Limit variable resistance upper bound?
Look at it this way: if the resistance of your sensor increases above 1.7 kohms, you want your complete circuit to still load the probing voltage source with the same current as if it were a 1.7 kohm resistor. 

So essentially you want a current source that only turns on if insufficient current is flowing through the variable resistor:







simulate this circuit &ndash; Schematic created using CircuitLab

Choosing the correct op-amp, transistor, and reference voltage depend on knowing the stimulus voltage, which you haven't shared. Also on how quickly the variable resistor value might be changing.