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AD8214 データシート(PDF) 11 Page - Analog Devices |
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AD8214 データシート(HTML) 11 Page - Analog Devices |
11 / 16 page AD8214 Rev. 0 | Page 11 of 16 COMPARATOR OFFSET AND HYSTERESIS The AD8214 features built-in hysteresis to minimize the effects of noise in the system. There is also a small offset at the input of the device. VOL VH VTH VOH VOS VOS = INPUT OFFSET VOLTAGE VH = HYSTERESIS VOLTAGE VTH = THRESHOLD VOLTAGE VOH = OUTPUT HIGH VOL = OUTPUT LOW Figure 29. Hysteresis and Input Offset Voltage Definition Figure 29 shows the relationship between the input voltage and the output current. The horizontal axis represents the voltage between the positive (+IN) and negative (–IN) inputs of the AD8214. The vertical axis shows the output current for a given input voltage. VTH represents the point where the inputs are at the same voltage level (+IN = –IN). The output of the AD8214 remains low (VOL) provided (–IN) is at a higher voltage potential than (+IN). As the input voltage transitions to +IN > –IN, the output switches states. Under ideal conditions, the output is expected to change states at exactly VTH. In practice, the output switches when the inputs are equal ± a small offset voltage (VOS). Once the output switches from low to high, it remains in this state until the input voltage falls below the hysteresis voltage. Typically, this occurs when +IN is 10 mV below –IN. SETTING THE INPUT THRESHOLD VOLTAGE The AD8214 features a 2.4 V series regulator, which can be used to set a reference threshold voltage with two external resistors. The resistors constitute a voltage divider, the middle point of which connects to +IN. The total voltage across the resistors is always 2.4 V. (See Figure 28 for proper resistor placement.) The values for these resistors can be chosen based on the desired threshold voltage using the equation: ) ( 4 . 2 IN TH V R1 R2 R1 + = × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + (1) For proper operation it is recommended that the internal 2.4 V regulator not be loaded down by using small R1 and R2 values. Figure 11 shows the proper range for the total series resistance. INPUT-REFERRED DYNAMIC ERROR Frequently, the dynamics of comparators are specified in terms of propagation delay of the response at the output to an input pulse crossing the threshold between two overload states. For this measurement, the rise time of the input pulse is negligible compared to the comparator propagation delay. In the case of the AD8214, this propagation delay is typically 100 ns, when the input signal is a fast step. The primary purpose of the AD8214 is to monitor for over- current conditions in a system. It is much more common that in such systems, the current in the path increases slowly; therefore, the transition between two input overload conditions around the threshold is slow relative to the propagation delay. In some cases, this transition can be so slow that the time from the actual threshold crossing to the output signal switching states is longer than the specified propagation delay, due to the comparator dynamics. If the voltage at the input of the AD8214 is crossing the set threshold at a rate ≤100 mV/µs, the output switches states before the threshold voltage has been exceeded by 15 mV. Therefore, if the input signal is changing so slowly that the propagation delay is affected, the error that accumulates at the input while waiting for the output response is proportionately smaller and, typically, less than 15 mV for ramp rates ≤100 mV/µs. |
同様の部品番号 - AD8214 |
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同様の説明 - AD8214 |
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