Free electron analog to digital conversion is accomplished by deflecting an electron beam bunch transversally by the voltage to be sampled and then quantizing the deflected angle in an array of electron detectors. Miniaturization of the whole system, especially the deflection plates, leads to an improved performance. A system restricted by the fundamental limit of diffraction could achieve a sampling rate of 1 THz with 8 bit resolution; the source requirements would be a brightness of 2x10(13) A cm(-2)/sr at 5 kV and a source jitter of 1 fs. A more practical system using a negative electron affinity photocathode electron source with a brightness of 1x10(7) A cm(-2)/sr at 5 kV will allow the system to sample 4 bits at 750 GHz or 6 bits at 450 GHz.