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Rarely Asked Questions – Issue 101, January 2014
Bits Per Second Make My Head Hertz
Q: I’m confused by the use of “Mbps” units when looking at digital isolator data sheets. Why not use MHz?
A: Digital isolators and optocouplers pass information between two circuits while maintaining galvanic isolation (preventing current flow between the circuits). The information passed is a digital state denoted by a logic level. Changes in the logic level occur with low-to-high or high-to-low transitions in the signal. Each state following a transition is a digital bit. The transitions may or may not occur at regular intervals.
By contrast, a continuously variable signal such as a sine or square wave has regular transitions between states. With a 50% duty cycle, the time the signal spends in each state is equal and constant from one cycle to the next. The frequency at which the signal changes state is often expressed in cycles per second, or hertz (abbreviated as Hz).
Since the digital data passed through a digital isolator isn’t necessarily a continuous signal, units of “bits per second” are used. There is, however, an important distinction to bear in mind. A continuous signal (specified in Hz) changes state twice per cycle. That means that a 50% duty cycle square wave at 1 MHz would present data to the digital isolator at 2 Mbps. Said another way, a digital isolator’s throughput rating must be twice that of the maximum sustained signal frequency it can support.
Let’s consider an example to drive the point home: isolating a serial peripheral interface (SPI). An isolated SPI bus generally includes four signals: the serial clock (SCLK), chip select (CS), serial data in (SDI) and serial data out (SDO). The output data (SDO) is latched on one edge of SCLK, while input data (SDI) is latched on the opposite edge. Here’s where confusion can set in: one input bit is latched on each clock cycle, so the SPI throughput (in Mbps) is equal to the clock frequency (in MHz). Therefore, a 1 MHz SCLK would transmit input data at 1 Mbps and output data at 1 Mbps. The SCLK signal is also isolated, however, so the digital isolator rating must be 2 Mbps (SCLK is toggling at 1 MHz). Users sometimes confuse the SPI data throughput in Mbps with the required throughput rating of the digital isolator.
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