Cascaded recursive transversal filter for sigma-delta modulators

A sigma-delta analog-to-digital converter including an output port at which the desired digital signals are generated, said sigma-delta analog-to-digital converter comprising:

• input summing means including a noninverting input port coupled to receive analog signals to be converted into digital signals, and an inverting input port coupled to receive reconstructed digital signals, to thereby produce difference signals;
• a digital-to-analog converter coupled to said output port of said sigma-delta analog-to-digital converter and to said inverting input port of said input summing means, for converting said desired digital signals into said reconstructed digital signals;
• a loop filter coupled to said input summing means for receiving said difference signals therefrom, and for filtering said difference signals with a frequency characteristic which includes N gain poles and N-1 gain zeroes, where N is an integer greater than one, to produce a resonated analog signal, said loop filter further including a cascade of a plurality M of feedback network stages, where M is greater than one, and P feedforward network stages, where P is at least one, each of said feedback network stages comprising a summer with a main input port and a summed signal output port, for receiving processed signal at said main input port from a preceding stage, and for, in the ith feedback stage, summing said processed signal with N.sub.i weighted, delayed versions of the signal appearing at said summed output port, where the sum of all N.sub.i for all stages equals N, and each of said feedforward network stages comprising a summer including a plurality of input ports and a summed signal output port, for, in the jth feedforward stage, receiving L.sub.j plural mutually delayed and weighted versions of processed signals from a preceding stage, and for summing together said L.sub.j mutually delayed and weighted versions in a feedforward manner, where the sum of all L.sub.j for all feedforward stages equals N-1;
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• analog-to-digital conversion means coupled to said loop filter and to said output port of said sigma-delta analog-to-digital converter, for converting said resonated analog signal into said desired digital signals at said output port of said sigma-delta analog-to-digital converter.

A sigma-delta analog-to-digital converter including an output port at which the desired digital signals are generated, said sigma-delta analog-to-digital converter comprising:

• input summing means including a noninverting input port coupled to receive analog signals to be converted into digital signals, and an inverting input port coupled to receive reconstructed digital signals, to thereby produce difference signals;
• a digital-to-analog converter coupled to said output port of said sigma-delta analog-to-digital converter and to said inverting input port of said input summing means, for converting said desired digital signals into said reconstructed digital signals;
• a loop filter coupled to said input summing means for receiving said difference signals therefrom, and for filtering said difference signals with a frequency characteristic which includes a plurality of gain poles and a plurality of gain zeroes, to produce a resonated analog signal;
• analog-to-digital conversion means coupled to said loop filter and to said output port of said sigma-delta analog-to-digital converter, for converting said resonated analog signal into said desired digital signals;
• said loop filter including a cascade of a plurality of filter stages, each of said filter stages including
• (a) an input summer coupled for receiving stage output signals from a preceding one of said filter stages in the case of all of said filter stages except a first one of said filter stages, and for receiving said difference signals from said input summing means in the case of said first one of said filter stages, for combining the signals applied thereto with delayed, weighted feedback signals, to thereby produce stage intermediate signals;
• (b) an output summer coupled for receiving weighted intermediate signals, for combining said weighted intermediate signals with delayed, weighted feedforward signals, to thereby produce said stage output signals;
• (c) stage coupling means coupled to said input and output summers, for coupling said intermediate signals from said input summer to said output summer, and for weighting said intermediate signals applied to said output summer;
• (d) delay means coupled to said stage coupling means for delaying said intermediate signals flowing in said stage coupling means, for generating at least one delayed intermediate signal;
• (e) feedback signal weighting means coupled to said delay means and to said input summer for weighting said delayed intermediate signals for generating said delayed, weighted feedback signals;
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• at least one of said filter stages further including feedforward weighting means coupled to said delay means and to said output summer, for weighting said delayed intermediate signals, for generating said delayed, weighted feedforward signals.

A sigma-delta analog-to-digital converter including an output port at which the desired digital signals are generated, said sigma-delta analog-to-digital converter comprising:

• input summing means including a noninverting input port coupled to receive analog signals to be converted into digital signals, and an inverting input port coupled to receive reconstructed digital signals, to thereby produce difference signals;
• a digital-to-analog converter coupled to said output port of said sigma-delta analog-to-digital converter and to said inverting input port of said input summing means, for converting said desired digital signals into said reconstructed digital signals;
• a loop filter coupled to said input summing means for receiving said difference signals therefrom, and for filtering said difference signals with a frequency characteristic which includes N gain poles and N-1 gain zeroes, where N is an integer greater than one, to produce a resonated analog signal, said loop filter further including M cascaded filter stages, where M is an integer not smaller than two and not larger than N, at least the ith one of said M filter stages including
• (a) an input summer coupled, in the case of the first one of said M stages, receiving said difference signals from said input summing means, and in the case of filter stages other than said first one of said filter stages, receiving output signals from a preceding one of said filter stages, for summing said one of said difference signals and said output signals from a preceding one of said stages with weighted, delayed feedback signals, to thereby produce stage intermediate signals;
• (b) an output summer coupled for receiving weighted intermediate signals, for combining said weighted intermediate signals with delayed, weighted feedforward signals, to thereby produce said stage output signals;
• (c) stage intermediate coupling means coupled to said input and output summers, for coupling said intermediate signals from said input summer to said output summer, and for weighting said intermediate signals to produce said weighted intermediate signals for application to said output summer;
• (d) delay means coupled to said stage intermediate coupling means for delaying said intermediate signals flowing in said stage intermediate coupling means, for generating N.sub.i delayed intermediate signals, where i designates the ith stage of said cascade of filter stages, and N.sub.i is an integer no less than one, and where the sum of the values of N.sub.i is N;
• (e) feedback signal weighting means coupled to said delay means and to said input summer for weighting said delayed intermediate signals for generating said delayed, weighted feedback signals for application to said input summer;
• (f) feedforward signal weighting means coupled to said delay means and to said output summer, for weighting said delayed intermediate signals for generating said weighted feedforward signals for application to said output summer;
• and, when M is greater than two, at least an other of said M filter stages including said input summer, said delay means, and said feedback weighting means, but not including said feedforward signal weighting means or said output summer, whereby the output of said other of said stages is the output of said stage intermediate coupling means;
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• analog-to-digital conversion means coupled to said loop filter and to said output port of said sigma-delta analog-to-digital converter, for converting said resonated analog signal into said desired digital signals at said output port of said sigma-delta analog-to-digital converter.