The Finite Difference Approximation (FDA) (§7.3.1) is a special case of the matched transformation applied to the point . To see this, simply set in Eq.(8.5) to obtain
Since the FDA equals the match z transformation for the point , it maps analog dc ( ) to digital dc ( ) exactly. However, that is the only point on the frequency axis that is perfectly mapped, as shown in Fig.7.15.