A Capacitively Coupled Digital Isolator with CMTI of 162 kV/μs and Data Rate of 230 Mbps

Abstract This paper presents a small-size on-chip capacitively coupled digital isolator with superior Common Mode Transient Immunity (CMTI) and high operational speed performances. It employs a gate-cross-coupled common gate pre-amplifier with an active zero load and a high-pass filter before the pre-amplifier. By applying the proposed technique, the high-frequency gain retains to high value, while the low-frequency gain is greatly attenuated. The high-pass filter, which is designed based on the technology structure of the on-chip isolation capacitance, is applied to further cancel the influence of Common Mode Transient (CMT) and aid in the transmission of high-frequency signal. Furthermore, differential On-Off Keying (OOK) modulation is adopted to eliminate CMT noise. For the purpose of increasing the operation speed performances, the work utilizes the Voltage-Current high-speed comparator with fast feed-forward path as Schmitt trigger and level shift, and employs the envelop-comparator, integrator and filter as demodulator. Thus, it significantly reduces the propagation delay and speeds up the data rate. As a result, both high CMTI and operational speed performances are achieved simultaneously. Fabricated in a 0.18 µm isolated CMOS technology, the area of the isolation element, i.e., the capacitor is 2×104 µm2. The measurement results of a test chip are an over 162 kV/µs CMTI, a 230 Mbps data rate, a 6 ns propagation delay, a 1.5 mA dynamic current and 14 kV isolation breakdown voltage. The characteristics are superior to those of conventional digital isolators.