DACs plus Shepard project

[gstopp at fibermux.com]

     Juergen's right - the discrete DAC is really easy and works great. I 
     used one recently in my Shepard function generator - a 74HCT245 
     driving a 100K/200K R-2R ladder into a buffer. I based the design on 
     John Simonton's PAIA web page schematics (thanks, Honcho!). Works 
     great.
     
     For those who are interested in my version of this module:
     
     I put all the logic (counters, EXOR gates, and adders) into a small 
     EPLD chip (Altera, you know my favorite logic chip) and added the 
     external buffer chip (I used the 245 because all the bits IN's and 
     OUT's are lined up on either side of the chip). I used a 4046 plus a 
     CV summer to provide a voltage-controlled clock. I used 2x gain buffer 
     amps on the triangles so that they go from ground to 5 volts. I added 
     a reversible attenuator between the DAC output and the sawtooth S/H 
     analog switch mux, so that all the sawtooths can go from negative 
     down-ramps to no signal to positive up-ramps with one pot sweep (a 
     DC-coupled four-quadrant multiplier here could make this feature 
     voltage-controlled).
     
     Other easy DACs - the AD7228 octal 8-bit is wonderful, but costly. The 
     AD558 is a nice single 8-bit chip, with it's own latch (great for 
     monophonic keyboard interfaces). I've recently dug up some old MC1408 
     and MC1406 chips, which used to be pretty popular, and they look 
     fairly useful on the scope.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: AW: Re: Midi to CV -> ZN426 DAC ?
Author:  Haible_Juergen#Tel2743 <HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de>
at ccrelayout
Date:    2/4/97 12:40 PM
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What about a discrete DAC? I built one for my poly keyboard. Just a few 
CMOS drivers and a handfull of hand-selected resistors - works great !
     
JH.