[sdiy] Re: Inexpensive 16 bit DAC?

Fri Apr 29 03:38:05 CEST 2005

Lowering the VREF of a multiplying D/A converter will degrade the INL and 
DNL performance...if you are going to sum a fine and coarse voltage 
together, better to scale the gain of the fine input...however while doing 
this will accomplish specific tasks...it will still not be a multi bit 
composite DAC (this scheme would be good for attempting to like say correct 
minor pitch deficiencies in a VCO for instance...).

At 10:45 AM 4/28/2005 -0700, Tim Parkhurst wrote:
>What about the trick Oberheim used on the Matrix 12? Or was it the OB
>series? Anyway, they used an 8 bit DAC, but in places where they need higher
>resolution (VCO fine tune, fer example), they used a fairly simple trick to
>get (approx) 12 to 13 bit resolution:
>
>Set DAC Vref to its 'normal' level. Say +10V
>Convert 8 MSB, store analog result in S/H
>Set DAC Vref to lower level. Say 1V
>Convert 8 LSB, add analog result to MSB conversion.
>
>Basically, you take two 8-bit samples, but you change the Vref for the LSB
>sample and add the two results. The voltage levels I have listed above are
>only examples, but you get the basic idea.
>
>
>Tim (lots of basic ideas) Servo
>
>"Imagination is more important than knowledge." - Albert Einstein
>
>**********************************
>
>
> > -----Original Message-----
> > From: Scott Gravenhorst [mailto:music.maker at gte.net]
> > Sent: Thursday, April 28, 2005 10:13 AM
> > To: synth-diy at dropmix.xs4all.nl
> > Subject: Re: [sdiy] Re: Inexpensive 16 bit DAC?
> >
> > Harry Bissell Jr <harrybissell at prodigy.net> wrote:
> > >As drawn, no advantage. Possiblt a little better
> > >symmetry, have to check the source / sink currents
> > >of the buffer to see if it would be truly
> > >symmetrical...
> >
> > I can't speak for the 4066, but I know that the 4069UB is
> > spec'd as having symmetrical transistors.  It's one of the
> > reasons I like them.
> >
> > >By extension, you sould replace the "+12V" going
> > >to the switches with a precision reference... or even
> > >a variable reference... making a multiplying DAC.
> > >(of course negative voltages are out).
> >
> > Truly.  For me, it was mental gymnastics.
> >
> > >
> > >Back to my ORIGINAL complaint and name of the
> > >thread... with 1M and 2M resistors you are going to
> > >not, NO WAY make your own 'inecpensive' 16 bit
> > >converter.  The archetecture is there, but the
> > >implementation is NOT !!!
> >
> > I am so out of touch, is this because they must be precision
> > resistors and prec. resistors of that value range are more
> > expensive?
> >
> >
> >
> > >--- Scott Gravenhorst <music.maker at gte.net> wrote:
> > >> It seems to me that the analog switches are overkill
> > >> and could be replaced by
> > >> a CMOS inverter gate (or buffer) per bit.  It looks
> > >> to me that the switches
> > >> are simply providing a solid 12 volts or a solid
> > >> ground to one end of each
> > >> resistor.  I'm really not sure why a 4069UB
> > >> wouldn't work as well at less
> > >> than half the part count.  Ok, you would want to do
> > >> TTL to CMOS level
> > >> conversion, but still it would be fewer parts.  A
> > >> 4066 is a bidirectional
> > >> device but you are using it as a unidirectional
> > >> device which is why I think a
> > >> 4069UB design would work just as well.  The 4069
> > >> would be powered from +12
> > >> and ground, the output of each gate hits the
> > >> resistor.  If the whole thing
> > >> were powered by +5 and ground, it would be even
> > >> simpler.
> > >>
> > >> Is there some advantage the 4066 provides that I'm
> > >> not seeing?
> > >>
> > >> "Ray Wilson" <raywilson at comcast.net> wrote:
> > >> >
> > >> >Here is a DAC with CD40106 and CD4066 chips. I have
> > >> demo'd 4 bits but you
> > >> >could go as many as you had patience to build.
> > >> >
> > >> >Ray
> > >> >
> > >> >----- Original Message -----
> > >> >From: "KA4HJH" <ka4hjh at gte.net>
> > >> >To: "Bob Weigel" <sounddoctorin at imt.net>
> > >> >Cc: <synth-diy at dropmix.xs4all.nl>
> > >> >Sent: Wednesday, April 27, 2005 3:24 AM
> > >> >Subject: Re: [sdiy] Re: Inexpensive 16 bit DAC?
> > >> >
> > >> >
> > >> >> >Usually you have to tell them that you are like
> > >> working on something
> > >> >>>where you might actually BUY some chips to get
> > >> samples...or does Analog
> > >> >>>just hand them out to anyone?  I could have been
> > >> getting samples all
> > >> >>>along for the project I'm working on from
> > >> microchip but I haven't even
> > >> >>>had time to ask for them and I feel bad doing it
> > >> because we really don't
> > >> >>>need them except that that company is out of
> > >> money I guess so maybe we
> > >> >>>do :-). -Bob
> > >> >>
> > >> >> They may ask you a few simple questions, like
> > >> "What's this for?" or
> > >> >> "What's
> > >> >> the goal of the project?". So you just answer
> > >> "experimental interface
> > >> >> design for musical instrument" or something lofty
> > >> you've daydreamed about
> > >> >> but will probably never get around to. Talk like
> > >> a junior in college--half
> > >> >> clueless but you know some buzzwords.
> > >> >>
> > >> >> Maxim is another one you want to hit up...they've
> > >> got good stuff...
> > >> >>
> > >> >> --
> > >> >>
> > >> >> Terry Bowman, KA4HJH
> > >> >> "The Mac Doctor"
> > >> >>
> > >> >> "You'd PAY to know what you REALLY think"--Dobbs
> > >> >
> > >> >
> > >>
> > >>
> > >---------------------------------------------------------
> > >> - Where merit is not rewarded, excellence fades.
> > >> - Hydrogen is pointless without solar.
> > >> - What good are laws that only lawyers understand?
> > >> - The media's credibility should always be
> > >> questioned.
> > >> - The only good terrorist is a dead terrorist.
> > >> - Governments do nothing well, save collect taxes.
> > >>
> > >> -- Scott Gravenhorst | LegoManiac / Lego Trains /
> > >> RIS 1.5
> > >> -- Linux Rex         | RedWebMail by RedStarWare
> > >> -- FatMan: home1.gte.net/res0658s/fatman/
> > >> -- NonFatMan: home1.gte.net/res0658s/electronics/
> > >> -- Autodidactic Master of Arcane and Hidden
> > >> Knowledge.
> > >>
> > >>
> > >
> >
> > ---------------------------------------------------------
> > - Where merit is not rewarded, excellence fades.
> > - Hydrogen is pointless without solar.
> > - What good are laws that only lawyers understand?
> > - The media's credibility should always be questioned.
> > - The only good terrorist is a dead terrorist.
> > - Governments do nothing well, save collect taxes.
> >
> > -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
> > -- Linux Rex         | RedWebMail by RedStarWare
> > -- FatMan: home1.gte.net/res0658s/fatman/
> > -- NonFatMan: home1.gte.net/res0658s/electronics/
> > -- Autodidactic Master of Arcane and Hidden Knowledge.
> >
> > --
> > No virus found in this incoming message.
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> >
>
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>
>
>
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         -Jim
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