Datasheet AD8072, AD8073 (Analog Devices) - 9

AD8072/AD8073 APPLICATIONS
On the other hand, the bandwidth of a current feedback ampli-
Overdrive Recovery
fier can be decreased by increasing the feedback resistance. This Overdrive of an amplifier occurs when the output and/or input can sometimes be useful where it is desired to reduce the noise range are exceeded. The amplifier must recover from this overdrive bandwidth of a system. As a practical matter, the maximum condition and resume normal operation. As shown in Figure 4, value of feedback resistor was found to be 2 kΩ. Figure 5 shows the AD8072 and AD8073 recover within 75 ns from positive the frequency response of an AD8072/AD8073 at a gain of two overdrive and 30 ns from negative overdrive. 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
VIN
due to the pole formed by the op amp’s output impedance and
V
the capacitor can cause peaking or even oscillation. The top trace
OUT
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
1V 25ns
of values to experiment with for capacitive loads of up to a few hundred pF. Figure 4. Overload Recovery; VS = ±5 V, VIN = 8 V p-p, RF = 1 kΩ, RL = 150 Ω, G = +2
RS = 0
Ω
Bandwidth vs. Feedback Resistor Value RS = 25
Ω 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
50mV
response of an AD8072/AD8073 at a gain of two with both feed-
20ns
back and gain resistors equal to 649 Ω. Figure 6. Capacitive Low Drive
6.1 7 1k
⍀
1k
⍀
6.0 6 R 5.9 S R 5 F = 649
⍀
dB dB – – VIN = 100mV p-p C R L L 5.8 4 50pF 1k
⍀
0.1 dB 50
⍀
DIV 5.7 3 VS =
ⴞ
5V A
Figure 7. Capacitive Load Drive Circuit
V = 2 GAIN FLATNESS 5.6 1 dB 2 RL = 150
⍀
DIV CLOSED-LOOP GAIN VO = 0.2V p-p 5.5 1 RF = 2k
⍀
5.4 0 0.1 1 10 100 500 FREQUENCY – MHz
Figure 5. Frequency Response vs. RF REV. D –9–