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Entry  Mon Dec 17 22:13:03 2018, anchal, DailyProgress, TempCtrl, Proposed circuit for cavity temperature sensor Part 2 Cavity_Temperature_Sensor_Calculations.zipCavTempSensorAcromagChannels.pdf
    Reply  Wed Dec 19 11:01:00 2018, awade, DailyProgress, TempCtrl, Proposed circuit for cavity temperature sensor Part 2 
       Reply  Wed Dec 19 14:00:55 2018, anchal, DailyProgress, TempCtrl, Proposed circuit for cavity temperature sensor Part 2 
Message ID: 2267     Entry time: Wed Dec 19 14:00:55 2018     In reply to: 2266
Author: anchal 
Type: DailyProgress 
Category: TempCtrl 
Subject: Proposed circuit for cavity temperature sensor Part 2 

About the current source:

  • The current excitation circuit is there, it is just in another page in the schematic. I'm uploading full schematic now for clarity. I used  PSL:2264 circuit for the purpose only.
  • Regarding the circuit, I think I found an error now in this schematic. I was implementing the circuit mentioned in the technical note you posted but I took it from another source and I failed to realize that I need to keep GND input of MAX6350 floating for it to work. I'll fix this in next version.
  • I remembered your point about 7V being more fundamental but I couldn't find a good voltage reference at this voltage. Probably industry is too biased towards using 5V. In MAX63xx, 7V is not available.

Safety resistors at outputs of OP27

  • I designed the circuit for use with acromags only so I didn't try to affect the output signal at all.
  • But you are right. Keeping in mind the mishaps I have seen myself, I will put a 100 Ohm resistors in the next version.

No of wires at RTD:

  • Yeah, I wanted to implement 4 wire method only. I by mistake grounded the incoming signal from RTD before InAmp.
  • You are right, We will miss out CMRR if I do not keep inputs to InAmp floating. I'll fix this in next version.

Best nominal RTD resistance:

I'll look into it but since the numbers work out for us for now (fortunately), I'll keep it at a lower priority. Having a good science reason will be good, I agree.

Channels:

I couldn't think of a strong argument against having extra channels. Plus we don't really know what temperatures the cavities go to (and will go to with new heat shield on). So for the first version at least, I wanted to keep extra channels and decide this answer in the future.


Next version required:

Unfortunately, I think the present version won't really work without a lot of ugly scratches on the pcb. I messed up this one. Thanks to JLCPCB, getting boards is cheaper and faster now. So I'll get to this right away to avoid Christmas delays.

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