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AD8571 Datasheet(PDF) 19 Page - Analog Devices |
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AD8571 Datasheet(HTML) 19 Page - Analog Devices |
19 / 24 page AD8571/AD8572/AD8574 Rev. C | Page 19 of 24 CAPACITIVE LOAD DRIVE The optimum value for the resistor and capacitor is a function of the load capacitance and is best determined empirically since actual C The AD857x have excellent capacitive load driving capabilities and can safely drive up to 10 nF from a single 5 V supply. Although the device is stable, capacitive loading limits the bandwidth of the amplifier. Capacitive loads also increase the amount of overshoot and ringing at the output. An RC snubber network, shown in LOAD includes stray capacitances and can differ substantially from the nominal capacitive load. Table 5 shows some snubber network values that can be used as starting points. Table 5. Snubber Network Values for Driving Capacitive Loads Figure 59, can be used to compensate the amplifier against capacitive load ringing and overshoot. C (nF) Rx (Ω) Cx LOAD 1 200 1 nF 5V VOUT VIN 200mV p-p AD8571/ AD8572/ AD8574 CL 4.7nF Cx 0.47µF Rx 60Ω + – 4.7 60 0.47 μF 10 20 10 μF POWER-UP BEHAVIOR On power-up, the AD857x settle to a valid output within 5 μs. Figure 61 shows an oscilloscope photo of the output of the amplifier along with the power supply voltage. Figure 59. Snubber Network Configuration for Driving Capacitive Loads Figure 62 shows the test circuit. With the amplifier configured for unity gain, the device takes approximately 5 μs to settle to its final output voltage, hundreds of microseconds faster than many other autocorrection amplifiers. Although the snubber does not recover the loss of amplifier bandwidth from the load capacitance, it does allow the amplifier to drive larger values of capacitance while maintaining a minimum of overshoot and ringing. Figure 60 shows the output of an AD857x driving a 1 nF capacitor with and without a snubber network. 5µs 1V VOUT V+ 0V 0V BOTTOM TRACE = 2V/DIV TOP TRACE = 1V/DIV 10μs 100mV VS = 5V CLOAD = 4.7nF WITHOUT SNUBBER WITH SNUBBER Figure 61. AD857x Output Behavior on Power-Up Figure 60. Overshoot and Ringing Are Substantially Reduced Using a Snubber Network 100kΩ 100kΩ VSY = 0V TO 5V VOUT AD8571/ AD8572/ AD8574 Figure 62. AD857x Test Circuit for Turn-On Time |
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