  In this work, we investigate two specific linear ADRC structures, namely
output- and error-based. The former is considered a 'standard' version of ADRC,
a title obtained primarily thanks to its simplicity and effectiveness, which
have spurred its adoption across multiple industries. The latter is found to be
especially appealing to practitioners as its feedback error-driven structure
bares similarities to conventional control solutions, like PI and PID. In this
paper, we describe newly found connections between the two considered ADRC
structures, which allowed us to formally establish conditions for their
equivalence. Furthermore, the conducted comprehensive performance comparison
between output- and error-based ADRCs has facilitated the identification of
specific modules within them, which can now be conveniently used as building
blocks, thus aiding the control designers in customizing ADRC-based solutions
and making them most suitable for their applications.
