Differential Data Transfer: What's the Difference?

The main function of the isolator is to transmit some form of information through the electrical isolation barrier and block the current at the same time. The isolator is made of insulating material to stop the current, and the isolator has coupling elements at both ends of the barrier. The information is usually encoded by the coupling element before the transmission passes through the barrier.

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

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

Differential data transmission uses a transformer in a true differential way. 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 up or down (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 single-ended methods is the low power consumption ratio at low data rates. This is because differential receivers require more DC bias current than CMOS Schmidt triggers used in single-ended receivers. However, the differential method consumes less power at higher throughput rates for two reasons: the drive level and the number of pulses. The drive level of the transformer can be lowered because the receiver only needs to determine the polarity and not whether there is a single pulse or two. 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 also reduce RF radiation. The cause of radiation is that the current pulse in the power supply causes the radiation of the printed circuit board structure. Because there are fewer pulses and the energy of each pulse is low, the RF radiation generated is significantly reduced.

Differential transport has two other advantages over single-ended systems: propagation latency and immunity. In a single-ended approach, there must be a specific timing relationship when creating a single pulse or two pulses, and the receiver must analyze the pulses within a specific time window. These requirements impose limits on encoding and decoding, ultimately limiting propagation delays through the device. This in turn limits the total throughput that the device can achieve. The differential method is less limited because it always uses a single pulse, resulting in lower propagation latency and higher throughput.

The differential receiver reliably detects the differential signal sent by the transmitter and suppresses the unwanted common-mode noise that is prevalent in isolation systems, resulting in a significant increase in common-mode transient immunity (CMTI). Differential receivers are not easily affected by power supply noise, so they have high immunity. The LEDs used in optocouplers are essentially single-ended, which is one of the reasons why optocouplers often have poor CMTI performance. Differential data transmission allows for a significant improvement in the performance of iCoupler digital isolators compared to optocouplers.

The data transfer method is also an option for designers to optimize the performance of digital isolators. Using true differential coupled devices as the basis for iCoupler technology provides a high degree of flexibility in this area, which is not typically possible with optocouplers and capacitively coupled devices.

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