) , while the switches driven by the control phase b j Φ2 and b j Φ2 connect the capacitor either to VI- or to V2- accordingly to the status of the j-th bit belonging to the set of the less significant bits LSB. A capacitor of minimum value C is always connected, during the time in which Φ2 is high, to the minimum voltage between V2- and VI- (that is always to VI-) for the same reasons described for the capacitor of minimum value belonging to the upper branch CP, that is to complete the charge injection proportional to VI- whatever is the input digital word. The operation of the converter according to the invention shall now be described making reference to Figures 3 and 4. Figures 3A and 3B better show the selection of the levels on the half-string. Let's assume that the input digital word corresponds to a voltage level included in the voltege interval identified by the two successive taps A and B on the half-string. This level is higher than Vgnd and lower than the middle voltage of the half-string as indicated in Fig. 3A by Vm. During the first half of the clock period, the selector SEL activates the clock controls which turn off switches MA and MB making available the ends Vj and Vj+1 of the voltage interval within which the output voltage shall be included. At the same time SEL activates the turning on of switches MC and MD which make available the voltage SUBSTITUTE SHEET levels corresponding to the ends of the voltage interval specular to the previous one compared to level Vm. In this hypothesis, the b ss bit is equal to 0, therefore the block MX closes switches MX1, MX2, MX3 and MX4 and determines the relation Vj = VI-, Vj+1 = V2- connecting therefore taps A and B of the half-string to the bottom plates of the capacitors forming the matrix of branch M. During the phase Φ2 the capacitors perform the injection of a charge proportional to said voltages, taking Vgnd as reference level (that is the voltage at which they were connected during the previous Φl phase) . This injection consists in the flow of positive charge through the feedback capacitor 2 K C giving rise to an output voltage Vo- lower than Vgnd, that is a negative voltage. At the same time the block MX also performs the connection Vi = V2+ and Vi+1 = V1+. The matrix capacitors belonging to the branch CP are so connected during Φ2 to taps D and C of the half-string. Their reference value is equal to Vref (voltage at which they were connected during phase Φl) . Their switching on V1+ and V2+ thus gives rise to the injection of a negative charge which, flowing through the reaction capacitor towards the output Vo+, produces a voltage higher than Vgnd on said output, that is a positive voltage. If, letting the other bits unchanged, it is assumed that bss=l, the selector SEL activates the same control phases closing the same switches MA, MB, MC and MD. The block MX receives therefore the same voltage at the input end, however, it turns off the switches MX5, MX6, MX7 and MX8 and establishes a new configuration setting Vi=Vl-, Vi+l=V2-,Vj=V2+, Vj+l=Vl+. Taps C and D are therefore connected to the capacitors of branch M and taps A and B to those of the branch P. Voltages Vo+ and Vo- have the same polarity as before but higher amplitude, having now to construct an output analog voltage having amplitude ranging between Vc and Vd by subtraction. This arrangement for the conversion of the MSB allows to simplify the logic circuitry forming the SEL block, to minimize the number of phases converging on switches connected to each tap of the half-string and the number of the switches themselves. Making reference to Fig. 4, structures AC and AP, performing the LSB conversion shall now be described more in detail. For simplicity sake, we shall describe only the upper branch CP, and the matrix of these capacitors is represented in brief, highlighting a generic capacitor of the matrix itself, indicated by 2JC. The switched capacitor structure of the previous Fig. 2B described the capacitive elements useful for the injection and integration of the charge, that is the charge proportional to the code and to the reference voltages. Fig. 4 shows now in more detail a preferred structure adopted in the invention, including also the capacitors Ca and Cf the role of which shall be described later on. During phase Φl, the input capacitor 2JC and the feedback one 2 K C discharge the charge they stored during the previous conversion cycle. Thanks to the inverting structure, only the charge required to update the offset voltage present at the input of the operational amplifier flows through the parking capacitor Cf. The input matrix and the feedback capacitor always hold charge with opposite sign and equal value. During the successive phase the input capacitor 2JC is connected to the higher or to the lower voltage of the available pair V1+ and V2+, the connection being determined by the bit of weight j . If, for instance, all LSB are equal to zero, then all the capacitors of the upper matrix are connected to V1+ causing a flowing of charge through each capacitor 2J ' c which originates an output amplitude Vo+ equal to [ (Vref) - (V1+) ] referred to Vgnd. At the same time, all the capacitors of the lower matrix CM, which switching is driven by the same less significant bits, are connected to VI-, giving rise to a negative output level (compared to Vgnd) equal to [ (V1-) - (Vgnd) ] . Since VI- and V1+ were specular on the half-string with respect to the middle level Vm, the two differential voltages corresponding to the input code have been constructed. It must be noted that up to now the conversion procedure (and therefore the circuit components implementing it) is independent of the sign bit b s . The information related to b s is processed at this stage of the conversion operation. As explained before, it is necessary to sample the signals Vo+ and Vo- of the amplifier AP during the phase Φ2 and hold them until the phase Φ2 of the subsequent conversion period is on. This operation is performed through two sample and hold blocks SHI and SH2 (indicated by SH as a whole in Fig. 1) , only one of which is shown in Fig. 6. This block includes an amplifier Al, two capacitors Csh and the switches S1-S4 controlled by signals derived from Φl and 2 and from the b s sign bit. If the sign bit is negative, the connections between these two blocks and the outputs Vo+ and Vo- of the