  This work presents the design of BUSARD, an application specific integrated
circuit (ASIC) for the detection of ionizing particles. The ASIC is a
monolithic active pixel sensor which has been fabricated in a High-Voltage
Silicon-On-Insulator (HV-SOI) process that allows the fabrication of a buried
N+ diffusion below the Buried OXide (BOX) as a standard processing step. The
first version of the chip, BUSARD-A, takes advantage of this buried diffusion
as an ionizing particle sensor. It includes a small array of 13$\times$13
pixels, with a pitch of $80\,\mu$m, and each pixel has one buried diffusion
with a charge amplifier, discriminator with offset tuning and digital
processing. The detector has several operation modes including particle
counting and Time-over-Threshold (ToT). An initial X-ray characterization of
the detector was carried out, obtaining several pulse height and ToT spectra,
which then were used to perform the energy calibration of the device. The
Molybdenum $\mathbf{K_{\alpha}}$ emission was measured with a standard
deviation of $127\,e^{-}$ of ENC by using the analog pulse output, and with
$276\,e^{-}$ of ENC by using the ToT digital output. The resolution in ToT mode
is dominated by the pixel-to-pixel variation.
