Hi,
I wonder who designed the "Recommended Circuit Diagram" of the application note.
In my eyes a "constant current circuit" should provide constant current regardless of supply voltage. But the given circuit does not. If the supply voltage changes by x%, then the (so called) constant current will change the same x%.
--> Replacing R2 with ZXRE125FF (or similar shunt reference) will truly generate
constant current of 122uA.
Also the whole circuit does not include any noise suppressing low pass fiilters, nor any power supply stabilizing capacitors.
(I´d never use this circuit "as is". It may work in laboratory environment, but may fail in the field.)
1)giving GND to Reference input
It should be the Reference_GND of the ADC. Wire this node directely to the ADC pin.
2) R6=10Kohms not 1Kohms which give me gain of 3
Makes no sense, since you worte you want gain of 100.
I recommend to do the "high gain" with the left hand two OPAMPs, by
reducing the value of R6.
Let the right hand OPAMP have gain of 1. (unless you have frequency response problems)
Klaus