[sdiy] Static ADC and DAC recommendations?

[Paul Burns]

Thank you for the historical insight, my uncle is one John W. Lacey who was
a top dog at CDC right from the outset ( he used to employ Seymour Cray ;-)
)
Regards
Paul Burns.


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Phillip Harbison
Sent: 17 January 2013 22:41
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Static ADC and DAC recommendations?

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


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