The 0-40 Ohms is NOT the signal. It controls the signal by varying voltage (or current) seen as inout by the sensor.
That in mind, and with your second post, it sounds like you want smoother steps between changes. O.K. if you use stepper to control the pot you need smaller steps. You might be able to get 0-40 out of a 5K digi-pot, but you probably won't get any better resolution.