I have designed a circuit for using RTDs for sensing temperature of the cavities.

Current Source:

Previously REF200 has been used for driving the RTDs. It is quite noisy and we can make a better current source using a good voltage reference chip and a FET opamp. PFA circuit schematic.

Following is some comparison to highlight improvements:

Comparison between REF200 and current source from MAX6325CPA+OPA827

Parameters	REF200	MAX6325CPA + OPA827	Units
Current	100	125
Current Accuracy	0.25	0.05	%
Temp. Drift	25	1.2	ppm/K
Output Impedance	0.1	10000
Noise BW: 0.1Hz to 10 Hz	1	0.078
Noise Density	20 @ 10kHz	6.42 @ 1kHz (Shot noise dominated)
Error in perceived temperature per Kelvin change in circuit temperature	6.5	0.31	mK/K

* 1k Platinum RTD is assumed (link).
** 20k resistor used in the schematic is 0.01% tolerance and 0.2ppm/K TCR metal foil resistor.

Instrumentation Amplifier:

I searched through various places to hunt for an instrumentation amplifier for our low noise and low-temperature drift requirements. I also checked equivalent features of instrumentation amplifiers made with various low noise opamps. See attached LISO+LTSpice analysis results. I have come to the conclusion to use AD8429BRZ for our purpose because of being single chip and low drift (AD8421BRZ is the best but it is not available until february. But I suggest future people to use AD8421BRZ). Following are some key features:

• Voltage offset = 50 uV which translates to signal offset of 103.9 mK. We can calibrate this in code.
• Voltage offset drift = 0.3 uV/K. This translates to 0.62 mK/K  error in perceived temperature per kelvin change of circuit temperature.
• Input offset current = 30 nA which translates to signal offset of 68.33 mK. Again, this is correctable easily.
• Input offset current drift = 15 pA/K. This translates to 34.17 uK/K. So no pains here.
• Input reffered voltage noise with 1k RTD input (See attached LTSpice plot, apologies for bad graphics.)
  Input ref. voltage noise density @ 1kHz is 4.87 .
  Input ref. voltage noise at BW 0.1 Hz to 10 Hz is 39.614  = 112.062  . This translates to 0.233  noise in perceived temperature.

Note:

• The RTD at input matters as input current noise in the bias current goes to ground through the RTD. The values in the datasheet are mentioned for zero impedance source.
• I am using the gain value of 81 in the instrumentation amplifier as this will give us an operation up to 99.4 . 
• 75 Ohm with 0.2ppm/K will be used for the gain setting resistor. This (along with AD8429's 2ppm/K gain coefficient) translates to 74 uK/K error in perceived temperature per Kelvin change in temperature.Edit

Edit Wed Dec 19 17:20:03 2018

There is an error in this schematic. Please use latest DCC LIGO-D1800308 doc.