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HCPL-0721 Datasheet(PDF) 11 Page - Agilent(Hewlett-Packard) |
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HCPL-0721 Datasheet(HTML) 11 Page - Agilent(Hewlett-Packard) |
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11 / 18 page  11 Figure 13. Propagation delay skew waveform. Figure 14. Parallel data transmission example. Propagation delay skew repre- sents the uncertainty of where an edge might be after being sent through an optocoupler. Figure 14 shows that there will be uncertainty in both the data and clock lines. It is important that these two areas of uncertainty not overlap, otherwise the clock signal might arrive before all of the data outputs have settled, or some of the data outputs may start to change before the clock signal has arrived. From these considerations, the absolute minimum pulse width that can be sent through optocouplers in a parallel application is twice tPSK. A cautious design should use a slightly longer pulse width to ensure that any additional uncertainty in the rest of the circuit does not cause a problem. The HCPL-772X/072X optocouplers offer the advantage of guaranteed specifications for propagation delays, pulse-width distortion, and propagation delay skew over the recommended temperature and power supply ranges. Pulse-width distortion (PWD) is the difference between tPHL and tPLH and often determines the maximum data rate capability of a transmission system. PWD can be expressed in percent by dividing the PWD (in ns) by the minimum pulse width (in ns) being trans- mitted. Typically, PWD on the order of 20 - 30% of the minimum pulse width is tolerable. Propagation delay skew, tPSK, is an important parameter to con- sider in parallel data applications where synchronization of signals on parallel data lines is a concern. If the parallel data is being sent through a group of optocouplers, differences in propagation delays will cause the data to arrive at the outputs of the optocouplers at different times. If this difference in propagation delay is large enough it will determine the maximum rate at which parallel data can be sent through the optocouplers. Propagation delay skew is defined as the difference between the minimum and maximum propa- gation delays, either tPLH or tPHL, for any given group of optocoup- lers which are operating under the same conditions (i.e., the same drive current, supply voltage, output load, and operating temperature). As illustrated in Figure 13, if the inputs of a group of optocouplers are switched either ON or OFF at the same time, tPSK is the difference between the shortest propagation delay, either tPLH or tPHL, and the longest propagation delay, either tPLH or tPHL. As mentioned earlier, tPSK can determine the maximum parallel data transmission rate. Figure 14 is the timing diagram of a typical parallel data application with both the clock and data lines being sent through the optocouplers. The figure shows data and clock signals at the inputs and outputs of the optocouplers. In this case the data is assumed to be clocked off of the rising edge of the clock. 50% 50% tPSK VI VO VI VO 2.5 V, CMOS 2.5 V, CMOS DATA INPUTS CLOCK DATA OUTPUTS CLOCK tPSK tPSK |
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