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

[Harry Bissell Jr]

Hi Roy (et al)

I wnat to tie two threads together here... the making
a 16 bit d/a and your idea.

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.

The technique for getting an expo response is usually
to use a multiplying dac, and feed some of the output
into the reference input. I have not seen the
schematics personally, but I think that later PAiA
designs used this technique.

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. 

(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.

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

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?
> 
> 
> 
> 
> 
> 
>