Area-efficient reconstruction filters, particularly for D/A current-driven converters

Abstract: An area-efficient low-pass, time-invariant, second-order reconstruction filter, particularly for current-driven digital-to-analog converters, comprising: a first resistor (R 1 ) and a first capacitor (C 1 ) which are parallel connected; an operational amplifier (3); a terminal of a second resistor (R 2 ) which is connected to the inverting input of the operational amplifier; another terminal of the second resistor which is connected to a common node of the first resistor (R 1 ) and the first capacitor (C 1 ); a second capacitor (C 2 ), which is fedback between the output of the operational amplifier and the inverting input; the filter further comprising an additional pair of resistors (R 3A , R 3B ) which are arranged so as to be fedback between the output and the inverting input, a current signal (I DAC ) arriving from a digital-to-analog converter arranged upstream of the reconstruction filter being fed to a common node of the additional pair of resistors.

Description: The present invention as it is defined in the appended claims relates to area-efficient reconstruction filters, particularly for current-driven digital-to-analog converters (DAC). Digital-to-analog converters are conventionally used very frequently in integrated circuits. Since these converters are sampled-data circuits, in addition to generating the intended analog signal in the correct frequency range or base band, they also produce in output an undesirable duplicate image of the signal, generally designated as "imaging", as shown in figure 1, which plots the output of the DAC as a function of the frequency f. The chart shows that in addition to the output signal, designated by S (where B is the base band), there is also a duplicate image of the signal S which is centered around the sampling frequency f S of the DAC. In order to eliminate this duplicate image, a continuous-time low-pass reconstruction filter is usually introduced and placed downstream of the DAC, as shown in figure 2. In this figure, the reference numeral 1 designates an N-bit DAC, where b 0 , b 1 , ..., b n-1 are the input bits of the DAC and V DAC and I DAC are, respectively, the output voltage and the output current of the DAC; the reference numeral 2 instead designates a continuous-time low-pass reconstruction filter arranged downstream of the DAC; V O is the output voltage. The reconstruction filter 2 must provide high attenuation for frequencies close to the sampling frequency f S of the DAC, but at the same time it must be efficient in terms of area occupation if the DAC is to be used in an integrated circuit, where of course the requirement of minimum area occupation is one of the most important factors. It is known to those skilled in the art that these are two mutually contrasting requirements. It is therefore necessary to achieve a compromise, shown in figures 3a and 3b. The filters shown in these figures are second-order low-pass filters; the solution shown in figure 3b, however, is the one that is practically mandatory when working with supply voltages of less than 3V. This solution is rather area-efficient when the input signal is a voltage, but it is highly insufficient when the input signal must be a current, as shown in figure 3c by applying only the Norton equivalent to the input of figure 3b. A numeric example is now described to clarify the above explanation. Assume that a DAC has been devised which has a full-scale voltage output V iFS and that a full-scale voltage in output from the reconstruction filter is required. Assume also that a cutoff frequency of approximately 270 kHz is chosen for the filter. The values of the components of figure 3b will be as follows: R 1 = R 2 = R 3 = 50 kohm C 1 = 25.2 pF C 2 = 5.6 pF Assume also that one intends to use a DAC with a full-scale current output I DACFS =160 µA and that one seeks a full-scale output voltage V OFS = 0.5 V from the reconstruction filter with a frequency response which is identical to that of the filter used previously with the voltage-output DAC. Since one must have R 3 I DACFS = V OFS , then R 3 = 3.125 kohm and therefore R 1 = R 2 = R 3 = 3.125 kohm. Therefore, in order to have the same frequency response as the preceding filter, the values of C 1 and C 2 must be 403.2 pF and 89.6 pF respectively. Accordingly, the area occupied on the silicon in order to integrate the values of these components is approximately sixteen times greater than the area occupied to integrate the components of figure 3b, due to the relatively low specific capacitance that can be provided in integrated circuits. Therefore, the reconstruction filter, in the case of a current input (and therefore of a current output of the DAC), is very wasteful from the point of view of the area occupied on the silicon wafer. The aim of the present invention is to provide an area-efficient reconstruction filter, particularly for current-driven digital-to-analog converters. Within the scope of this aim, an object of the present invention is to provide a reconstruction filter for current-driven digital-to-analog converters which is optimized in view of its integration in an integrated circuit. Another object of the present invention is to provide a reconstruction filter for current-driven digital-to-analog converters in which the reconstruction filter with current input has the same area occupation as the corresponding reconstruction filter with voltage input. Another object of the present invention is to provide a reconstruction filter with current input having the same transfer function as a similar reconstruction filter with voltage input. Another object of the present invention is to provide a reconstruction filter which is highly reliable, relatively easy to produce and at competitive costs. This aim, these objects and others which will become apparent hereinafter are achieved by an area-efficient low-pass, time-invariant, second-order reconstruction filter, particularly for current-driven digital-to-analog converters, comprising: a first resistor and a first capacitor which are parallel connected; an operational amplifier; a terminal of a second resistor which is connected to the inverting input of said operational amplifier; another terminal of said second resistor which is connected to a common node of said first resistor and said first capacitor; a second capacitor, which is fedback between the output of said operational amplifier and said inverting input; characterized in that it comprises an additional pair of resistors which are arranged