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Difference Integration Spectrum Limiting D/A Converter

IP.com Disclosure Number: IPCOM000106043D
Original Publication Date: 1993-Sep-01
Included in the Prior Art Database: 2005-Mar-20
Document File: 2 page(s) / 76K

Publishing Venue

IBM

Related People

Ferry, M: AUTHOR

Abstract

Disclosed is a circuit that converts a digital input into an analog output with a spectrum content above the Nyquist frequency improved respective to a conventional staircase converter.

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Difference Integration Spectrum Limiting D/A Converter

      Disclosed is a circuit that converts a digital input into an
analog output  with  a  spectrum content above the Nyquist frequency
improved respective to a conventional staircase converter.

      The high frequency spectrum of such a conventional converter
can be attributed to the abrupt changes of the converter output when
the digital input value changes.  For this reason,  a  conventional
D/A  con verter is  always  followed by a more or less complex low
pass filter the task of which is to smooth out the converter  output.
Whereas  at low frequencies techniques exist that allow to integrate
such a filter on  silicon,it  is no longer the case at high
frequencies.  It is the purpose of the present disclosure to describe
a converter that  gener ates a smoothed signal and has consequently a
limited high frequency spectrum.  Further, the described converter is
well suited  to  silicon integration.

      The  time  domain  output  of a conventional D/A can be seen as
the convolution of the digital input signal with a gate function that
is 0 before time 0, 1 between 0 and T (sampling time interval) and 0
after T : the result is a staircase function, each stair having the
value of the digital  input.  The frequency domain output of such a
converter, ac cording to Fourrier, is the product of the input signal
spectrum  by the transform of the gate function (
sin(Pi*f*T)/(Pi*f*T) = sinc f ).

      Let  one  now  convolute  2 times the digital input by the
previous gate function, the result can also be seen as the
convolution  of  the input  by  the self convolution of the gate
function.  This self convolu tion is a triangle of height 1 and base
2T.  Convoluting this triangle with  the  digital  input gives a
curve with linear interpolation be tween the digital samples value.
The frequency spectrum of the ...