  The upgrade to the High Luminosity Large Hadron Collider will pose
unprecedented challenges to the tracking systems of all experiments. Recent
advancement of active pixel detectors designed in CMOS processes provide
attractive alternatives to the well-established hybrid design using passive
sensors. This article presents studies with a high-voltage CMOS active pixel
sensor designed for the ATLAS tracker upgrade. The sensor is glued to the
read-out chip of the Insertable B-Layer. The device under test is structured in
smaller pixels than those of the attached read-out chip with information on the
hit CMOS pixel encoded in the output signal (subpixel encoding). Test beam
measurements mapping recorded hits in the read-out chip to CMOS pixels, thus
validating the subpixel encoding, are described.
