zuzu,
Several things.
Note in the data sheet, the op amp is running on +16V so the output has enough swing to drive the MOSFET.
The A301 data sheet
http://www.tekscan.com/pdf/A301-force-sensor.pdf
is very poor, but looks like a variable resistor. It says the max recommended current is 2.5 mA, well
within the ADR3450 spec without extra circuitry (and cost)
There are several ways of measuring resistance. For two ways, see data sheets for
AD7793 and AD5933.
Harry