build a constant current supply
I need a 100uA constant-current source to use for calibration


To calibrate it, connect one current-sensing pin to ground on a power supply. Connect the other current-sensing pin via a precision resistor to a suitable voltage on the power supply. Maybe consider 10 volts as the excitation voltage.

10V across a resistor of 100 kohm produces 100.0000 uA. Even if you added the 1.2 milli ohms into the equation the current would be 99.999999 uA.

All you need next is a precision voltage source of 10V so buy a voltage reference chip with as much accuracy as you can afford.