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Digital milli ohm source rquired

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pokiri

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I require some design reference on digitally simulating resistance for a calibration source. I have to control resistance from 0 ohm to 1000 ohm in one milli ohm steps. Please let me know whether there are any application note or other circuit reference available.

Thanks.
Pokiri
 

silvio

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Hi pokiri,

Your application belongs much closer to metrology field. What you want it's not easy to achieve. Bear in mind that you must provide special input guard circuit in order to maintains accuracy by minimising the effect of probe or cable leakage.

If I well understand you want one digitally controlled potentiometer in 1 mili ohms increment. You'll apply a voltage across the pot and then throgh the cursor, fed a DVM's input that you want to calibrate ?

If that you have in mind, we can find a better solution in order to avoid expensive and high accuracy resistance bridge. Please come back with an enhanced description of your application.

Regards,
Silvio
 

pokiri

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Hi Silvio,

You are right. What I need exactly is a digital Potentiometer with 1 milli ohm setp. I do not think that there is any ready made commercial digital pot is available with that accuracy. My application is to control the value of the resistence under software control for calibrating the RTD sensors.

If you have any suggestion please let me know.

Regards
Pokiri
 

silvio

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Hi pokiri,

A good source of information is : http://www.temperatures.com/calb.html

For some ready made devices you can take a look at: http://www.transcat.com/TechReference/PDFs/Application Notes/RTD_Instrumentation.pdf

or http://www.thermoworks.com/products/calibration/piecal521.html
Perfect for checking the accuracy of any thermocouple instrument.
Simulates any of the 8 most popular thermocouple types and millivolts.
For some hints about the thermocouple types : http://www.temperatures.com/tctables.html

Look here in order to see how they succeed to manage with RTD sensor. You can find some formula, read carefuly:
http://www.microdaq.com/product/data-aquisition/webdaqHELPmanual/rtd.htm
You don't need a webDAQ/100 device. Look at the Mathematical details at bottom of page. You need only a good precision resistor for the known resistance values in the circuit, and also to choose a resistor rated for a low temperature coefficient (change in resistance with temperature). However, you don't need to rely on the resistor's labelled value and accuracy, you can measure its actual resistance with a good accurate multimeter (use one with at least 4.5 digits of precision).
See the additional wiring tips at the bottom of article.

and here as well: http://www.elecdesign.com/1998/apr0698/ifd/3IFD.pdf

Regards,
Silvio
 

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