Currently I have prepared a circuit which gives me constant current source of 10 micro ampere. 2 Volt DC source and 200 k Ohm resistor in series. What if the output volt of DC power supply fluctuates a little? Does it affect the output voltage generated by RTD sensor?
Second thing, For buying excitation source, What are the ready made constant current sources available in market? Make and Model? How much it costs?
Each and everything has a resistance. Current fluctuations cause voltage fluctuations (and vice-versa). There are linear changes: dU = R * dI
IN addition, diodes are non-linear elements, which results in changes that cannot be expressed in such a simple formula.
Regarding buying current sources: there is a plethora of devices available. Strating from integrated circuits (choose your semiconductor manufacturere - at least all "analog" suppliers got current sources) to complete "units" (what spring into mind: Agilent)...
You will have to find out shat you need in terms of "costant" (regarding current, temperature, precision and so on) and then do a bit of search to find out what "will do".
wikipedia ist your friend :) (why not look up 'current source' ?)
Google too.
Everything U.Dreher said is accurate, but your question was what if supply VOLTAGE fluctuated, and DC constant voltage supplies are common, regulation being .01% on most.
Second, linear RTD devices are available as thermistors, they are cheap,small, and come in several ranges, 0 to 1k, 100 to 10k,1k to 100k, and so on from many manufacturers in your area.
With a 2V driving voltage you would be more concerned with changes in current with changing voltage from the diode (or other sensor). Consider using a large voltage and larger resistor (an ideal current source has an "infinite" source resistance). The actual current through the sensor is found by measuring the voltage across the known series resistance, independent on actual source voltage and sensor voltage. Remember to check the how the resistance change with (room) temperature as well. Everything depends on how accurate you want the temperature measurement to be...
And to Geirfinn's point, silicon diodes have a forward voltage drop as much as.7 volts, where as thermistors are purely resistive. So if you must use a diode then consider a schottkey barrier diode, voltage drop is far less, around .1 volts, and temperature changes are fairly linear, at least compared to silicon diodes.
Sometimes its hard to answer these questions because we may know a better process, or circuit, but we must first answer the question that was asked.
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