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

[Harry Bissell Jr]

--- "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


> > 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...
> 
> 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" ???

> > 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.

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

H^) harry