REF200 current source

Hi,

We are developing a cryogenic temperature controller, and we are using Lakshore's Silicon diode sensor  for temperature measurement, this diode sensor takes a four wire configuration, as shown in below fig-1 , and it requires a excitation current of 10μA , we are using REF200 for generating the required excitation current, the circuit used is also shown below, i'm getting I+ from the Iout of the circuit shown below in fig-2, i have a small question ,can i connect the I- terminal of the diode to gnd or i should have similar setup which gives me I-, Pls help me in clearing this silly doubt . Further for obtaining the voltage from the diode sensor we need to have a high impedance voltage measurement source like voltmeter etc., for this measurement we are using PGA204 an instrumentational amplifier which is shown below in fig-4, Pls suggest the best way to measure the voltage or can we proceed with the PGA. Pls help me clearing these doubts. Looking forward for positive and speedy reply .

Thanking you

Karthik.R

             Fig-1                                                      

Fig-2

  Fig-3

                                                      Fig-4