[sdiy] Static ADC and DAC recommendations?

[Paul Anderson]

Maybe drive the 555 with the signal, and use the PWM output it produces?

--------
Paul Anderson -- VE3HOP

On 2013-01-17, at 6:11 PM, cheater cheater <cheater00 at gmail.com> wrote:

> Hi guys,
> Thanks for the great replies. Some great info there. Especially liked
> the bits on high-precision DMM's and on analog computers :)
> 
> I've read up on successive approximation some time ago, and just
> refreshed myself to make sure I'm not missing something. It's
> different from the chain-of-comparators; in fact I don't see a way to
> go from one to the other. Successive approximation is a feedback chain
> with a DAC (or a number of digital switches) and a single comparator
> at the end; this is read by logic (possibly an MCU) which adjusts the
> DAC/switches accordingly. Here, we're talking about a single bit being
> represented by a simple comparator.
> 
> Dual-slope is also different: it involves a voltage reference and a
> timing circuit (usually based off a clock and counter) and some
> arithmetic logic.
> 
> On Thu, Jan 17, 2013 at 11:40 PM, Neil Johnson
> <neil.johnson97 at ntlworld.com> wrote:
>> Plus the fact that you're having to implement precise gains at every stage
>> otherwise performance suffers, especially at the LSB end which is where it
>> matters most
> 
> when has insufficient THD+N stopped anyone from using a Tube Screamer?
> :) Seriously, guys, don't concentrate on performance. Quotes like the
> above or this one...
> 
> On Thu, Jan 17, 2013 at 11:40 PM, Phillip Harbison <alvitar at xavax.com> wrote:
>> An ADC capable of 200K samples
>> per second should be good enough for audio.
> 
> ...are great, thanks for the insight, but that's not really the point.
> And I know this is unusual, since normally when someone's designing an
> ADC or DAC the only thing that matters is quality, but this is not the
> case here :) And the direction the conversation went is not surprising
> since everyone I asked was in this exact same mindset :)
> 
> The idea isn't to build a circuit that will be good enough to
> transcribe audio in pristine condition. The idea is to take a flawed
> approach - comparator-based, low component count, no clocking, no
> sample holding - and see what disadvantages it brings and how they can
> be used creatively. So what if I can't make a perfect 2x multiplier -
> it's probably better this way. So what if I can't exactly match the
> comparators. So what if it's only 6 bit? Let's not think about how to
> build yet another boring box.. :) A circuit which will create
> reproduction in every way superior to the approach I am trying to go
> for can fit on silicon the size of a pin head and can be found in
> billions of $0.99 portable music players people throw away without
> thinking. No reason to recreate them, and if I wanted I couldn't even
> begin to compete, so I'd just buy an existing high-performance IC.
> 
> I idly wonder if you could do this (compare and multiply-by-2) with a
> 555 per bit (plus some other stuff).... :) But that's only a fun
> thought, nothing more.
> 
> On Thu, Jan 17, 2013 at 11:40 PM, Phillip Harbison <alvitar at xavax.com> wrote:
>> 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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