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AD8023 Fiches technique(PDF) 9 Page - Analog Devices |
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AD8023 Fiches technique(HTML) 9 Page - Analog Devices |
9 / 11 page –9– REV. A AD8023 ACL G 1 + SC T ( RF + Gn rin ) where: CT = transcapacitance 1 pF RF = feedback resistor G = ideal closed loop gain Gn = 1 + R F R G = noise gain rin = inverting input resistance 150 Ω ACL = closed loop gain The –3 dB bandwidth is determined from this model as: f3 1 2 π C T ( RF + Gn rin ) This model will predict –3 dB bandwidth to within about 10% to 15% of the correct value when the load is 150 Ω and VS = ±7.5 V. For lower supply voltages there will be a slight decrease in bandwidth. The model is not accurate enough to predict either the phase behavior or the frequency response peaking of the AD8023. It should be noted that the bandwidth is affected by attenuation due to the finite input resistance. Also, the open-loop output resistance of about 6 Ω reduces the bandwidth somewhat when driving load resistors less than about 150 Ω. (Bandwidths will be about 10% greater for load resistances above a couple hundred ohms.) Table I. –3 dB Bandwidth vs. Closed-Loop Gain and Feedback Resistor, RL = 150 (SOIC) VS – Volts Gain RF – Ohms BW – MHz ±7.5 +1 2000 460 +2 750 240 +10 300 50 –1 750 150 –10 250 60 ±2.5 +1 2000 250 +2 1000 90 +10 300 30 –1 750 95 –10 250 50 Driving Capacitive Loads When used in combination with the appropriate feedback resistor, the AD8023 will drive any load capacitance without oscillation. The general rule for current feedback amplifiers is that the higher the load capacitance, the higher the feedback resistor required for stable operation. Due to the high open-loop transresistance and low inverting input current of the AD8023, the use of a large feedback resistor does not result in large closed- loop gain errors. Additionally, its high output short circuit current makes possible rapid voltage slewing on large load capacitors. For the best combination of wide bandwidth and clean pulse response, a small output series resistor is also recommended. Table II contains values of feedback and series resistors which result in the best pulse responses. Figure 28 shows the AD8023 driving a 300 pF capacitor through a large voltage step with virtually no overshoot. (In this case, the large and small signal pulse responses are quite similar in appearance.) FREQUENCY – MHz +1 –8 1 500 10 100 –4 –5 –6 –7 –2 –3 0 –1 GAIN PHASE G = –10 RL = 150 VS = 7.5V VS = 2.5V VS = 2.5V –9 0 –90 –180 Figure 30. Closed-Loop Gain and Phase vs. Frequency, G = –10, RL = 150 Ω General The AD8023 is a wide bandwidth, triple video amplifier that offers a high level of performance on less than 9.0 mA per amplifier of quiescent supply current. The AD8023 achieves bandwidth in excess of 200 MHz, with low differential gain and phase errors and high output current making it an efficient video amplifier. The AD8023’s wide phase margin coupled with a high output short circuit current make it an excellent choice when driving any capacitive load up to 300 pF. It is designed to offer outstanding functionality and performance at closed-loop inverting or noninverting gains of one or greater. Choice of Feedback and Gain Resistors Because it is a current feedback amplifier, the closed-loop band- width of the AD8023 may be customized using different values of the feedback resistor. Table I shows typical bandwidths at different supply voltages for some useful closed-loop gains when driving a load of 150 Ω. The choice of feedback resistor is not critical unless it is desired to maintain the widest, flattest frequency response. The resistors recommended in the table (chip resistors) are those that will result in the widest 0.1 dB bandwidth without peaking. In applications requiring the best control of bandwidth, 1% resistors are adequate. Resistor values and widest bandwidth figures are shown. Wider bandwidths than those in the table can be attained by reducing the magnitude of the feedback resistor (at the expense of increased peaking), while peaking can be reduced by increasing the magnitude of the feedback resistor. Increasing the feedback resistor is especially useful when driving large capacitive loads as it will increase the phase margin of the closed-loop circuit. (Refer to the Driving Capacitive Loads section for more information.) To estimate the –3 dB bandwidth for closed-loop gains of 2 or greater, for feedback resistors not listed in the following table, the following single pole model for the AD8023 may be used: |
Numéro de pièce similaire - AD8023 |
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Description similaire - AD8023 |
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