From: Daniel, Erik S. (Daniel.Erik@mayo.edu)
Date: Tue Apr 24 2001 - 05:55:53 PDT
The high effective bit, high bandwidth ADC systems we have been involved
with are multi-bit sigma-delta systems. The sigma-delta architectures are
fairly forgiving except for a couple key places where coupling of noise is a
killer. We have begun general investigations of single-mode and
differential coupling at the 70-100 dB level for circuits operating in the
mid 100s - low 1000s of MHz. I think it's fair to say that we don't have a
clear understanding of all of the coupling mechanisms in this regime, but we
have definitely found that simple models (e.g., TEM models of t-line
coupling) are insufficient. Unfortunately, we haven't had the resources to
do a thorough analysis. I think this is a challenging problem.
Erik Daniel, Ph.D. Voice: (507) 538-5461
Mayo Foundation Fax: (507) 284-9171
200 First Street SW E-mail: firstname.lastname@example.org
Rochester, MN 55905 Web: www.mayo.edu/sppdg/
> -----Original Message-----
> From: Larry Miller [mailto:email@example.com]
> Sent: Friday, April 20, 2001 9:07 AM
> To: firstname.lastname@example.org
> Subject: [SI-LIST] : D/A interfacing
> In the recent exchange with Dr Eric Daniel the topic of high
> bit count A/D
> converters came up.
> The interface between analog circuits and digital circuits
> seems to have
> always been a troublesome area, even back to the days of
> converters made out
> of discrete parts (resistors, caps, etc.).
> In most systems through the 80's the "standard"
> parallel-interface DAC and
> ADC range was on the order of 10 to 14 bits. This was
> governed as much as
> anything by the limits of the converters' internal noise
> ("good" converters
> being hybrid thick film devices and rather poor by comparison
> mixed signal chips). By and large, with 14 bits of converter
> resolution most
> people were lucky to realize 10 to 12 bits of actual usable
> range, which was
> usually enough for control systems.
> ***Of course, the SI reflector did not exist and even the
> notions of SI that
> we discuss and use were in a very rudimentary stage of evolution!***
> The first 16-bit applications I heard of were for digital
> audio, and a whole
> new class of devices and noise control techniques evolved for this
> application. Later on, much larger ranges (up to 24 bits) were found
> necessary for CT (Computer Tomography). Companies like
> Analog Devices and
> Linear Technology came up with very inexpensive sigma-delta converters
> (which also won out in the audio CD arena).
> None of these high bit-count converters operated at very high
> speeds (i.e.,
> not much above audio signal processing). That made use of
> between the analog and digital areas of the system feasible,
> and I believe
> that this is the standard method used in CD players. (It also
> works great in
> motor control systems!)
> Video "flash" converters typically had much lower bit counts.
> 8 bits is
> better than most CRTs (black & white OR color) can display.
> Indeed, many PC
> video DACs have much fewer than 256 output levels; this is
> easily shown by
> playing with the color toy in Windows or whatever system you
> are running. 40
> dB down on analog noise is not that hard to come by (though,
> truth to tell,
> many PCs don't make it) and is OK for consumer video, but it
> is not good
> enough for your sound card, as your ears tell you. You may
> set the sound
> card for 16 bits but you do not get 16 bits of dynamic range
> by a long shot.
> Now, with digital video (and whatever Eric is working on)
> raising the bar on
> performance AND speed, the SI problems continue to increase.
> Most DAC/ADC manuals give some rules for the interface
> between the analog
> and digital systems ("connect grounds only at one place",
> yada yada yada
> yada), but these often do not work. Also, if you have multiple systems
> distributed over space in a system, what do you do? "Connect
> together at
> only one place" is not on the menu.
> Some years ago I ran across a novel approach described in a paper
> attractively titled, "Method for Noiseless Digital/Analog
> Interfacing in
> Complex Systems Using An Intermediate Ground Plane" and a
> Google search
> showed that it is on the Web.
> I never tried this scheme. Has anyone used this or something similar?
> Cellular phones use something like this to achieve the 120+
> dB isolation
> required between the transmitter and adjacent receiver.
> I would be curious to hear what has worked for people. I have
> seen questions
> about this from time to time on this reflector, but very
> little follow-on.
> (Maybe people who find the answer also find a new career as a
> Inquiring minds want to know,
> Larry Miller
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