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AD824 Folha de dados(PDF) 11 Page - Analog Devices |
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AD824 Folha de dados(HTML) 11 Page - Analog Devices |
11 / 16 page REV. C AD824 –11– A current-limiting resistor should be used in series with the input of the AD824 if there is a possibility of the input voltage exceeding the positive supply by more than 300 mV or if an input voltage will be applied to the AD824 when ±VS = 0. The amplifier will be damaged if left in that condition for more than 10 seconds. A 1 k W resistor allows the amplifier to withstand up to 10 V of continuous overvoltage and increases the input volt- age noise by a negligible amount. Input voltages less than –VS are a completely different story. The amplifier can safely withstand input voltages 20 V below the minus supply voltage as long as the total voltage from the positive supply to the input terminal is less than 36 V. In addition, the input stage typically maintains picoamp level input currents across that input voltage range. OUTPUT CHARACTERISTICS The AD824’s unique bipolar rail-to-rail output stage swings within 15 mV of the positive and negative supply voltages. The AD824’s approximate output saturation resistance is 100 W for both sourcing and sinking. This can be used to estimate output saturation voltage when driving heavier current loads. For instance, the saturation voltage will be 0.5 V from either supply with a 5 mA current load. For load resistances over 20 k W, the AD824’s input error voltage is virtually unchanged until the output voltage is driven to 180 mV of either supply. If the AD824’s output is overdriven so as to saturate either of the output devices, the amplifier will recover within 2 ms of its input returning to the amplifier’s linear operating region. Direct capacitive loads will interact with the amplifier’s effective output impedance to form an additional pole in the amplifier’s feedback loop, which can cause excessive peaking on the pulse response or loss of stability. Worst case is when the amplifier is used as a unity gain follower. TPC 4 and 6 show the AD824’s pulse response as a unity gain follower driving 220 pF. Configu- rations with less loop gain, and as a result less loop bandwidth, will be much less sensitive to capacitance load effects. Noise gain is the inverse of the feedback attenuation factor provided by the feedback network in use. Figure 3 shows a method for extending capacitance load drive capability for a unity gain follower. With these component val- ues, the circuit will drive 5,000 pF with a 10% overshoot. VOUT VIN 1/4 AD824 –VS +VS 20k 20pF 0.01 F 0.01 F 8 4 100 CL Figure 3. Extending Unity Gain Follower Capacitive Load Capability Beyond 350 pF APPLICATION NOTES INPUT CHARACTERISTICS In the AD824, n-channel JFETs are used to provide a low offset, low noise, high impedance input stage. Minimum input common-mode voltage extends from 0.2 V below –VS to 1 V less than +VS. Driving the input voltage closer to the positive rail will cause a loss of amplifier bandwidth. The AD824 does not exhibit phase reversal for input voltages up to and including +VS. Figure 2a shows the response of an AD824 voltage follower to a 0 V to 5 V (+VS) square wave input. The input and output are superimposed. The output tracks the input up to +VS without phase reversal. The reduced bandwidth above a 4 V input causes the rounding of the output wave form. For input voltages greater than +VS, a resistor in series with the AD824’s noninverting input will prevent phase reversal at the expense of greater input voltage noise. This is illustrated in Figure 2b. 10 0% 100 90 1V 1V 10µs 1V 10 0% 100 90 1V 2µs 1V GND GND +VS +5V RP VOUT VIN Figure 2. (a) Response with RP = 0; VIN from 0 to +VS (b) VIN = 0 to + VS + 200 m V VOUT = 0 to + VS RP = 49.9 k W Since the input stage uses n-channel JFETs, input current during normal operation is positive; the current flows out from the input terminals. If the input voltage is driven more positive than +VS – 0.4 V, the input current will reverse direction as internal device junctions become forward biased. This is illustrated in TPC 8. (b) (a) |
Nº de peça semelhante - AD824 |
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Descrição semelhante - AD824 |
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