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HCPL-0872 Datasheet(PDF) 10 Page - AVAGO TECHNOLOGIES LIMITED |
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HCPL-0872 Datasheet(HTML) 10 Page - AVAGO TECHNOLOGIES LIMITED |
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10 / 12 page  10 Figure 4. Pre-Trigger Modes 0, 1, and 2. Pre-Trigger Mode The pre-trigger mode refers to the operation of a PLL- based circuit that affects the sampling behavior and conversion time of the A/D converter when channel 1 is selected. The PLL pre-trigger circuit has two modes of operation; the first mode allows more precise control of the time at which the analog input voltage is effectively sampled, while the second mode essentially eliminates the time between when the external convert start command is given and when output data is available (reducing it to less than 1µs). A brief description of how the A/D converter works with the pre-trigger circuit disabled will help explain how the pre-trigger circuit affects operation when it is enabled. With the pre-trigger circuit is disabled (pre-trigger mode 0), Figure 4 illustrates the relationship between the convert start command, the weighting function used to average the modulator data, and the data ready signal. The weighted averaging of the modulator data begins immediately following the convert start command. The weighting function increases for half of the conversion cycle and then decreases back to zero, at which time the data ready signal is given, completing the conversion cycle. The analog signal is effectively sampled at the peak of the weighting function, half-way through the conver- sion cycle. This is the default mode. If the convert start signal is periodic (i.e., at a fixed frequency) and the PLL pre-trigger circuit is enabled (pre- trigger modes 1 or 2), either the peak of the weighting function or the end of the conversion cycle can be aligned to the external convert start command, as shown in Figure 4. The Digital Interface IC can therefore synchro- nize the conversion cycle so that either the beginning, the middle, or the end of the conversion is aligned with the external convert start command, depending on whether pre-trigger mode 0, 1, or 2 is selected, respectively. The only requirement is that the convert start signal for channel 1 be periodic. If the signal is not periodic and pre-trigger mode 1 or 2 is selected, then the pre-trigger circuit will not function properly. WEIGHTING FUNCTION CONVERT START - CS DATA READY - SDAT A) PRE-TRIGGER MODE 0 B) PRE-TRIGGER MODE 1 C) PRE-TRIGGER MODE 2 An important distinction should be made concerning the difference between conversion time and signal delay. As can be seen in Figure 4, the amount of time from the peak of the weighting function (when the input signal is being sampled) to when output data is ready is the same for all three modes. This is the actual delay of the analog signal through the A/D converter and is independent of the “conversion time,” which is simply the time between the convert start signal and the data ready signal. Because signal delay is the true measure of how much phase shift the A/D converter adds to the signal, it should be used when making calculations of phase margin and loop stability in feedback systems. There are different reasons for using each of the pre-trigger modes. If the signal is not periodic, then the pre-trigger circuit should be disabled by selecting pre-trigger mode 0. If the most time-accurate sampling of the input signal is desired, then mode 1 should be selected. If the shortest possible conversion time is desired, then mode 2 should be selected. The pre-trigger circuit functions only with channel 1; the circuit ignores any convert start signals while channel 2 is selected with the CHAN input. This allows conversions on channel 2 to be performed between conversions on channel 1 without affecting the operation of the pre-trigger circuit. As long as the convert start signals are periodic while channel 1 is selected, then the pre-trigger circuit will function properly. The three different pre-trigger modes are selected using bits 6 and 7 of register 3, as shown in Table 3 below. |
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