[sdiy] resistors and 4066 gates (and 16 bit...)

[Roy J. Tellason]

On Monday 25 April 2005 06:02 pm, Harry Bissell Jr wrote:
> --- "Roy J. Tellason" <rtellason at blazenet.net> wrote:
> > I think I could probably do with a whole lot less accuracy than most
> > applications that would use this sort of a part. Look at typical pots, 
> > for example -- they have a tolerance of what,  20% typically?
>
> Don't agree... Pots have a magic feedback device.. the operator. Even a 20%
> pot can probably be set to 1% if you are touchy with it...  ;^P

<shrug>

> > > If you wanted to, you could make resistive dividers on the far side of
> > > the switch (to set bit weights) and then sum them together into an
> > > opamp.
> >
> > My thinking was running along those lines,  yeah.
> >
> > > Locating the switch at the inverting input would guarantee that the
> > > voltage would be close to zero. You'd need to set the supplies to the
> > > 4066 to either +/-7.5 (with logic level problems) or cheat and run the
> > > negative pin one diode drop below ground (-.7 to +15 perhaps).  This
> > > would allow you to trim out any errors on a bit by bit basis.
> >
> > More ambitious than I want to get with this stuff.
> >
> > :-)
>
> If you really want a 'pot' this might be the way to go...as the signal input
> could be the reference. Long as you don't reverse bias the switches...

Yep.  Got lots of resistors around here...

> > I could always use a single-supply op amp,  too. Give me something to do
> > wtih my 324s ferinstance...
>
> Yack !!!   You are desperate aren't you. Even see Spock say "Captain...that
> would be the equivalent of making a nemantic converter using stone knives
> and bear-skins" ???

You don't like that chip then?  :-)

> > > The Sequential Circuits Model 700 programmer used this type of DAC,
> > > with .01% hand matched resistors and parallel'd gates where higher
> > > currents were expected.
> > >
> > > Early ProV used this as well iirc.
> >
> > That's definitely *way* more ambitious than I want to get to give me
> > something that's essentially electronic volume controls and pan pots. 
> > What did they use this stuff for?  Waveform generation?  Something
> > else?
>
> They used it for making Pitch CV voltages.  You only need 7 bits (128 notes)
> but they really, really need to be the correct notes. Low bit count, but
> precise linearity needed.

Sure.  I remember being able to _hear_ the difference when a CV moved by a mV 
or two.  But I scarcely think my application is that sensitive.

> For setting non-critical parameters, not needed. Still, a monolythic DAC of
> DAC-08 ilk is not a bad choice.

True.  But I don't have any on hand here,  nor am I currently in a position to 
buy parts just to fool around with,  unlike some folks in here.  I'm just 
thinking about ways to use what I've got,  most of which is what I've been 
able to salvage.  Every c64 board that got scrapped had two 4066 chips on it 
(along with all the rest of the parts)...

Maybe I can do something with one of the ROMs as a pseudo-random number 
generator?  :-)