The main function of an isolator is to transmit some form of information through an electrical isolation barrier while blocking current flow. The isolator is made of insulating material that blocks current flow, and there are coupling elements across the isolation barrier. Information is usually encoded by the coupling element before being transmitted through the isolation barrier.

Analog Devices' iCoupler® digital isolators use chip-scale microtransformers as coupling elements to transmit data across high-quality polyimide isolation barriers. There are two main data transfer methods used in iCoupler isolators: single-ended and differential. When selecting a data transfer mechanism, engineering design trade-offs are required to optimize the desired end product characteristics.

In single-ended data transmission, we use a transformer with one end of the primary winding connected to the ground. Logic transitions in the input signal are encoded as pulses, always positive with respect to ground, on the transmitter chip. This is also known as "one pulse, two pulses" because rising edges are encoded as two consecutive pulses while falling edges are represented as a single pulse (see the top of Figure 1). The receiver on the other side of the isolation barrier receives the signal and determines whether one or two pulses were sent; it will then reconstruct the output accordingly.

Differential data transmission uses a true differential transformer. In this case, a single pulse is always sent when the input edge is detected, but the polarity of the pulse determines whether the transition is rising or falling (bottom of Figure 1). The receiver is a true differential structure and updates the output according to the pulse polarity.

One of the main advantages of the single-ended approach is lower power consumption at low data rates. This is because differential receivers require more DC bias current than CMOS Schmitt triggers used in single-ended receivers. However, the differential method consumes less power at higher throughput rates for two reasons: drive level and number of pulses. The drive level of the transformer can be reduced because the receiver only needs to determine the polarity, not whether there is a single pulse or two pulses. Single-ended systems require an average of 1.5 pulses per edge, while differential transmission requires 1 pulse per edge (a 33% reduction).

Reduced drive levels and fewer pulses can also reduce RF radiation. The radiation is caused by current pulses in the power supply causing radiation from the printed circuit board structure. With fewer pulses and lower energy per pulse, significantly less RF radiation is generated.

Differential transmission has two other advantages over single-ended systems: propagation delay and noise immunity. In the single-ended method, a specific timing relationship must exist when creating a single pulse or two pulses, and the receiver must analyze the pulses within a specific time window. These requirements place constraints on encoding and decoding, and ultimately, the propagation delay through the device. This in turn limits the overall throughput that the device can achieve. The differential method is less restrictive because it always uses a single pulse, resulting in lower propagation delay and higher throughput.

Differential receivers can reliably detect the differential signal sent by the transmitter, and also reject unwanted common-mode noise ubiquitous in isolated systems, resulting in significantly improved common-mode transient immunity (CMTI). Differential receivers are also less susceptible to power supply noise and therefore have higher noise immunity. The LEDs used in optocouplers are single-ended in nature, which is one reason why optocouplers typically have poor CMTI performance. Differential data transmission enables iCoupler digital isolators to significantly improve performance over optocouplers.

A data transfer method is also an option for the designer to optimize the performance of the digital isolator. Using a true differential coupling element as the basis of iCoupler technology provides a high degree of flexibility in this area, which is usually not possible with optocouplers and capacitive coupling devices.

Summarized by Easybom.