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Organization: Queen's University at Kingston
Date: Thu, 15 Apr 1993 22:57:07 EDT
From: <LEEK@QUCDN.QueensU.CA>
Message-ID: <93105.225707LEEK@QUCDN.QueensU.CA>
Newsgroups: sci.electronics
Subject: Re: ADC card for computer
Distribution: world
References:  <1qi8e3$b5e@lll-winken.llnl.gov>
Lines: 54

In article <1qi8e3$b5e@lll-winken.llnl.gov>, you say:
>
>I would like to digitize the output of a SQUID magnetometer (range -10 V
>to +10 V) and do digital signal processing in a computer, say a Macintosh
>II or a 486 PC.  I would like a good 16 bit ADC with good linearity and a
>high conversion speed, at least 50 kHz, preferably 200 kHz.  Other concerns
>
>(2)  Must I use an ADC external to my computer to avoid digital noise
>     feedback into my sensitive SQUID electronics?

Might be a good idea...  The resolution you requested is about 0.3mV
In order to get what you've paid for, noise level better be lower than
that.  It is kind of hard to do it in a noisy box like you can expect
inside a PC.

Before you pay $$$ for a PC card, test it out by sampling a low
distortion sine wave (I think there is a sine wave on a CD.  Digital
Domain ?  There are possibly other low THD sources)  Run the digitized
waveform through a FFT transform and take alook at the noise floor on
the spectrum.  That's should give you a good indication of the design.
(That's what I am doing to test a data acquistion system I have designed
- I got the idea from MAXIM data sheet.)

If you can live with 14 bit resolution, I would recommend looking at
the MAX121 from MAXIM.  It is a high speed (308KHz) complete
sampling A/D with DSP interface.  The input range is +/- 5V and it
uses a serial interface (which can easily be optically isolated
from the computer to elinimate a major noise source)  The Analog design
guide I got from them shows a -100db noise level.  They claim a -77db
max (-85 typ.) THD.  Looks pretty good for the $12 @ 1000 pieces

A evaluation kit is available.  Might want to give these nice folks a
call.  1-800-998-8800 or fax: (408)737-7194 and (408) 737-7600 ext4000
for application assistance.

This assumes that you can build your own DAS and write your own software.
(Hey you can get the MAX121 as a free sample just by calling the 1-800 #)

>I would appreciate discussion of your personal experience with a Mac or PC-
>based ADC system.

I would recommend you to find out the resolution that can be gotten out
of your system by looking at the noise level, otherwise you might be
throwing out your money.

>Charles Cunningham
>cec@imager.llnl.gov

K. C. Lee
Elec. Eng. Grad. Student

I have no connection with MAXIM except I do in general recommend
companies that give samples to students to others.  I feel they
deserve that for being nice to me.
