Capacitive isolation technology is based on AC signal transfer across a dielectric using schemes such as on-off keying, phase-shift keying, edge-based transfer or other types of higher-order modulation, as the capacitor inherently blocks DC signals. Figure 5 illustrates a very basic modulator/demodulator pair using differential signaling through a series capacitive isolation barrier. These capacitors can send data and a very limited amount of power. Figure 5 shows two capacitors used to construct the isolation barrier, but depending on the product requirements and the desired isolation rating, one capacitor may easily suffice.

A series capacitive isolator is a multichip module comprising a transmitter (the left die) and a receiver (the right die). As shown in Figure 6, each die has a dedicated capacitor to provide high-voltage isolation and electrical shock protection while meeting reinforced isolation equivalent to two levels of basic isolation.

It is possible to place multiple capacitive channels into a single IC package where either side can be the transmitter or receiver, thus enabling bidirectional signal communication. Capacitive isolators have low propagation delay, can transfer data exceeding 150 Mbps, and consume less bias current compared to optocouplers – but still require separate bias supply voltages for each side of the isolation boundary.

TI’s capacitive isolators are constructed using a SiO₂ dielectric (see Figure 7), which has the highest dielectric strength of the materials listed in Table 3. In addition to having the highest dielectric strength among other insulators, SiO₂ is also an inorganic material and therefore very stable over moisture and temperature. TI’s proprietary methodology for multilayered capacitor and multilayer passivation improves isolator quality and reliability by reducing the dependence of high-voltage performance on any single layer. This technology supports working voltages (V_IOWM) of 2 kV_RMS, withstands isolation voltages (V_ISO) of 7.5 kV_RMS and has a surge voltage capability of 12.8 kV_PK.

Isolators must have long lifetimes – well beyond those of nonisolated components – to protect circuitry from faults. TI tests rigorously to the IEC standards listed in Table 2.