[sdiy] Static ADC and DAC recommendations?

[Phillip Harbison]

cheater cheater wrote:
> Wait a sec guys - I thought the usual approach was folding
> converters. That is, you take the input signal, scale it to
> e.g. 0-1V, and see if it's the upper or lower half [...]

That essentially describes one type of ADC, the successive
approximation variety. The other type is a flash converter,
where you have enough comparators on the chip to do this all
in parallel. The problem is flash converters obviously get
exponentially complex as you increase the accuracy. You'll
need twice as many comparators to go from 4 bits to 5 bits,
so anything beyond about 8 bits is prohibitive, and I've
only seen it used in extreme applications like video ADCs
where 8 bits per color is usually sufficient.

Another problem is even with a flash ADC, you'll need some
settling time so there's still the issue of delay time even
if you don't consider it clocked. I don't think a continuous
output is possible or at least not realistic. I second the
suggestion of using a sample rate high enough that you don't
notice it is not continuous. An ADC capable of 200K samples
per second should be good enough for audio.

If you really need continuous signal processing, that is why
we still have analog computers. In my younger days I had the
opportunity to visit the McMorrow Labs, a research center on
Redstone Arsenal (Huntsville, AL) that does simulations to
test missile guidance systems. I was majoring in computer
engineering so this was quite a thrill. They had a CDC-7600
which was the fast supercomputer in those days (the Cray-1
was not yet shipping). Connected to the 7600 was a CDC-6600
which interfaced to a room of analog computers about the size
of a hockey rink. I would guess there were about 200 to 300
analog computers each about the size to two 19" x 84" racks.
I asked why they needed all these analog computers, which I
considered to be fossils, when they had the fastest computer
in the world downstairs. My guide explained that solving any
one of these complex differential equations, each of which
was handled by an analog computer, would bring that 7600 to
its knees. The analog computers were the right tool for the
job. The 7600 ran a Fortran program that read descriptions
of differential equations to patch layouts for the analog
computers. OK, it did a few other things. :)

"If the only tool you have is a hammer, every problem looks
like a nail." -- me

-- 
Phil Harbison