Even though both capacitive digital isolators and optocouplers offer similar functionality, these devices are quite different in construction and working principle. Optocouplers use an LED to transmit digital or analog information across an isolation (or insulation) barrier (often just an air gap). Some optocouplers use epoxy as the insulating material which offers slightly better dielectric strength than air, as shown in Figure 1-1. Conversely, capacitive digital isolators are constructed with two series isolation capacitors using SiO₂ as the dielectric, as shown in Figure 1-2. SiO₂ offers one of the highest dielectric strengths among insulating materials and is significantly stronger compared to dielectrics used by competing isolation technologies, as shown in Table 1-1.

Isolators are extensively used in many industrial and automotive applications where isolation of data, control or status signals is needed. To enable processing of the isolated data, control, or status signals in a timely manner, it is critical for the isolator to have optimum switching characteristics, minimizing the impact on the overall system timing performance. Optocouplers fare very poorly when it comes to switching characteristics whereas digital isolators offer one of the best switching characteristics in the industry, enabling more systems to meet their performance requirements.

General purpose optocouplers usually do not have any supported data rates mentioned in their data sheets, making it difficult to know their suitability for a given application. Most of these optocouplers also have an open-collector output, due to which they are only characterized to a few select pullup or load resistor values. One of TI’s latest digital isolators, ISO6741, has its maximum supported data rate clearly specified in the data sheet as 50Mbps, which makes it easy to know its suitability for a given application. Unlike optocouplers, digital isolators do not require any external pullup resistors for operation and the maximum data rate is not heavily dependent on external components.

Table 2-1 compares timing specifications of a general purpose optocoupler with TI digital isolators. The information in the table also estimates the asynchronous and synchronous data rates that are achieved using the data sheet timing specifications. Table 2-1 shows that the data rate achieved using a general purpose optocoupler is much lower than what can be achieved using digital isolators. The two pullup resistor options listed with R_L = 100 Ω and R_L = 1.9 kΩ for optocoupler consume significantly higher current compared to digital isolators, making them unsuitable for many applications.

High-speed optocouplers offer better switching characteristics compared to general-purpose optocouplers. Table 2-2 compares a typical high-speed optocoupler with TI digital isolators in which the asynchronous and synchronous data rates for the devices are estimated using the timing specifications given in their respective data sheets. As shown in the comparison table, digital isolators still support much higher data rate compared to the high-speed optocoupler.

Time dependent dielectric breakdown (TDDB) test is an industry standard accelerated stress test for determining lifetime of a dielectric as a function of voltage. The test consists of applying various stress voltages across the isolation barrier of a device that are much higher than the typical working voltages and monitoring the amount of time it takes for the dielectric to break down. These voltage vs time coordinates are plotted on an appropriate graph, and the coordinates are extrapolated to lower stress voltages to determine expected dielectric lifetimes for the suitable working voltages.

Figure 3-1 compares TDDB plot of a TI digital isolator against a popular optocoupler, it can be noticed that the average TDDB line of optocoupler is about 2 divisions (100 times) lower than digital isolator average TDDB line. The primary reason for such a large difference in TDDB lifetimes of the two technologies is the large difference in dielectric strengths of the insulating material they use (refer Table 1-1). It can also be noticed that the lifetime of an optocoupler for a given stress voltage varies considerably from one sample to another while the same is consistent across samples for the digital isolator.

An optocoupler works on the principle of converting electrical signal into light and then back into electrical signal to achieve isolation. This limits the choice of dielectric that can be used for insulation to the ones that are optically transparent like air and epoxy. Since the dielectric strengths of air and epoxy are significantly low, they occupy considerable amount of space in a single-channel package, thereby limiting the maximum number of channels that can be fit into a given optocoupler device.

Also, digital isolators use SiO₂ as a dielectric, which has significantly higher dielectric strength and occupies a much lower space to realize a single isolation channel, hence multiple channels can be easily integrated into a small package. A typical single channel optocoupler is usually available in a package size of 3.7 mm × 4.55 mm whereas ISO7762 with SSOP package can fit 6 high-performance channels in a small package area of
4 mm × 5 mm.

Figure 4-1 compares amount of space occupied by eight single-channel optocouplers and four dual-channel optocouplers each with two ISO6741 devices to realize an eight-channel isolation solution. The figure also places ISO7762, six-channel digital isolator, by the side showing the highest channel density achieved in a wide-body SOIC-16 package.