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???
08/31/05 17:03
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 		 #100380 - Look at existing products for ideas
Responding to: ???'s previous message
Ti (Burr Brown) makes several RTD to 4-20mA converters. The principle of the RTD signal conversion is to provide a constant current source to the RTD and second current source to a local resistor. The circuit measures the voltage differential using an Instrumentation Amplifier (IA). An IA has a much better performance than just an op amp connected differentially especially from noise rejection considerations. It is fairly simple to extrapolate this concept through a mux arrangement to allow for multiple RTDs driving a single IA and fed from a single current source.

Check out the front end description of the XTR103 (even though it's now obsolete- there are others but you are really only interested in the description)
http://focus.ti.com/lit/ds/symlink/xtr103.pdf

-Aubrey

List of 37 messages in thread
TopicAuthorDate
Analog mux for Pt100            01/01/70 00:00      
   Precision            01/01/70 00:00      
      Precision is the same?            01/01/70 00:00      
         Err            01/01/70 00:00      
            Inertia!            01/01/70 00:00      
               Precision.            01/01/70 00:00      
                  Precision            01/01/70 00:00      
                     PT100 sensors            01/01/70 00:00      
   try another way, have a look at Linear T            01/01/70 00:00      
      No choice            01/01/70 00:00      
         it is still valid            01/01/70 00:00      
         The usual solution is...            01/01/70 00:00      
            As I said before            01/01/70 00:00      
               Whats the problem ?            01/01/70 00:00      
                  Acceptable limits            01/01/70 00:00      
                     hooking the PT100s up to differential in            01/01/70 00:00      
                        Thanks            01/01/70 00:00      
                           4 wire ?            01/01/70 00:00      
                     Filtering MUST help. So, there's a mista            01/01/70 00:00      
                        Ditto, filtering must work            01/01/70 00:00      
                           Exactly!            01/01/70 00:00      
                     Errm.            01/01/70 00:00      
   Look at existing products for ideas            01/01/70 00:00      
      that will give you more problems than it            01/01/70 00:00      
         It's not what I suggested            01/01/70 00:00      
            No chance            01/01/70 00:00      
               Again, not what I meant            01/01/70 00:00      
                  Sorry            01/01/70 00:00      
   Switched Higher Current does help            01/01/70 00:00      
      I am interested, it took some search, fo            01/01/70 00:00      
      I heavily doubt this            01/01/70 00:00      
         not as to noise, but it does have the ef            01/01/70 00:00      
         higher current does help            01/01/70 00:00      
            You can use pulsing and filtering            01/01/70 00:00      
               It's the same as DC measurement            01/01/70 00:00      
                  How pulsing improves performance            01/01/70 00:00      
   Averaging(filtering) has the same effect            01/01/70 00:00      

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