Low-Cost, High-Speed, Single-Supply, Gain of +2 Buffers with Rail-to-Rail Outputs in SOT23 MAX4014 MAX4017 500Ω 500Ω MAX4019 MAX4022 IN+ RISO OUT VOUT MAX40_ _ VIN CL RTIN 50Ω IN- 500Ω 500Ω Figure 5. Input Protection Circuit Figure 7. Driving a Capacitive Load through an Isolation Resistor conditions, the input protection diodes will be forward biased, lowering the disabled output resistance to 500Ω. 6 Output Capacitive Loading and Stability 5 CL = 15pF The MAX4014/MAX4017/MAX4019/MAX4022 provide 4 maximum AC performance with no load capacitance. 3 This is the case when the load is a properly terminated 2 CL = 10pF transmission line. However, they are designed to drive up 25pF of load capacitance without oscillating, but 1 GIAN (dB) with reduced AC performance. 0 Driving large capacitive loads increases the chance of -1 CL = 5pF oscillations occurring in most amplifier circuits. This is -2 especially true for circuits with high loop gains, such as -3 voltage followers. The buffer’s output resistance and -4 the load capacitor combine to add a pole and excess 100k 1M 10M 100M 1G phase to the loop response. If the frequency of this MAX4014/MAX4017/MAX4019/MAX4022 FREQUENCY (Hz) pole is low enough to interfere with the loop response and degrade phase margin sufficiently, oscillations can occur. Figure 6. Small-Signal Gain vs. Frequency with Load Capacitance and No Isolation Resistor A second problem when driving capacitive loads results from the amplifier’s output impedance, which Disabled Output Resistance looks inductive at high frequencies. This inductance forms an L-C resonant circuit with the capacitive load, The MAX4014/MAX4017/MAX4019/MAX4022 include which causes peaking in the frequency response and internal protection circuitry that prevents damage to the degrades the amplifier’s gain margin. precision input stage from large differential input volt- ages, as shown in Figure 5. This protection circuitry con- Figure 6 shows the frequency response of the MAX4014/ sists of five back-to-back Schottky diodes between IN_+ MAX4017/MAX4019/MAX4022 under different capacitive and IN_-. These diodes lower the disabled output resis- loads. To drive loads with greater than 25pF of capaci- tance from 1kΩ to 500Ω when the output voltage is 3V tance or to settle out some of the peaking, the output greater or less than the voltage at IN_+. Under these requires an isolation resistor like the one shown in 10______________________________________________________________________________________
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