Differential Data Transfer: What's the Difference?

The primary function of an isolator is to transmit some form of information through a galvanic isolation barrier while blocking current. The isolator is made of insulating material to block electric current, and there are coupling elements at both ends of the isolation barrier. Information is usually encoded by coupling elements before being transmitted through the isolation barrier.

Analog Devices' iCoupler® digital isolators use chip-scale microtransformers as coupling elements to transmit data through a high quality polyimide isolation barrier. There are two main data transmission methods used in iCoupler isolators: single-ended and differential. When choosing 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, one end of the primary winding is grounded. The logic conversion in the input signal is encoded as a pulse, which is always positive with respect to ground, 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 top). The receiver at the other end of the isolation barrier receives the signal and determines whether one or two pulses are sent; It will then reconstruct the output accordingly.

Differential data transmission uses transformers in true differential mode. In this case, a single pulse is always sent when an input edge is detected, but the polarity of the pulse determines whether the transition rises or falls (bottom of Figure 1). The receiver is a true differential structure and updates the output based on pulse polarity.

 

 

 

One of the main advantages of the single-ended approach is the lower power consumption at low data rates. This is because differential receivers require more dc bias current than the CMOS Schmitt trigger used in single-ended receivers. However, the differential method dissipates 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 also reduce RF emissions. Radiation is generated because current pulses in the power supply cause radiation to the printed circuit board structure. Because there are fewer pulses and the energy of each pulse is lower, the RF radiation generated is significantly reduced.

Differential transmission has two additional advantages over single-ended systems: propagation delay and immunity. In the single-ended method, when creating a single pulse or two pulses, there must be a specific timing relationship, and the receiver must analyze the pulse within a specific time window. These requirements limit encoding and decoding, ultimately limiting the propagation delay through the device. This, in turn, limits the total throughput that the device can achieve. The differential method is less constrained 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 also rejects unwanted common-mode noise that is prevalent in isolated systems, resulting in significantly higher common-mode transient immunity (CMTI). Differential receivers are also less susceptible to power supply noise and therefore have higher 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 transfer results in a significant improvement in the performance of iCoupler digital isolators over optocouplers.

Data transfer methods are also an option for designers to optimize digital isolator performance. Using true differential coupling components as the basis for iCoupler technology provides a high degree of flexibility that optocouplers and capacitively coupled devices typically do not reach.

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