differential amplifier AP are reversed, thus performing the inversion of the final analog voltage. In other words the conversion of the sign bit just involves the addition of two switches. The advantages of the new proposed conversion procedure, are fully applied by the two-phase mixed architecture described, representing the currently preferred realization form of the invention. The halving of the number of components of the resistive string involves a save in the silicon area, particularly manifest when the required linearity is high, and this leads to the use of polysilicon integrated resistances, which present a low specific resistance. The reduction in the string extension gives rise to a better resistive matching (at equal technological characteristics) and consequently to a better absolute linearity. The selection logic results simpler and the differential topology, besides the known intrinsic advantages of better power supply noise rejection, represents a more simple and direct implementation of the proposed conversion procedure. It must be noted that the selection logic could have been furtherly simplified if on one of the two branches CP or CM the role of phases Φl and Φ2 was exchanged, performing therefore the connection to the taps of the half-string during phase *1, and the offset redding- discharging operation during phase Φ2. According to this possibility, the differential voltages Vout+ and Vout- could be constructed using the same levels on the half- string, that is by selecting one single voltage pair on the string. However, the configuration here proposed allows to obtain a great advantage when combined with a compensation procedure of the clockfeedthrough effect, this procedure forming the object of another patent application deposited by the Applicant, under the title "Procedure for the compensation of the clockfeedthrough effect in switched capacitor circuits". As already known, opening a switch originates a phenomenon of .parasitic charge injection which, in high impedance nodes (for instance the virtual ground of an operational amplifier) , gives rise to an error voltage or clockfeedthrough voltage. The proposed inverting configuration enables to open before the useful phase (phase Φ2) only switches biased at reference voltages and not at signal voltages. The injection of parasitic charge which arises following this switching is therefore constant for any input digital word. The clockfeedthrough voltage is SUBSTITUTE SHEET moreover a common mode voltage (even if in part due to mismatches between the two paths) and is therefore partially rejected by the common mode feedback in the differential amplifier stage. The connection of capacitors to the half-string only during the second half of the clock period, when the digital portion (selector SEL, switches on the half-string sockets, block MX) has completed its transitions, avoids the direct coupling of switching noise coming from the digital portion. The reduction of the residual clockfeedthrough effect and of the more important one (since it depends on the signal) originated by the switches connecting the outputs Vo+ and Vo- to the two blocks SH, has been made by applying the compensation technique forming the object of the above mentioned patent application, reference to which has to be made for further details. The switched capacitor structure presents further considerable advantages which shall be described in brief later on, always referring to Fig. 4. The offset voltage, due to mismatch between the two amplification paths of the differential amplifier, is stored by the input capacitors (2 J C) and by the feedback one (2 K C) during the phase Φl, in such a way the useful charge injection in virtual ground during the successive phase Φ2 results being independent of the offset voltage. Moreover the dc transmission zero so obtained by this configuration allows to limit the equivalent input noise 1/f of the operational amplifier. The parasitic capacitors existing between the plates of the integrated capacitors and the substrate do not affect the charge injection since injection and integration capacitors present one plate always connected to the virtual ground, and the other one switched between low impedance points. During the offset reading phase, the operational amplifier is closed in feedback through Cf which is charged at the signal voltage stored during the previous Φ2 phase. This prevents the outputs from going again to the analog ground level, as it would happen if the amplifier, during Φl, was closed in a unit feedback configuration through a switch. Large output swings would bring about a high slew rate specification. The C f capacitor, besides limiting the slew rate time of the operational amplifier, allows to compensate the incomplete discharge error due to the fact that the operational amplifier gain is actually high but of finite value. Finally the C a capacitor makes the retroaction continuously closed, preventing large output fluctuations which could occur during the disoverlap interval of phases, during which the amplifier would be in open loop in lack of C a . These fluctuations could involve an increase in the slew rate time. Fig. 8 shows the diagram of the arrangement relative to a single-ended conversion which is useful, for instance, when there is no severe S/N ratio requirement, which present a simpler structure compared to the differential one, and therefore covers a smaller area on the chip in the integration. In this case one input of the operational amplifier AS is connected to the analog ground voltage Vgnd, and the other input is alternatively connected to the signal and to one of Vgnd or Vref voltages, depending on the sign bit of the code word. This is obtained through the switches shown in Fig. 8 which are controlled by the sign bit b s . The output voltage Vo, present on the single terminal and referred to ground, is obtained by using as reference Vgnd if it is negative (b s =l makes the related switch closed during phase Φl) , and Vref if it is positive. The Vo voltage has module equal to Vpp/2 if Vpp is the peak-to- peak amplitude of the sinusoid to reconstruct. Although the invention has been described and detailed referring in particular to the preferred realization forms, it extends to cover the implicit variants and modification and the adaptations which should be clear to the technician in this field.

