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Thread: [EE] 24-bit A/D. Are We in the Twilite Zone Here?
picon face BY : Russell McMahon email (remove spam text)


Here is the latest response from my friend who has extensive experience
delaing with signals in very real world situations at levels well below what
you are dealing with. I can vouch for his experience and success
commercially. His comment on direct conversion without amplification using a
commercial Sigma Delta chip should be given some consideration.


               Russell McMahon



I don't mean to be rude but maybe this guy is flogging a dead horse.

Why doesn't he jut get rid of the op-amp front-end and use a sigma-delta
converter chip with the low-level signal conditioning built in.  At well
under US$15 in moderate quantities the AD7730 is very cost-efective when you
consider that it needs nothing in the way of analog support chips and (with
care) is capable of very high performance.

Analog Devices and Crystal Semiconductor (and probably others) have a whole
range of sigma-delta ADC devices with varying cost/performance tradeoffs,
including several which will give usable input-referred resolutions well
under 0.1uV.  I have used the AD7730, AD7714, and AD7715 and recommend them
highly for sensor applications where resolution is more important than high
speed conversion.

I have also used the AD7750 for power metering  - it is by far the easiest
and cheapest way to build an accurate AC power (or more correctly energy)
metering device.  I believe there is now an improved version (AD7751 ?)
which can better handle variable-frequency inputs (the AD7750 only works of
a narrow frequency range).

I'm not trying to sell AD parts here  - merely to point out that they (along
with other manufacturers) have some very useful devices which though not
cheap, may represent cost-effective solutions both in terms of the
performance they provide and in the substantial reduction in design time
they offer (especially important for low-volume niche products).

On the other subject of engineers doing silly (and often dangerous)
hings  - there seems to be a lot of it about and much of it is absolutely
hilarious.  Maybe there is a commercial opportunity here (modern versions of
Victorian morality tales ?)  - how about acting as a publishing editor for a
book of short stories (promoted via snippets posted on the web).

Sounds as though you could come close to filling it by yourself but I'm sure
there would be plenty of willing contributors.  Like the time I nearly
electrocuted myself under Mums' house while investigating a water leak, or
"the home made arc lamp incidents" (yes two separate ones  - melting the
glass in my bedroom window and burning a hole right through by bedding and
foam mattress to the wire-wove supporting it), or exploding insects on the
back lawn using a 1500uF capacitor bank charged to 2kV and a set of old
multimeter test leads, or the Tesla coil incident (or maybe I should let
Chris Paice tell his own story), or the pirate FM transmitter mounted on a
ramp on top of the U of A chemistry building with a remote release as an aid
to preventing it falling into the hands of the radio inspectors (involving
persons who are now respectively head of an F&P division and a senior
engineer at Talon).

Gavin Higgie could tell the story of the EPROM programmer and the 230VAC
supply (and an engineer at DSE whose name I can't recall could tell a
similar story involving literally hundreds of very expensive (and very dead)
NMOS chips in a top-secret digital signal processing system  - being a
military establishment the poor individual was required to wear the results
of his labours on an arm-band for several days).

You could even cast the net wider and allow non-electrical themes.  That
would admit a large number of "bomb" stories (including of course "RDM and
the Headmaster"), allow Ross to relate his story of the dangers of making
smoke bombs by melting ingredients on a gas stove, not to mention the
dangerous combination of boredom and compressed air (as epitomised while I
was working overtime assembling Eveready torches at Fountain Electronics
when I was 15), the
total destruction of the school chem lab fume cupboard (a small 2H + 0
explosion), and how the front 20 rows at the school drama production were
showered with burning newspaper.

Then there is just plain dumb stuff like having to explain to the NZ Forest
Products Ltd. Accomodation Officer a disassembled (and very oily) motorcycle
engine in the lounge of the company house provided for student vacation
employees, or the dye capsules which mysteriously found their way into the
public swimming pool, or the time I helped roll a spare wheel down one of
Wellington's steeper streets (no real harm done but the consequences could
have been horrendous), or the attempt to shoot a large rat with a spear gun
(stupid at the best of times but doubly so considering we were both in a
small tin dinghy at the time), or the time I broke the fish tank and flooded
the doctors waiting room.

There is also the "penguin" incident  - person on holiday at East Cape
sleeping peacefully in back of small van is woken up in pitch dark by cold
wet flapping fishy-smelly alien monster thing (introduced by so-called
"friends")  - penguin survives ordeal relatively intact but person sustains
deep gash on head which bleeds profusely and by light of feeble torches
makes entire scene look like horror movie.

Now there just has to be a market for book of that kind of stuff.


       Ken Mardle

<008001bfb2a0$f3259060$623561cb@mkbs> 7bit

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