so as to be fedback between said output and said inverting input, a current signal arriving from a digital-to-analog converter arranged upstream of said reconstruction filter being fed to a common node of said additional pair of resistors. Further characteristics and advantages of the present invention will become apparent from the description of a preferred but not exclusive embodiment of the reconstruction filter according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a chart that plots the signal in output from a DAC; figure 2 is a block diagram of a DAC with a reconstruction filter arranged downstream thereof; figures 3a and 3b are circuit diagrams of conventional low-pass, continuous-time, second-order, voltage-input reconstruction filters used downstream of the DACs, as shown in figure 2; figure 3c is a circuit diagram of a conventional low-pass, continuous-time, second-order current-input reconstruction filter, conceptually similar to the filter shown in figure 3b; and figure 4 is a circuit diagram of a low-pass, continuous-time, second-order current-input reconstruction filter according to the present invention. With reference to the above figures, figures 1 to 3c are not described further herein, since reference has already been made thereto in the description of the prior art and of the problems related thereto. In the various figures, identical reference numerals and letters designate identical elements. The only reference to figures 3a-3c relates to the definition of the transfer functions of said filters, which are respectively the following. For the filter of figure 3a: V 0 (s)/V i (s) = 1/[1+sC 1 (R 1 +R 2 ) + s 2 C 1 C 2 R 1 R 2 ] For the filter of figure 3b: V 0 (s)/V i (s) = (R 3 /R 1 )/[1+sC 2 (R 2 +R 3 +R 2 R 3 /R 1 ) + s 2 C 1 C 2 R 2 R 3 ] And finally, for the filter of figure 3c: V 0 (s)/I DAC (s) = R 3 /[1+sC 2 (R 2 +R 3 +R 2 R 3 /R 1 ) + s 2 C 1 C 2 R 2 R 3 ] where R 1 I DAC = V i (s). The reconstruction filter according to the invention, shown in figure 4, is of the low-pass, second-order, continuous-time type and comprises an operational amplifier 3 which is advantageously provided in an inverting configuration, as in figures 3a-3c, and the non-inverting input whereof is connected to the ground. A first resistor R 1 is parallel-connected to a first capacitor (C 1 ) and a second resistor (R 2 ) is connected with one terminal to a common node of the first resistor and of the first capacitor and, with its other terminal, to the inverting input of the operational amplifier 3. A second capacitor C 2 is feedback-connected between the output V O of the operational amplifier 3 and the inverting input, and two resistors R 3A and R 3B are respectively connected in parallel to said second capacitor; said resistors are arranged in series to each other, and the current I DAC is fed to a common node thereof. The comparison between this circuit solution (shown in figure 4) and the circuit solution of figure 3c shows that the resistor R 3 of figure 3c has been divided into the series of two resistors R 3A and R 3B (third and fourth resistors, respectively), so that the sum of these last two resistors arranged in series is equal in value to the resistor R 3 . The current I DAC enters directly the common node between the two resistors R 3A and R 3B . Calculation of the transfer function of the filter according to the invention shows that also in the case of a reconstruction filter with current input (I DAC ), the transfer function of the filter does not change and is always equal to that of the filter of figure 3b; but most of all, the area occupation is considerably smaller than that of the filter of figure 3c (it is in fact similar to that of the filter of figure 3b). The transfer function is therefore V O (s)/I DAC (s) = R 3B /[1+sC 2 (R 2 +R 3A +R 3B +R 2 (R 3A +R 3B )/R 1 ) + S 2 C 1 C 2 R 2 (R 3A +R 3B ] where therefore R 3A + R 3B = R 3 and R 3B I DAC = V 0FS . In practice it has been observed that the reconstruction filter according to the invention fully achieves the intended aim and objects, since it allows to optimize the area occupied by the filter with current input, in a manner similar to what occurs with a voltage-input reconstruction filter, thus allowing effective use thereof in an integrated circuit. The reconstruction filter thus conceived is susceptible of numerous modifications and variations, some of which may be within the scope of the inventive concept as defined in the appended claims ; all the details may also be replaced with other technically equivalent elements. In practice, the materials employed, so long as they are compatible with the specific use, as well as the dimensions, may be any according to requirements and to the state of the art. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims: An area-efficient low-pass, time-invariant, second-order reconstruction filter, particularly for current-driven digital-to-analog converters, comprising: a first resistor (R 1 ) and a first capacitor (C 1 ) which are parallel connected; an operational amplifier (3); a terminal of a second resistor, (R2) which is connected to the inverting input of said operational amplifier (3); another terminal of said second resistor which is connected to a common node of said first resistor and said first capacitor; a second capacitor (C2), which is fedback between the output of said operational amplifier (3) and said inverting input; characterized in that it comprises an additional pair of resistors (R3a,R3b) which are arranged so as to be fedback between said output and said inverting input, a current signal (Idac) arriving from a digital-to-analog converter arranged upstream of said reconstruction filter being fed to a common node of said additional pair of resistors (R3a, R3b). A reconstruction filter according to claim 1, characterized in that said additional pair of resistors comprises a third resistor and a fourth resistor which are mutually series-connected. A reconstruction filter according to claim 2, characterized in that the value of said fourth resistor multiplied by the value of said full-scale current signal is equal to the intended full-scale output voltage. A reconstruction filter according to claim 1, characterized in that the transfer function of said current-input reconstruction filter is similar to the transfer function of a reconstruction filter which is identical but has a voltage input.

Inventors: CONFALONIERI PIERANGELO, NICOLLINI GERMANO, CONFALONIERI, PIERANGELO, NICOLLINI, GERMANO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: H03H1
IPC codes: H03H11, H03M1