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bambino
04-27-2008, 05:00 AM
Would this circuit work on the prop. The 220 ohm I think would protect the pin, but would it make an accurate rcTime to temperature conversion?

Tracy Allen
04-28-2008, 12:04 AM
I used that circuit on the Stamp for the Applied Sensors series. It is on the edge. The AD592 specs suggest at least 4 volts across the chip to support correct linear operation, but at the 1.6 volt Prop threshold, that leaves only 3.4 volts across the AD592. It would be better if the top of the AD592 could come from a higher voltage. I found that it typically did still work, though, with 5 volts into the Stamp's 1.4 volt threshold. The result depends on the tolerances and drifts of all the components, including the drift and noise of the prop threshold. It is important to avoid ground loops. Don't expect more than a couple of degrees accuracy.

You could also have the AD592 feed a sigma-delta converter, which would I think be considerably more accurate.

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Tracy Allen
www.emesystems.com (http://www.emesystems.com)

bambino
04-28-2008, 03:39 AM
Thank you Tracy for your insight!
I beleive I'll take your advice and go with a convertor.

bambino
04-28-2008, 06:44 AM
Tracy,
I just found this:http:
//www.national.com/mpf/LM/LM19.html
Would this not be the 3.3v compatable version of the 592?

Tracy Allen
04-28-2008, 07:34 AM
Well, the LM19 is a voltage output device, so it would be compatible with a voltage input ADC. Another option if you want a current output that could feed a Prop sigma delta or RCtime would be the venerable LM334. It can operate down to less than 1.5 V of overhead. Here (http://www.emesys.com/EarthM/TK334fab.pdf) are ideas for constructing an LM334 probe.

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Tracy Allen
www.emesystems.com (http://www.emesystems.com)