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Old 5th December 2002, 03:42 PM   #1
haldor is offline haldor  United States
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Default Audio DAC as signal generator (Need DC output).

I just got a request to help find a way to reproduce a very intermittant field problem with one of our products that so far we have not been able to reproduce in our lab.

My employer makes digital weight scales. We are digitizing signals from DC up to around 100 Hz. We have a method to capture data into a file on a PC when the problem occurs. What we want to do is recreate the signal from the data in file here in our lab and feed it into an identical scales A/D converter so we can figure out what is happening.

Time is more important to us than cost so rather than making a DAC I was wondering if there is an a commercially available audio DAC that can output low noise, high resolution signals down to DC? We are resolving the signal to 20 bit precision.

Also can anyone suggest a program to convert data in a file into a wave file? I can massage the file data into whatever format the program is expecting.

PS. Please forgive me if this issue has already been discussed. I tried searching for "DAC AND DC", but the search engine refused to accept DC since it isn't 3 letters long.

Thanks for any help you can give me.

Phil
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Old 5th December 2002, 04:35 PM   #2
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I don't know if there are any DC-coupled soundcards. I tested
this on my soundcard just a couple of days ago, and it was not
DC-coupled. There are various PC-cards both with D/A and A/D
converters intended for measurement and control. For instance,
Advantech has a bunch of cards with 12-bit DACs in the range
$300 - $500 here in Sweden. I think this might be a better way
to go than using a soundcard. I am sure there are cards with
better than 12-bit reolution also if this is not enough.

For the wave files, if you are prepared to do a little bit of
programming i can tell you the file format. It is very
straightforward and easy to use.
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Old 5th December 2002, 05:45 PM   #3
haldor is offline haldor  United States
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Hi Christer,

I am looking for 16 bit at a minimum and prefer 20 bit or better. That's why I was thinking about using an outboard audio DAC instead of a PC sound card.

The signal I am looking at is a 10 mV differential signal riding on top of a 2.5 V common voltage. The A/D conversion is to 20 bit resolution.
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Old 5th December 2002, 05:54 PM   #4
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I had a look at Advantechs own web pages. They do have
16-bit cards, eg. this one
http://www.advantech.com/products/Mo...id=1-8IXVM&bu=
I didn't find any 20-bit cards, but there might be others making
such cards (there ought to be).
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Old 5th December 2002, 06:30 PM   #5
haldor is offline haldor  United States
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Hi Christer,

Thanks for the info. I found one audio DAC that on paper looks like it will work. Less than $400 too, although price is not a major concern.

http://www.audioadvisor.com/store/pr...asp?sku=MSBLD3

Comments anyone?

Suggestions on how to go about attacking the data conversion issues?

Phil
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Old 5th December 2002, 07:20 PM   #6
Rarkov is offline Rarkov  England
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Hi,
Correct me if I'm wrong, but a DAC requires a Vref- and a Vref+...The way I see it is to set Vref- at 2.5V and Vref+ at 2.51V (you'll have to work out how it get it that accurate!) and as far as I can tell, you'll get output in steps of 0.00000244140625V. That's 4096 steps!

Isn't that accurate enough?!


Edit:
Forgot to add that that was based on a 12Bit DAC chip. 2^12=4096. 10mV/4096 equals result above
Gaz
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Old 5th December 2002, 07:28 PM   #7
haldor is offline haldor  United States
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One displayed increment size in this particular product is 0.45 uV. We maintain an internal working resolution of 10 times that = 0.045 uV per minor increment.

Phil
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Old 5th December 2002, 08:08 PM   #8
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Hi,

Have a look on the M-Audio Audiophile 2496 24 bit soundcard:
http://www.m-audio.com/products/m-audio/audiophile.php
This card goes to 0.5 Hz Ė3dB. If you short the input caps you can measure DC. It is centred on 2.5V DC so you have to use a low noise op-amp eventually or some offset compensation. I donít know how stable DC drift will be. You can record and output signals just with the Win sound panels on a PC or the Mac sound software on a Mac.

It uses a AKM AK4528 codec chip. A datasheet can be downloaded here:
http://www.asahi-kasei.co.jp/akm/en/product/audio.html

Also download the demo board .PDF, it contains additional information. On the audio card the application example given by AKM is exactly copied.

A program that convert .wav to .text and vice versa (and many other formats) is Goldwave. 16 bit only as far as I know. It is shareware:
http://www.goldwave.com/

An other program that can maybe of use is audiotester:
http://www.sumuller.de/audiotester/

It contains an oscilloscope as well.


Chaio
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Old 5th December 2002, 08:10 PM   #9
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Default signal generator

Haldor,

have you thought about an instrumentation card or unit like those made by National Instruments? I'm sure they have 20 bit units going to DC. They also have a large library of signal manipulation routines, don't know if wavetables are included but it might be worth a check.

Jan Didden
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Old 5th December 2002, 08:25 PM   #10
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Hi Jan,

For sure that is a far more better option but it is much more expensive. Is has also a longer learning curve.

For professional use I would personally choose National Instuments.
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