[1] CNR, IASF, Sez Bologna, I-40029 Bologna, Italy
[2] Politecn Milan, Dipartimento Elettron & Informaz, I-20133 Milan, Italy
[3] Smart Silicon Syst, Lausanne, Switzerland
[4] Univ Bologna, Dipartmento Fis, I-40127 Bologna, Italy
[5] CNR, IASF, Sez Milano, I-20133 Milan, Italy