IEEE Transactions on Instrumentation and Measurement vol:40 issue:3 pages:659-660
In a recent paper  a method has been presented for the design of amplitude- and phase-compensating IIR filters. The procedure is based on inversion of the transfer function, identified in the z-domain. Unstable poles are reflected into the unit circle, and the spoiled phase response is equalized again using allpass sections. This paper points to the fact that this method is not optimal in the sense that it does not reach the minimum error for a given order (or provides the desired error compensation by using a too high order). By refitting the compensating filter, a lower order filter is obtained with the same performance. In the investigated case the order is reduced from 34/34 to 26/26.