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Entry  Wed Nov 9 17:43:59 2016, awade, Summary, TempCtrl, Three wire temperature sensor circuit for shields 20161109_Sketch3wireRTD2VoltageCircuit.pdf
    Reply  Tue Nov 29 23:17:14 2016, awade, DailyProgress, TempCtrl, Three wire temperature sensor circuit for shields IMG_0556.JPG
Message ID: 1778     Entry time: Tue Nov 29 23:17:14 2016     In reply to: 1767
Author: awade 
Type: DailyProgress 
Category: TempCtrl 
Subject: Three wire temperature sensor circuit for shields 

I've now built this circuit out onto a small breadboard patterned proto-board as a tester.  It is configured with a offset resistor of 7.31 kΩ and a AD620 gain resistor of 9.7kΩ (I would have liked 10kΩ but we're out). The intention is to intitially test its performance for 10kΩ thermister (I have one from the PSL lab), we may eventually go for the standard Pl 100Ω RTD. The only thing that will need to change is the offset resistor and the gain at the instrument amplifier stage. The board has +/- 15 V regulators and another step down to +5V with similar 0.1A regulators. Seems a bit crude to voltage adjust like this but the datasheet suggests it should still have good rejection and the REF200 chip offers a further stage of supply noise rejection. 

I'm debating whether I should add a sallen-key style active LP filter. Maybe at 1 Hz.

Single channel build RTD to voltage transducer. 



At some stage we will need to install new temperature sensors on the thermal shields.  Tara et al. had installed AD590s in vacuum (and did a similar thing for the pervious longer cavities), these are now broken. They give currents proportional to applied voltage in the forward direction (see: PSL:1173). One theory is that they were overheated during soldering. Another possibility is that that the ICs didn't fare well in vacuum.  Either way it would be best to have passive components that we know are vacuum compatible and low noise.  

Previous rana post here on temperature sensor noise plots: PSL:1205

We need a robust low noise circuit to turn resistance into a ADC usable voltage.  I've attached a sketch below. It uses a IC dual current source regulator (REG200) and a low noise instrument amplifier (AD620) to convert resistance of a PT100 sensor (like the one used in the TCS, see PSL:1700 and subsequent posts) in a three wire scheme. I've yet to compute a noise budget, for now I've penciled in some specs for noise I've drawing from their data sheets. This scheme would require three wires per sensor (but it might be possible to double up the grounding line between sensors). 


Note: the R1 resistor provides an offset so that the Vout is within a useable range for high gain. R_g sets gain, where G = 49.4kΩ/R_g +1.


edit: added not on function of R1 and R_g - awadeWed Nov 9 17:47:14 2016


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