AW: [En-Nut-Discussion] PT100 Sensor on ethernut

[James A van Zee]

Ole,

You didn't indicate the temperature range of interest, nor the precision/accuracy required.  Although the DS1822 is available in a TO92 package and can thus be used as an external probe (I have used it in this way), it suffers fatally from the fact that it is only an 'approximatly-correct' sensor, while a PT100 sensor can offer 'NBS-quality' accuracy.

One must apply curvature correction to get the full accurcy of a PT100 sensor, and thus one needs 12+ bits of resolution (14-16, IMHO) to fully exploit this device.  Given that it is a resistor, one needs a 'balanced bridge' circuit (i.e. an A/D whose reference is derived from the supply voltage) to get precise measurements.  Attempts to amplify the signal before feeding it to an A/D only introduce offset/gain errors, leading to the notion that the best way to go is a Delta-Sigma converter.  Linear Technology has several low-cost versions that will do the job (e.g. the LT2421/2 if the probe is far from the A/D, and the LT2420 if you don't need fully differential input).  These have an 'SPI-style' interface.  Be aware that they are SLOW (7,5Hz), but that's usually a perfect match for temperature sensors.  No amplifier is needed.  There are many other Delta-Sigma convertors out there; Analog Devices offers some that can do a first-order correction (scale+offset) in the converter itself, but they are much harder to program compared the L.T. devices.  Burr-Brown (now Texas Instruments) has some nice devices, and even Intersil has one (out-of-date, IMHO).  L.T.'s newest models can compensate for long lead wires - I don't know if that's an issue for your system.

                                                             Sincerely,

                                                             Jim van Zee
                                                             Pacific Technologies
                                                             Redmond, WA, USA

----- Original Message -----
From: "Ole Reinhardt" <ole.reinhardt at kernelconcepts.de>
To: "Ethernut User Chat (English)" <en-nut-discussion at egnite.de>
Subject: Re: AW: [En-Nut-Discussion] PT100 Sensor on ethernut
Date: Sat, 17 Sep 2005 23:59:00 +0200

> 
> Hi Oliver,
> 
> > Why not using directly a Dallas DS1820 digital sensor? Much easier to
> > connect, b/c to analog amp stuff is needed, and you can connect several
> > DS1820 to one input pin. The source code for read is also finished and
> > working...
> 
> sorry, I have to use a pt100 sensor, it is a necessary requirement
> because of the way the sensor should be mounted... In other words, I
> need to use an external sensor, not a pc mountable one.
> 
> Bye,
> 
> Ole
> 
> 
> 
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