Claims: CLAIMS 1. Process for the conversion of a signed binary digital code formed by N bits, one of which represents the sign, characterized by the fact to involve the following operations: a. to construct two positive unipolar voltages (Vo+, Vo-) ranging between the analog ground level (Vgnd) and the upper full scale level (Vref) , the maximum range of each one of these voltages being equal to 1/4 of the peak-to- peak voltage to reconstruct (Vpp) ; b. to shift downward the second one (Vo-) of these voltages, by a quantity equal to its maximum range, as to obtain two still unipolar voltages, the first one having always positive polarity (Vo+) , while the second one (Vo-) having always negative polarity, both having amplitude equal to the halved module of the final voltage; c. to make the algebraic difference between these two voltages (Vo+, Vo-) , with the sign bit determining which voltage is time by time the minuend and the subtrahend. 2. Process according to claim 1, characterized by the fact that said two positive unipolar voltages (Vo+, Vo-) are symmetric compared to a level equal to [(Vgnd) + Vpp/8] and 2 N " 1 conversion weights are employed only. 3. Process according to claim 1 or 2, applied in a mixed conversion architecture, characterized by the fact to involve the module bits decoding to construct the two unipolar voltages (Vo+, Vo-) , and the separate decoding of the sign bit (b s ) . 4. Process according to claim 3, characterized by the fact that the level shifting of one of the two unipolar voltages (Vo+, Vo-) is obtained by changing the reference level of an input terminal in a charge switching structure of the differential type. 5. Process for the conversion of a singed binary digital code of N bits, one of which representing the sign, characterized by the fact to construct a voltage (Vo) equal to the module of the voltage to reconstruct and to shift the reference level (Vgnd, Vref) according to the sign bit (bs) of the digital code. 6. Digital-to-analog converter with fully differential structure, of the type including a resistive string for the decoding of the more significant M bits (MSB) and a matrix of binary weighetd capacitors and divided in two branches (CP, CM) one end of each branch being connected to one of the inputs of a differential amplifier (AP) , while the other plate of the capacitors can be alternatively connected to one or to the other between a constant voltage pair and a voltage pair depending on the configuration of the less significant K bits, characterized by the fact that said resistive string (Rl,...Rx) includes 2 M /2 resistive elements of equal value, which ends are connected to an upper reference voltage (Vref) and to an analog ground voltage (Vgnd) forming said pair of constant voltages, and by the fact that the plates of the capacitors of one branch (CP) can be selectively connected to a first pair of voltages (V1+, V2+) identified by the (M-l) more significant bits on the resistive string, while the plates of the other branch (CM) can be selectively connected to a second pair of voltages (VI-, V2-) specularly symmetric of said first pair of voltages, compared to the middle point of the resistive network. 7. Converter according to claim 6, characterized by the fact that the outputs of the operational amplifier (AP) are connected to two sampling and hold circuits (SH) controlled by the sign bit. 8. Converter according to claim 6 or 7, characterized by the fact to involve a matrix of switches (MX) controlled of the (b ss ) bit having the more significant weight immediately after the sign bit. 9. Converter according to claim 6 or 7, characterized by the fact that each one of the branches P and M includes a set of capacitors with value (C, C, 2C, 4C, ...2 K - 1 C) connected in parallel, and the capacitors have the common top plate connected to the corresponding input of the operational amplifier (AP) and the bottom plate selectively connected, through electronic switches to one of the four voltages belonging to said two pairs (V1+, V2+, and VI-, V2-) . 10. Digital-to-analog converter of the type including a resistive string for the decoding of the (M-l) more significant bits (MSB) and a binary weighted capacitors matrix for the decoding of the less significant K bits (LSB) , said matrix being connected at one end to the input of a single ended amplifier (AS) , and at the other end alternatively connectable to one or to the other between a constant voltage pair, the other input of the differential amplifier being connected to the analog ground voltage (Vgnd) , characterized by the fact that said resistive string (Rl, ...Rx) includes 2 H /2 resistive elements of equal value, which ends are connected to an upper reference voltage (Vref) and to the analog ground voltage (Vgnd) forming said constant voltage pair, and by the fact that the connection of the capacitor plates to one or to the other constant voltage pair (Vref, Vgnd) is made with switches controlled by the sign bit (bs) of the code word.

Inventors: COPPERO LUCIANA, MALOBERTI FRANCO, PALMISANO GIUSEPPE, COPPERO, LUCIANA, MALOBERTI, FRANCO, PALMISANO, GIUSEPPE
Applicants: ITALTEL SPA, ITALTEL SOCIETA' ITALIANA TELECOMUNICAZIONI S.P.A.
Application year: 1991
Main classification: H03M1
IPC codes: H03M1