  At SRON we are developing the Frequency Domain Multiplexing (FDM) for the
read-out of the TES-based detector array for the future infrared and X-ray
space mission. We describe the performances of a multiplexer designed to
increase the experimental throughput in the characterisation of ultra-low noise
equivalent power (NEP) TES bolometers and high energy resolving power X-ray
microcalorimeters arrays under ac and dc bias. We discuss the results obtained
using the TiAu TES bolometers array fabricated at SRON with measured dark NEP
below $5\cdot 10^{-19}W/Hz^{1/2}$ and saturation power of several fW
