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AD8618AR-REEL 데이터시트(PDF) 11 Page - Analog Devices |
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AD8618AR-REEL 데이터시트(HTML) 11 Page - Analog Devices |
11 / 20 page AD8615/AD8616/AD8618 Rev. E | Page 11 of 20 APPLICATIONS INFORMATION INPUT OVERVOLTAGE PROTECTION The AD8615/AD8616/AD8618 have internal protective circuitry that allows voltages exceeding the supply to be applied at the input. It is recommended, however, not to apply voltages that exceed the supplies by more than 1.5 V at either input of the amplifier. If a higher input voltage is applied, series resistors should be used to limit the current flowing into the inputs. The input current should be limited to <5 mA. The extremely low input bias current allows the use of larger resistors, which allows the user to apply higher voltages at the inputs. The use of these resistors adds thermal noise, which contributes to the overall output voltage noise of the amplifier. For example, a 10 kΩ resistor has less than 13 nV/√Hz of thermal noise and less than 10 nV of error voltage at room temperature. OUTPUT PHASE REVERSAL The AD8615/AD8616/AD8618 are immune to phase inversion, a phenomenon that occurs when the voltage applied at the input of the amplifier exceeds the maximum input common mode. Phase reversal can cause permanent damage to the amplifier and can create lock ups in systems with feedback loops. TIME (2ms/DIV) VIN VOUT VS = ±2.5V VIN = 6V p-p AV = 1 RL = 10kΩ Figure 36. No Phase Reversal DRIVING CAPACITIVE LOADS Although the AD8615/AD8616/AD8618 are capable of driving capacitive loads of up to 500 pF without oscillating, a large amount of overshoot is present when operating at frequencies above 100 kHz. This is especially true when the amplifier is configured in positive unity gain (worst case). When such large capacitive loads are required, the use of external compensation is highly recommended. This reduces the overshoot and minimizes ringing, which in turn improves the frequency response of the AD8615/AD8616/ AD8618. One simple technique for compensation is the snubber, which consists of a simple RC network. With this circuit in place, output swing is maintained and the amplifier is stable at all gains. Figure 38 shows the implementation of the snubber, which reduces overshoot by more than 30% and eliminates ringing that can cause instability. Using the snubber does not recover the loss of bandwidth incurred from a heavy capacitive load. TIME (2µs/DIV) VS = ±2.5V AV = 1 CL = 500pF Figure 37. Driving Heavy Capacitive Loads Without Compensation V+ 200Ω 500pF 500pF V– VEE VCC 200mV – + – Figure 38. Snubber Network TIME (10µs/DIV) VS =±2.5V AV = 1 RS = 200Ω CS = 500pF CL = 500pF Figure 39. Driving Heavy Capacitive Loads Using the Snubber Network |
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