Abstract:
An area-efficient reconstruction filter removes undesirable sample images produced by current-driven digital-to-analog converters. The reconstruction filter includes: an input node for receiving the input current signal; an operational amplifier having first and second inputs and an output at which the output voltage signal is produced; a first resistor coupled between the output of the operational amplifier and the input node; a second resistor coupled to the first input of the operational amplifier; and a third resistor coupled between the input node and the second resistor. The reconstruction filter may also include a fourth resistor coupled between the input node and a reference voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of application Ser. No. 09/129,450 filed Aug. 4, 1998, U.S. Pat. No. 6,201,438. 
    
    
     TECHNICAL FIELD 
     The present invention relates to area-efficient reconstruction filters, particularly for current-driven digital-to-analog converters (DAC). 
     BACKGROUND OF THE INVENTION 
     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 FIG. 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 FIG.  2 . 
     In this Figure, the reference numeral  1  designates an N-bit DAC, where b 0 , b 1 , . . . , b a −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  1  and V 0  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 FIGS. 3 a  and  3   b.  The filters shown in these figures are second-order low-pass filters. The solution shown in FIG. 3 b,  however, is the one that is practically mandatory when working with supply voltages of less than 3V 
     The solution of FIG. 3 b  is rather area efficient when the input signal is a voltage, but it would be highly insufficient when the input signal must be a current, as shown in FIG. 3 c  by applying only the Norton equivalent to the input of FIG. 3 b.    
     A numeric example is now described to clarity the above explanation. 
     Assume that a DAC has been devised which has a full-scale voltage output V iPS =0.5 V and that a full-scale voltage V GPS =0.5 V from the reconstruction filter is required as output. Assume also that a cutoff frequency of approximately 270 kHz is chosen for the filter. 
     The values of the components of FIG. 3 b  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 CFS =0.5 V from the reconstruction filter with a frequency response that 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 FIG. 3 b,  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. 
     SUMMARY OF THE INVENTION 
     An 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 an area occupation at least as good 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 that is highly reliable, relatively easy to produce, and cost-competitive. 
     This aim, these objects and others which will become apparent hereinafter are achieved in one embodiment by an area-efficient reconstruction filter, particularly for current-driven digital-to-analog converters, including: an input node for receiving the input current signal; an operational amplifier having first and second inputs and an output at which the output voltage signal is produced; a first resistor coupled between the output of the operational amplifier and the input node; a second resistor connected to the first input of the operational amplifier; and a third resistor connected between the input node and the second resistor. The reconstruction filter may also include a fourth resistor coupled between the input node and a reference voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further characteristics and advantages of the present invention will become apparent from the description of preferred but not exclusive embodiments of a reconstruction filter according to the invention, illustrated only by way of non-limitative example in the s accompanying drawings. 
     FIG. 1 is a chart that plots the signal in output from a DAC. 
     FIG. 2 is a block diagram of a DAC circuit having a DAC stage and a reconstruction filter arranged downstream thereof. 
     FIGS. 3 a  and  3   b  are circuit diagrams of conventional low-pass, continuous-time, second-order, voltage-input reconstruction filters used downstream of a DAC stage. 
     FIG. 3 c  is a circuit diagram of a conventional low-pass, continuous-time, second-order current-input reconstruction filter, conceptually similar to the filter shown in FIG. 3 b.    
     FIG. 4 is a circuit diagram of a low-pass, continuous-time, second-order current-input reconstruction filter according to a first embodiment of the present invention. 
     FIG. 5 is a circuit diagram of a reconstruction filter according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the various Figures, identical reference numeral and letters designate identical elements. 
     The transfer functions of the reconstruction filters of FIGS. 3 a - 3   c  are respectively the following. 
     For the filter of FIG. 3 a:   
     
       
           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 FIG. 3 b:   
     
       
           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 FIG. 3 c:   
     
