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

[Roy J. Tellason]

On Monday 25 April 2005 02:21 pm, Harry Bissell Jr wrote:
> Hi Roy (et al)
>
> I wnat to tie two threads together here... the making a 16 bit d/a and your
> idea.

Ok...

Though my thinking is that 16 bits wouldn't really be required.  For fairly 
crude uses, 8 might be enough.  For most others,  10 or 12 should be.

> Your idea will not work directly. The steps need to be 1/2 of the next step,
> or they will fail to line up. what works at one end of the range will cause
> holes in the other.

I'm not sure I understand how you mean this.  If I take an op amp with some 
resistor setting the gain (say with unity gain as maximum) and the input 
impedance is a known value,  then what's the problem with connecting various 
other resistors to give a log response?

> The technique for getting an expo response is usually to use a multiplying
> dac, and feed some of the output into the reference input.

Well,  I guess that's one way to do it.

> I have not seen the schematics personally, but I think that later PAiA
> designs used this technique.

Something to look at,  for sure.  But a multiplying DAC seems like a little 
money I could perhaps avoid spending,  if I don't have to.

> 4066 switches could be used, but you need to make the resistors MUCH bigger
> than the on resistance of the switch of you get a lot of error.

Seems to me those things have an on resistance that's way lower than their 
4016 predecessor.  Something like 50 ohms?

There _was_ an article in EW about those,  or maybe it was about FETs in 
general,  I forget,  but I don't know how seriously I want to take their 
concerns about distortion,  since they were the same folks that published 
another article about different kinds of capacitors having measurably 
different distortion levels in another,  earlier article -- and the numbers I 
remember being tossed around were *way* low,  way smaller than I'd ever 
expect to actually hear.

> (now joining the other thread)...
>
> Setting the resistor to a size large enough to get low error on the MSB
> means that the resistor size will be really high on the LSB.   With eight
> bits it will be possible.
>
> with 16 bits, the range of resistance values, or the currents neede to drive
> the ladder will get out of hand.

Isn't that why they came up with the R2R ladder in the first place?

> Look at some of the early Sequential Circuits designs where they made
> discrete DACs.  They would parallel gates to get enough current to drive the
> MSBs.  A single CMOS output was not enough. They were just trying for a good
> EIGHT bits.

I never worked on that much of their stuff,  which ones are you thinking of?  
And do you have any links to schematics?

> I'd buy a 16 bit converter if I needed one.

I'm not sure that I do,  for what I have in mind.  Time to play around on a 
breadboard,  I guess...

> H^) harry
>
> --- "Roy J. Tellason" <rtellason at blazenet.net> wrote:
> > I've seen a number of different web pages which detail making a
> > dirt-simple D/A converter,  usually involving either a number of
> > resistors that double in value or an R/2R ladder.
> >
> > I was thinking that it probably should be possible to do something similar
> > only tailoring the response the same way an "audio taper" pot is tailored,
> > exponentially rather than linearly.
> >
> > What I'd like is the ability to switch,  route,  and control the level of
> > audio signals.  The stuff I am thinking of would be more along the lines
> > of what you'd find as front panel controls on a big mixer,  rather than
> > the sort of precise responses to CVs that one would tend to need in the
> > context of a typical VCA.
> >
> > I also have a whole mess of 4066 chips,  and am thinking that these should
> > be somehow useful in this context.
> >
> > Comments?