Two components make up the resistance at any given setting: the polysilicon or thin film resistors (step resistor, RS), and the CMOS switch resistor (RSW = 50 Ω at 5 V supply). Together, these components add up such that
where N is the number of bits of the digital potentiometer.
The TC of the step resistor, which is published in the relevant product data sheet, is typically −35 ppm/°C for thin film resistors or 600 ppm/°C for polysilicon. The resistance of the switch, on the other hand, doubles at 100°C. As a result, the overall TC is nonlinear and it is worse off at low value codes where the switch resistance dominates. See the TC graphs in the relevant product data sheet for more detailed information.