  The first evaluation of an ultra-high granularity digital electromagnetic
calorimeter prototype using 1.0-5.8 GeV/c electrons is presented. The
$25\times10^6$ pixel detector consists of 24 layers of ALPIDE CMOS MAPS
sensors, with a pitch of around 30~$\mu$m, and has a depth of almost 20
radiation lengths of tungsten absorber. Ultra-thin cables allow for a very
compact design. The properties that are critical for physics studies are
measured: electromagnetic shower response, energy resolution and linearity. The
stochastic energy resolution is comparable with the state-of-the art resolution
for a Si-W calorimeter, with data described well by a simulation model using
GEANT and Allpix$^2$. The performance achieved makes this technology a good
candidate for use in the ALICE FoCal upgrade, and in general demonstrates the
strong potential for future applications in high-energy physics.
