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

[Roy J. Tellason]

On Monday 25 April 2005 05:00 pm, Harry Bissell Jr wrote:
> --- "Roy J. Tellason" <rtellason at blazenet.net> wrote:
> > > 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?
>
> Thunk of the point where you do the transition from 1/2 scale to the next
> step. (from 10000000 to 10000001)
>
> The major step will be OK, because you sized the resistor correctly. But the
> LSB will be wrong... you chose the value at the low end of the scale and its
> a very small step.  After this change it should be a much BIGGER step.  No
> matter which way you slice it, you lose.

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?

> PMI made a 'companding DAC' back in the 80's... They used an number of
> "cords" which set the upper and lower limits to a series of linear 5 bit
> "steps" so it was a piecewise linear approximation to an expo function.

I might even have some data on that somewhere,  I have some fairly old 
databooks around here.  I'll have to take a look.  I never did pay all that 
much attention to the conversion parts back then,  focusing mostly on the 
logic...

> > > 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?
>
> Yes... something LIKE 50 ohms but not stable with applied voltage for one
> thing... and not guaranteed to be 50 ohms, and not matched from package to
> package...

Yeah,  but if the circuit resistances around it are up in the 1000s of ohms,  
what difference will it make?

> Still you'd need to probably be in the 5K range (minimum) for your
> resistors... so that the 50 ohms (maybe) is a small fraction of the total,
> and the voltage drop across the switch remains small.

Just so.

> 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.  :-)

I could always use a single-supply op amp,  too.  Give me something to do wtih 
my 324s ferinstance...

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

> In this case you need to precisely source, or sink currents into that node.
> The resistors are the same size, but you still rely on being able to
> precisely hold the point high, or low... without modulating the
> voltage on the CMOS output from the state of other stages... not easy.

True.  But if I'm not _that_ worried about accuracy what's the big deal about 
it?

> 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?  I don't see a 
need for it in what I'm talking about...