AD8072JR-REEL 查看數據表（PDF） - Analog Devices

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AD8072JR-REEL

Low Cost, Dual/Triple Video Amplifiers

Analog Devices 

AD8072/AD8073

APPLICATIONS

Overdrive Recovery

Overdrive of an amplifier occurs when the output and/or input

range are exceeded. The amplifier must recover from this overdrive

condition and resume normal operation. As shown in Figure 4,

the AD8072 and AD8073 recover within 75 ns from positive

overdrive and 30 ns from negative overdrive.

VIN

VOUT

1V

25ns

Figure 4. Overload Recovery; VS = ±5 V, VIN = 8 V p-p,

RF = 1 kΩ, RL = 150 Ω, G = +2

Bandwidth vs. Feedback Resistor Value

The closed-loop frequency response of a current feedback amplifier

is a function of the feedback resistor. A smaller feedback resistor

will produce a wider bandwidth response. However, if the feed-

back resistance becomes too small, the gain flatness can be

affected. As a practical consideration, the minimum value of

feedback resistance for the AD8072/AD8073 was found to be

649 Ω. For resistances below this value, the gain flatness will be

affected and more significant lot-to-lot variations in device per-

formance will be noticed. Figure 5 shows a plot of the frequency

response of an AD8072/AD8073 at a gain of two with both feed-

back and gain resistors equal to 649 Ω.

6.1

7

6.0

6

5.9

RF = 649⍀ 5

5.8

5.7

VS = ؎5V

AV = 2

5.6 RL = 150⍀

VO = 0.2V p-p

5.5

0.1 dB

DIV

4

3

1 dB

2

DIV

1

5.4

0.1

RF = 2k⍀

1

10

100

FREQUENCY – MHz

0

500

Figure 5. Frequency Response vs. RF

On the other hand, the bandwidth of a current feedback ampli-

fier can be decreased by increasing the feedback resistance. This

can sometimes be useful where it is desired to reduce the noise

bandwidth of a system. As a practical matter, the maximum

value of feedback resistor was found to be 2 kΩ. Figure 5 shows

the frequency response of an AD8072/AD8073 at a gain of two

with both feedback and gain resistors equal to 2 kΩ.

Capacitive Load Drive

When an op amp output drives a capacitive load, extra phase shift

due to the pole formed by the op amp’s output impedance and

the capacitor can cause peaking or even oscillation. The top trace

of Figure 6, RS = 0 Ω, shows the output of one of the amplifiers of

the AD8072/AD8073 when driving a 50 pF capacitor as shown in

the schematic of Figure 7.

The amount of peaking can be significantly reduced by adding

a resistor in series with the capacitor. The lower trace of Figure 6

shows the same capacitor being driven with a 25 Ω resistor

in series with it. In general, the resistor value will have to be

experimentally determined, but 10 Ω to 50 Ω is a practical range

of values to experiment with for capacitive loads of up to a few

hundred pF.

RS = 0Ω

RS = 25Ω

50mV

20ns

Figure 6. Capacitive Low Drive

1k⍀

1k⍀

VIN = 100mV p-p

50⍀

RS

CL

50pF

RL

1k⍀

Figure 7. Capacitive Load Drive Circuit

REV. D

–9–

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