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AD8644AR Datenblatt(PDF) 9 Page - Analog Devices |
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AD8644AR Datenblatt(HTML) 9 Page - Analog Devices |
9 / 16 page AD8614/AD8644 Rev. B | Page 9 of 16 THEORY OF OPERATION The AD8614/AD8644 are processed using Analog Devices high voltage, extra fast complementary bipolar (HV XFCB) process. This process includes trench-isolated transistors that lower parasitic capacitance. Figure 26 shows a simplified schematic of the AD8614/AD8644. The input stage is rail-to-rail, consisting of two complementary differential pairs, one NPN pair and one PNP pair. The input stage is protected against avalanche breakdown by two back-to- back diodes. Each input has a 1.5 kΩ resistor that limits input current during overvoltage events and furnishes phase reversal protection if the inputs are exceeded. The two differential pairs are connected to a double-folded cascode. This is the stage in the amplifier with the most gain. The double-folded cascode differentially feeds the output stage circuitry. Two complemen- tary common emitter transistors are used as the output stage. This allows the output to swing to within 125 mV from each rail with a 10 mA load. The gain of the output stage, and thus the open-loop gain of the op amp, depends on the load resistance. The AD8614/AD8644 have no built-in short-circuit protection. The short-circuit limit is a function of high current roll-off of the output stage transistors and the voltage drop over the resistor shown on the schematic at the output stage. The voltage over this resistor is clamped to one diode during short-circuit voltage events. OUTPUT SHORT-CIRCUIT PROTECTION To achieve a wide bandwidth and high slew rate, the output of the AD8614/AD8644 is not short-circuit protected. Shorting the output directly to ground or to a supply rail can destroy the device. The typical maximum safe output current is 70 mA. In applications where some output current protection is needed, but not at the expense of reduced output voltage headroom, a low value resistor in series with the output can be used. This is shown in Figure 25. The resistor is connected within the feedback loop of the amplifier so that if VOUT is shorted to ground and VIN swings up to 18 V, the output current does not exceed 70 mA. For 18 V single-supply applications, resistors less than 261 Ω are not recommended. AD86x4 VIN 261Ω VOUT 18V Figure 25. Output Short-Circuit Protection VCC – + 1.5kΩ VEE VOUT VCC VCC 1.5kΩ Figure 26. Simplified Schematic |
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