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

[0002] 1. Field of the Invention

[0003] The present invention relates to area-efficient reconstruction filters, particularly for current-driven digital-to-analog converters (DAC).

[0004] 2. Discussion of the Related Art

[0005] Digital-to-analog converters are conventionally used very frequently in integrated circuits.

[0006] 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 an undesirable duplicate image of the signal, generally designated as “imaging”, as shown in FIG. 1, which plots the output of the DAC as a function of the frequency f.

[0007] 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_sof the DAC.

[0008] 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 FIG. 2.

[0009] In this figure, the reference numeral 1 designates an N-bit DAC, where b₀, b₁, . . . b_n-1are the input bits of the DAC and V_DACand I_DACare, 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_ois the output voltage.

[0010] The reconstruction filter 2 must provide high attenuation for frequencies close to the sampling frequency f_sof 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.

[0011] It is known to those skilled in the art that these are two mutually contrasting requirements.

[0012] It is therefore necessary to achieve a compromise, shown in FIGS. 3aand 3b.The filters shown in these figures are second-order low-pass filters. The solution shown in FIG. 3b,however, is the one that is practically mandatory when working with supply voltages of less than 3V.

[0013] 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 FIG. 3cby applying only the Norton equivalent to the input of FIG. 3b.

[0014] A numeric example is now described to clarify the above explanation.

[0015] Assume that a DAC has been devised which has a full-scale voltage output V_iFSand that a full-scale voltage from the reconstruction filter is required as output. Assume also that a cutoff frequency of approximately 270 kHz is chosen for the filter.

[0016] The values of the components of FIG. 3bwill be as follows:

[0017] R₁=R₂=R₃=50 kohm C₁=25.2 pF C₂=5.6 pF

[0018] 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.

[0019] Since one must have R₃I_DACPS=V_OFS, then R₃=3.125 kohm and therefore R₁=R₂=R₃=3.125 kohm.

[0020] Therefore, in order to have the same frequency response as the preceding filter, the values of C₁and C₂must be 403.2 pF and 89.6 pF respectively.

[0021] 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 FIG. 3b, due to the relatively low specific capacitance that can be provide in integrated circuits.

[0022] 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.

[0023] An aim of the present invention is to provide an area-efficient reconstruction filter, particularly for current-driven digital-to-analog converters.

[0024] 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 integrated circuit.

[0025] 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 a current input occupies the same area as the corresponding reconstruction filter with a voltage input.

[0026] Another object of the present invention is to provide a reconstruction filter with a current input having the same transfer function as a similar reconstruction filter with a voltage input.

[0027] Another object of the present invention is to provide a reconstruction filter which is highly reliable, relatively easy to produce and at competitive costs.

[0028] 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 feedback-connected between the output of said operational amplifier and said inverting input; and an additional pair of resistors which are arranged so as to provide feedback 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 resistor.

[0029] 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:

[0030] FIG. 1 is a chart that plots the signal in output from a DAC;

[0031] FIG. 2 is a block diagram of a DAC with a reconstruction filter arranged downstream thereof;

[0032] FIGS. 3aand 3bare circuit diagrams of conventional low-pass, continuous-time, second-order, voltage-input reconstruction filters used downstream of the DACs, as shown in figure 2;

[0033] FIG. 3cis a circuit diagram of a conventional low-pass, continuous-time, second-order current-input reconstruction filter, conceptually similar to the filter shown in FIG. 3b; and

[0034] FIG. 4 is a circuit diagram of a low-pass, continuous-time, second-order current-input reconstruction filter according to the present invention.

[0035] With reference to the above figures, FIGS. 1 to 3care not described further herein, since reference has already been made thereto in the description of the prior art and of the problems related thereto.

[0036] In the various figures, identical reference numerals and letters designate identical elements.

[0037] FIGS. 3a-3crelate to the definition of the transfer functions of said filters, which are respectively the following.

[0038] For the filter of FIG. 3a:

[0039] V₀(s)/V_i(s)=1/[1+sC₁(R₁+R₂)+S²C₁C₂R₁R₂]

[0040] For the filter of FIG. 3b:

[0041] V₀(s)/V₁(s)=(R₃/R₁)/[1+sC₂(R₂+R₃+R₂R₃/R₁)+s²C₁C₂R₂R₃]

[0042] And finally, for the filter of FIG. 3c:

[0043] V₀(s)/I_DAC(s)=R₃/[1+sC₂(R₂+R₃+R₂R₃/R₁)+s²C₁C₂R₂R₃]

[0044] where R₁I_DAC=V_i(s).

[0045] The reconstruction filter according to the invention, shown in FIG. 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 FIGS. 3a-3c, and the non-inverting input whereof is connected to the ground.

[0046] A first resistor R₁is parallel-connected to a first capacitor (C₁) and a second resistor (R₂) 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.

[0047] A second capacitor C₂is feedback-connected between the output V_oof the operational amplifier 3 and the inverting input, and two resistors R_3Aand R_3Bare respectively connected in parallel to said second capacitor; said resistors are arranged in series to each other, and the current I_DACis fed to a common node thereof. The comparison between this circuit solution (shown in figure 4) and the circuit solution of FIG. 3cshows that the resistor R₃of FIG. 3chas been divided into the series of two resistors R_3Aand 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 R₃.

[0048] The current I_DACenters directly the common node between the two resistors R_3Aand R_3A.

[0049] 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 FIG. 3b; but most of all, the area occupied is considerably smaller than that of the filter of FIG. 3c(it is in fact similar to that of the filter of FIG. 3b).

[0050] The transfer function is therefore

[0051] V₀(s)/I_DAC(s)=R_3B/[1+sC₂(R₂+R_3A+R_3B+R₂(R_3A+R_3B)/R₁)+s²C₁C₂R₂(R_3A+R_3B]

[0052] where therefore R_3A+R_3B=R₃and R_3BI_DAC=V_OFS.

[0053] In practice it has been observed that the reconstruction filter according to the invention fully achieves the intended aim and objects, since it allows optimization of the area occupied by the filter with a current input, in a manner similar to what occurs with a voltage-input reconstruction filter, thus allowing effective use thereof in an integrated circuit.

[0054] The reconstruction filter thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept; all the details may also be replaced with other technically equivalent elements.

[0055] 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.

[0056] Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.