DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT FIG.
7 shows a block diagram of the inventive sigma delta modulator.
A comparison of FIG.
7 and FIG.
2 indicates that the inventive modulator has an additional filter element, 712, in the feedback loop.
In particular, the analog input on lead 700 is applied, via sampling switch 702, to a summing junction 704.
The output 706 of summing junction 704 is applied to filter unit 708 and the output 714 of this latter filter unit is, in turn, applied to A/D converter 720.
Converter 720 is controlled to operate at the sampling clock frequency via the sampling clock provided on lead 718.
The output of converter 720 on lead 724 is the pulse density modulated output Y(z).
This output is fed back, via the output 716 of D/A converter 722, to filter unit 712.
The output 710 of filter unit 712 is, in turn, applied to summing junction 704.
From conventional theory, it can easily be shown that the linear forward transfer function is: ##EQU3## and the noise transfer function (assuming that the quantization noise introduced by A/D converter 720 is equivalent to additive white noise at the position of converter 720 is: ##EQU4## These transfer functions have the general shape shown in FIGS.
3A and 3B, respectively.
However, the transfer functions now depend on both of the functions G(z) and H(z).
Consequently, by proper choice of the functions G(z) and H(z), the forward transfer function can be specified independently of the noise transfer function.
In accordance with another embodiment of the invention, as shown in FIG.
8A, both the forward filter circuit and the feedback filter circuit can be implemented as distributed circuits.
In particular, as shown in FIG.
8A, the forward filter function G(z) shown in FIG.
7 as circuit 708 can be divided into two or more discrete functions G1(z)-G3(z) corresponding to circuits 808, 820 and 830.
Similarly, the feedback filter function H(z) shown in FIG.
7 as circuit 710 can be divided into two or more portions H1(z)-H3(z) corresponding to circuits 812, 822 and 832
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I/O Systems 