       
           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). 
     A reconstruction filter  10  according to a first embodiment of the invention is shown in FIG.  4 . The reconstruction filter  10  is of the low-pass, second-order, continuous-time type and comprises an operational amplifier  3  that is advantageously provided in an inverting configuration, as in FIGS. 3 b - 3   c.  The operational amplifier  3  has a non-inverting input that is connected to the ground. 
     The reconstruction filter  10  includes a first resistor R 1  parallel-connected to a first capacitor C 1 . A second resistor R 2  has a first terminal connected to a common node between the first resistor and the first capacitor and a second terminal connected to the inverting input of the operational amplifier  3 . 
     A second capacitor C 2  is feedback-connected between the output V 0  of the operational amplifier  3  and the inverting input. Two resistors R 3A  and R 3B  are connected in series with each other between the output of the operational amplifier and the first terminal of the second resistor R 2 . An input node  12  is coupled to a DAC stage  14  that feeds a current I DAC  to the input node. 
     A comparison between the reconstruction filter  10  shown in FIG.  4  and the circuit of FIG. 3 c  shows the resistor R 3  of FIG. 3 c  has been divided into the series-connected 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 . In addition, the current I DAC  enters directly at input node  12  between the two resistors R 3A  and R 3B  rather than at the junction between the first capacitor and the first, second, and third resistors R 1 ,R 2 ,R 3  as in the circuit of FIG. 3 c.    
     Calculation of the transfer function of the reconstruction filter  10  shows that in the case of a reconstruction filter with a current input (I DAC ), the transfer function of the filter does not change and is always equal to that of the filter of FIG. 3 b.  Moreover, the area occupation of the reconstruction filter  10  is considerably smaller than that of the filter of FIG. 3 c  (it is in fact similar to that of the filter of FIG. 3 b ). 
     The transfer function for the reconstruction filter  10  is: 
     
       
           V   0 ( 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 R 3A +R 3B =R 3  and R 3B I DAC =V OFS . 
     In practice it has been observed that the reconstruction filter  10  fully achieves the intended aim and objects discussed above, since it optimizes 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. 
     A reconstruction filter  20  according to a second embodiment of the invention is shown in FIG.  5 . The reconstruction filter  20  differs from the reconstruction filter  10  of FIG. 4 in that rather than the first resistor R 1  being in parallel with the first capacitor C 1 , the reconstruction filter  20  includes a first resistor R 4  coupled directly to the input node  12 . In other words, the first resistor R 4  of the reconstruction filter  20  is placed in parallel with the DAC stage  14 , The resistors R 5 , R 6 , and R 7  are positioned like the resistors R 2 , R 3A , and R 3B , respectively of FIG. 4, and the capacitors C 3  and C 4  are positioned like the capacitors C 1  and C 2 , respectively of FIG.  4 . 
     In order to give the reconstruction filter  20  of FIG. 5 the same transfer function and frequency response as the reconstruction filter  10  of FIG. 4, the values of the resistors and capacitors of the reconstruction filter  20  should be resized compared to those of the reconstruction filter  10 . The transfer function of the reconstruction filter  20  is:              V   0          (   s   )           I   DAC          (   s   )         =         R   7             1   +       sC   4          [         R   5          (     1   +       R   7       R   4         )       +     R   6     +     R   7     +         R   6          R   7         R   4         ]       +                 s   2          C   3          C   4            R   5          (       R   6     +     R   7     +         R   6          R   7         R   4         )                 .                            
     Equalizing the transfer functions of the reconstruction filters  10 ,  20  of FIGS. 4 and 5 and resolving the resulting system one obtains:          R   5     =           R   2     k                     R   7       =     R     3      B                   R   4     =       R     3      B         k        [       (     2   -     1   k       )     +           R     3      A       +     R     3      B           R   2            (     1   -   k     )         ]                   R   6     =           k        (       R     3      A       +     R     3      B         )       -     R     3      B           k        [     2   +           R     3      A       +     R     3      B           R   2            (     1   -   k     )         ]         .                            
     By opportunely choosing k, it can be demonstrated that the sum of the resistance values so obtained for the reconstruction filter  20  (R 4 +R 5 +R 6 +R 7 ) is much less than that of the reconstruction filter  10  (R 1 +R 2 +R 3A +R 3B ). For example, if a cut-off frequency of 270 KHz is desired for the reconstruction filters  10 ,  20 , with I DACFS =160 μA and V aFS =0.5 V, the reconstruction filter  10  of FIG. 4 would require: 
     R 1 =R 2 =50 KΩ R 3A =46.875 KΩ R 3B =3.125 KΩ C 1 =25.2 pF C 2 =5.6 pF. 
     In contrast, the reconstruction filter  20  of FIG. 5 would require: 
     R 4 =R 7 =3.125 KΩ R 5 =50 KΩ R 6 =23.4375 KΩ C 1 =25.2 pF C 2 =5.6 pF. 
     As a result, the total resistance for the reconstruction filter is 150 KΩ, while the total resistance for the reconstruction filter  20  is only 78 KΩ. Such smaller resistance values results in a silicon area savings of about 50% for the resistors of the reconstruction filter  20  compared to the ones of the reconstruction filter  10  and the prior art voltage-input reconstruction filter of FIG. 3 b.    
     The reconstruction filters  10 ,  20  discussed above are susceptible to 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. 
     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. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims and equivalents thereto.