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From: Bill Davy on 29 Sep 2009 08:40
Can anyone point me at something? Not going fast (480 samples/sec). Must be
available immediately (going down an oil well just after Christmas).

TIA,
Bill


From: Mark Borgerson on 29 Sep 2009 10:14
In article <7iedi5F2vcad9U1(a)mid.individual.net>, Bill(a)SynectixLtd.com
says...
> Can anyone point me at something? Not going fast (480 samples/sec). Must be
> available immediately (going down an oil well just after Christmas).
>
> TIA,
> Bill

There are a number of things you need to watch out for, in addition to
the temperature specification:

Very few ADCs will give you 24 bits of noise free data at 480 samples/s.

multi-channel Delta-sigma ADCs often incur a significant timing penalty
when switching between channels. To collect two channels at
480samples/s you may actually have to use a sampling rate 3 to 5 times
higher to allow the internal digital filters to settle after switching
the multiplexer.

Your best bet might be to use two separate ADCs such as the CS5532.
That would allow you to avoid the timing penalty incurred when switching
channels.

Mark Borgerson
From: -jg on 30 Sep 2009 00:18
On Sep 30, 12:40 am, "Bill Davy" <B...(a)SynectixLtd.com> wrote:
> Can anyone point me at something? Not going fast (480 samples/sec).  Must be
> available immediately (going down an oil well just after Christmas).
>
> TIA,
>     Bill

TI shows 42 candidates on their selection guide, and this one sounds a
good fit

http://focus.ti.com/docs/prod/folders/print/ads1278-ht.html

[" * Available in Extreme (–55°C/210°C)
Temperature Range(1)
# APPLICATIONS * Down-Hole Drilling "] bingo!

-jg
From: Bill Davy on 30 Sep 2009 06:29
"-jg" <jim.granville(a)gmail.com> wrote in message
news:30f081a3-2dfb-4cec-b825-e86d22373f0e(a)y10g2000prg.googlegroups.com...
On Sep 30, 12:40 am, "Bill Davy" <B...(a)SynectixLtd.com> wrote:
> Can anyone point me at something? Not going fast (480 samples/sec). Must
> be
> available immediately (going down an oil well just after Christmas).
>
> TIA,
> Bill

TI shows 42 candidates on their selection guide, and this one sounds a
good fit

http://focus.ti.com/docs/prod/folders/print/ads1278-ht.html

[" * Available in Extreme (�55�C/210�C)
Temperature Range(1)
# APPLICATIONS * Down-Hole Drilling "] bingo!

-jg


Thanks for this. Looks good (low power, good temperature range).
Bill


From: Bill Davy on 30 Sep 2009 06:32
"Mark Borgerson" <mborgerson(a)comcast.net> wrote in message
news:MPG.252bb61bb210a8fa98990f(a)news.eternal-september.org...
> In article <7iedi5F2vcad9U1(a)mid.individual.net>, Bill(a)SynectixLtd.com
> says...
>> Can anyone point me at something? Not going fast (480 samples/sec). Must
>> be
>> available immediately (going down an oil well just after Christmas).
>>
>> TIA,
>> Bill
>
> There are a number of things you need to watch out for, in addition to
> the temperature specification:
>
> Very few ADCs will give you 24 bits of noise free data at 480 samples/s.
>
> multi-channel Delta-sigma ADCs often incur a significant timing penalty
> when switching between channels. To collect two channels at
> 480samples/s you may actually have to use a sampling rate 3 to 5 times
> higher to allow the internal digital filters to settle after switching
> the multiplexer.
>
> Your best bet might be to use two separate ADCs such as the CS5532.
> That would allow you to avoid the timing penalty incurred when switching
> channels.
>
> Mark Borgerson


Hmmm, they seem to draw 500 mW (max) each! Also temperature range limited
to -40�C to +85�C. Still, I will point this device out.

Still, very interesting.

I've looked at how much time jitter is allowed to avoid disturbing LSB with
maximum swing. It's not very much! Perhaps the application really calls
for 17 bits. Still, I am only a programmer :-)

Bill


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