Electrical isolators such as digital isolators are widely employed in electric vehicles, industrial automation, solar inverters and other applications that require the isolation of high-voltage and low-voltage areas [1-6]. Digital isolators with exceptional Common Mode Transient Immunity (CMTI), high operational speed performances, low power consumption and robust stability in the most noisy harsh environment are becoming increasingly significant. It means that the signal must be precisely transmitted while remaining immune to very quick Common Mode Transient (CMT). Furthermore, the isolator should have a low propagation delay and high-speed data rata under the condition of a low current power consumption. In addition, to safeguard the internal circuit, the isolation barrier must be able to withstand high VPK surges.
In comparison to optocoupler isolators, transformer isolators [1, 2, 4, 7-17] and capacitive isolators [5, 6, 8-23] are the two types of usual isolation architectures because of their advantages of smaller size, faster speed, lower power consumption, and better CMTI.
Fig. 1 shows a polyimide-based digital isolator using on-chip inductor as transformer and OOK architecture in . In which, the On-Off Keying (OOK) transmitter (TX) directly resonates with the transformer to generate a high frequency carrier signal. For high CMTI, the TX is designed based on the negative-Gm oscillator, which can contribute more gm to sustain the oscillation during the CMT event. Moreover, in front of the RX, an AC-coupled bias network is used to attenuate the CMT noise without impacting the carrier signal. Therefore,  can achieve 200 kV/μs CMTI. However, the data rate is unavoidable to decrease due to the building time of the oscillator and comparing time contributed by the AC-coupled bias network. In addition, the diameter of micro-transformer is usually larger than 250 μm, occupying too much area and even need another die to put it . For solving the problem of large occupied area of the transformer,  presents a SiO2-based digital isolator using smaller transformers and pulse generation and detection scheme. However, with a transformer of 230 μm diameter, it requires a high-gain receivers to compensate for poor gain. Therefore, it has developed a GHz-band signal generation and detection technique with a 5 V CMOS technology, unavoidably, which degrades the CMT and noise immunity critical for isolators. In addition, pulse modulation, which is sensitive to signal edge, has extremely low static power consumption and high data rate, however, its anti-interference ability is inferior than OOK modulation. Hence,  achieves 1.6 mA power consumption and 250 Mbps data rate, however, but only provides 35 kV/μs CMTI due to the small transformer and the limited of pulse modulation. For both minimal isolation element area and high CMTI,  presents a SiO2 based digital isolator using capacitors and OOK architecture. Using isolation capacitance, its isolation element area is 3.00×104 μm2, which is about 1/5 that of traditional micro-transformer. However, capacitive isolators are limited to CMTI of 100 kV/μs even with the OOK data architecture [1, 5].
There is a trade-off between the area of isolation element and the accurate transmission with high-speed data rate when a fast CMT surges.
In this work, a SiO2-based capacitively coupled digital isolator using OOK architecture with an active zero load pre-amplifier and a high-pass filter is presented that features high CMTI and operational speed performances. The pre-amplifier is designed into a gate-cross-coupled common gate amplifier. By applying the proposed active zero load, the high-frequency gain retains to high value, while the low-frequency gain is greatly attenuated. Besides, the high-pass filter before pre-amplifier, based on the technology structure, is applied to further cancel the influence of CMT. Finally, both high gain for high-frequency carrier signal and low gain for CMT and noise is achieved simultaneously.
For fast transient response, it utilizes the Voltage-Current high-speed comparator with fast feed-forward path as Schmitt trigger and level shift. Besides, it employs the envelop-comparator, integrator and filter as demodulator, which is beneficial to reduce the propagation delay and improve the transmission accuracy with extra boost of the CMTI. Finally, forming the core of the OOK modulation scheme, the oscilalator is designed as a fast start-up ring oscillator with frequency feedback for high accuracy to fulfill the demand of propagation delay and data rate.
This paper is organized as follows. Section II introduces the proposed architecture of the capacitively coupled digital isolator for providing high CMTI from transmitter and receiver. Section III describes the detailed circuit implementation of pre-amplifier, high-pass filter, demodulation and oscillator, all of which are designed for the improvement of CMTI and operational speed performances. Section IV presents the measurement results and finally a conclusion is given.