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 Mon Dec 17 22:13:03 2018, anchal, DailyProgress, TempCtrl, Proposed circuit for cavity temperature sensor Part 2 Wed Dec 19 11:01:00 2018, awade, DailyProgress, TempCtrl, Proposed circuit for cavity temperature sensor Part 2 Wed Dec 19 14:00:55 2018, anchal, DailyProgress, TempCtrl, Proposed circuit for cavity temperature sensor Part 2
Message ID: 2266     Entry time: Wed Dec 19 11:01:00 2018     In reply to: 2265     Reply to this: 2267
 Author: awade Type: DailyProgress Category: TempCtrl Subject: Proposed circuit for cavity temperature sensor Part 2

## Missing current excitation to RTD?

I don't know if I'm missing it, or if you've yet to add it, but can't see current excitation into the RTD.  I remember you were looking into REF200 and some other voltage-> current highly stable sources (PSL:2264),  that part will be key to actually getting a high precision temperature readout.  I didn't understand how the op amp stage in your schematic (PSL:2264) worked; it looks like you pinned the inverting input to ground but then also connected the output pin for feedback.  Doesn't really make sense to me.

Do you want something more like this: technical note. There they use the op amp to bootstrap the voltage reference so that the actual applied voltage rises to whatever is necessary to get the design current fixed through the reference resistor AND sink it through the load. The point of the that design is that the resistance of the sensor and its cabling does not affect the current: current is set only by the reference resistor. The stablity of the excitation current will hinge on the goodness of the voltage reference and that of the reference resistor.

Also, as a side note, you were proposing a 5V reference.  I thought I read somewhere that the standard buried zenor reference in these types of voltage references were actually typically 7 V and the 5 V and 10 V versions some how stepped up/down using a precision resistor network buried within the chip.  My understanding was that 7V references were the best because there was no additional drift.  If the specs of the MAX6325 are good, then maybe its well compensated and its nothing to worry about.  It might be worth checking if that series of chips has other discreet​ voltage options that perform better.

## Add some impedance to output of OP27s

Its a good idea to add about 100 Ω (or more) of resistance to output of OP27s. Otherwise poor little chips are going to overdrawn in current for low impedance loads. Probably won't break them, but it will produce bogus voltages. The acromags are 10 kΩ impedance but you can never predict if someone is going to plug in a 50 Ω oscilloscope at some point.  Check what max current of OP27 is and adjust so that at full range you wont be overdrawing with 50 Ω load.

## How many wires?

RTD is pinned to ground on one side.  Not sure what your intending for the wiring scheme here? 3 wires, 4 wires.  Ideally you want to excite current through two wires and read back voltage drop across another two.  But if you're pinning one side to ground then you've already effectively reduced it to a three wire scheme: ok but not best.  I guess you want to think about how this scheme is going to minimize your uncertainty/sensitivity with respects to wiring (see maybe this). Four wires is best.  Be best.

Also are we going to miss out on the excellent CMRR of the instrument amplifier if it isn't a truly differential readout on the front end of the circuit?

## Best RTD nominal resistance

You've looked at a bunch of numbers for choice of RTD resistance @ 25 C.  From the numbers we looked at 1 kΩ seemed like the best for order 100 µA excitation (this is the break point with Johnson noise). It might be nice to have a plot or a table that shows the tradeoff off of sensing SNR (for various noise sources) for different choices of RTD nominal resistance, i.e. 100 Ω, 1 kΩ, 5 kΩ, 10 kΩ.   The excitation current and resistance determine the tradeoff points and the right choice is the one that gets below/close to the nominal input referred noise of available pre-amp stages. That would be good for framing a good science reason for choosing a particular RTD (with a particular slope).

## So many channels

Still not sure you need so many channels.  If we already have a good idea of the operating range, then a one high precision channel will do.  We are almost certainly not going to need all that dynamic range and changing resistors is easy.

 Quote: The second stage of the amplifier for temperature sensor would be subtractor circuits using OP27G. PFA the circuit schematic consisting of the instrumentation amplifier and different circuits for acromag channels. There would be 1 low-resolution channel with a range from about 15 $^\circ C$ to 95 $^\circ C$ with a response: 0.238053435115*T - 13.167938931297705 Rest would high-resolution channels with a smaller range. These channels would give us about 1mV/mK slope so that we can read with mK precision with 16 bit $\pm 10$ V range. Channel 1: From 15 $^\circ C$ to 35 $^\circ C$ with a response: 0.952875*T - 22.5 Channel 2: From 35  $^\circ C$ to 55 $^\circ C$ with a response: 0.943758551683*T - 43.329447115384596 Channel 3: From 55  $^\circ C$ to 75 $^\circ C$ with a response: 0.955040625*T - 61.9375 Channel 4: From 75  $^\circ C$ to 95 $^\circ C$ with a response: 0.9429496875*T - 79.74466991341995 I'm attaching calculations done in the jupyter notebook as zip file.

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