A combination includes a first device and a second device, wherein both devices comprise at least one surface and the surface of the first device comprises at least one recess. The gas sample extraction apparatus is disposed in a flow of fluid to be sampled, and the fluid sample flows through the sample chamber via the fluid inlet and outlet ports. A servo control senses the lateral shift excursion of the defined servo track; determines a maximum positive peak and a maximum negative peak of the lateral shift excursion; and positions the coarse actuator substantially at a midpoint of the maximum positive peak and the maximum negative peak of lateral shift excursion of the defined servo track. An analog-to-digital converter converts the mixed analog signal into a converted digital signal.