The specification table in the datasheet shows the Dynamic Range as a function of the Softspan range. As shown in Figure 1, each Softspan range has a flat input referred noise, independent of the input voltage. However, the AD4858 has a feature called Seamless High Dynamic Range (SHDR) that can internally change the front-end gain on a sample-by-sample basis. With SHDR enabled (blue line), as the input voltage gets smaller, the AD4858 changes the range automatically resulting in a lower input referred noise for small input voltages while still capable of acquiring larger signal, i.e. increases the dynamic range of the device. The output code will stick to the Softspan range selected by the user, so it implies no need for calculating the input voltage as the transfer function is the same for all samples. See the Application Note available in the AD4858’s product page for more information on SHDR.
Figure 1. Input referred noise vs differential input voltage, with SHDR ON and OFF.