  A prototype of a Si-W EM calorimeter was built with Monolithic Active Pixel
Sensors as the active elements. With a pixelsize of 30 $\mu$m it allows digital
calorimetry, i.e. the particles' energy is determined by counting pixels, not
by measuring the energy deposited. Although of modest size, with a width of
only four Moliere radii, it has 39 million pixels. We describe the construction
and tuning of the prototype and present results from beam tests and compare
them with predictions of GEANT-based Monte Carlo simulations. We show the shape
of showers caused by electrons in unprecedented detail. Results for energy and
position resolution will also be given.
