  TES-based bolometer and microcalorimeter arrays with thousands of pixels are
under development for several space-based and ground-based applications. A
linear detector response and low levels of cross talk facilitate the
calibration of the instruments. In an effort to improve the properties of
TES-based detectors, fixing the TES resistance in a resistance-locked loop
(RLL) under optical loading has recently been proposed. Earlier theoretical
work on this mode of operation has shown that the detector speed, linearity and
dynamic range should improve with respect to voltage biased operation.
  This paper presents an experimental demonstration of multiplexed readout in
this mode of operation in a TES-based detector array with noise equivalent
power values (NEP) of $3.5\cdot 10^{-19} $W/$\sqrt{\mathrm{Hz}}$. The measured
noise and dynamic properties of the detector in the RLL will be compared with
the earlier modelling work. Furthermore, the practical implementation routes
for future FDM systems for the readout of bolometer and microcalorimeter arrays
will be discussed.
