AD811 Data Sheet
APPLICATIONS INFORMATION
GENERAL DESIGN CONSIDERATIONS
The AD811 is a current feedback amplifier optimized for use in
high performance video and data acquisition applications.
Because it uses a current feedback architecture, its closed-loop
−3 dB bandwidth is dependent on the magnitude of the
feedback resistor. The desired closed-loop gain and bandwidth
are obtained by varying the feedback resistor (R
FB
) to tune the
bandwidth and by varying the gain resistor (R
G
) to obtain the
correct gain. Table 3 contains recommended resistor values for
a variety of useful closed-loop gains and supply voltages.
Table 3. −3 dB Bandwidth vs. Closed-Loop Gain and
Resistance Values
V
S
= ±15 V
Closed-Loop Gain R
FB
R
G
−3 dB BW (MHz)
+1 750 Ω 140
+2 649 Ω 649 Ω 120
+10 511 Ω 56.2 Ω 100
−1
590 Ω 590 Ω 115
−10 511 Ω 51.1 Ω 95
V
S
= ±5 V
Closed-Loop Gain R
FB
R
G
−3 dB BW (MHz)
+1 619 Ω 80
+2 562 Ω 562 Ω 80
+10 442 Ω 48.7 Ω 65
−1
562 Ω 562 Ω 75
−10
442 Ω 44.2 Ω 65
V
S
= ±10 V
Closed-Loop Gain R
FB
R
G
−3 dB BW (MHz)
+1 649 Ω 105
+2 590 Ω 590 Ω 105
+10 499 Ω 49.9 Ω 80
−1 590 Ω 590 Ω 105
−10 499 Ω 49.9 Ω 80
Figure 17 and Figure 18 illustrate the relationship between the
feedback resistor and the frequency and time domain response
characteristics for a closed-loop gain of +2. (The response at
other gains is similar.)
The 3 dB bandwidth is somewhat dependent on the power
supply voltage. As the supply voltage is decreased, for example,
the magnitude of the internal junction capacitances is increased,
causing a reduction in closed-loop bandwidth. To compensate
for this, smaller values of feedback resistor are used at lower
supply voltages.
ACHIEVING THE FLATTEST GAIN RESPONSE AT
HIGH FREQUENCY
Achieving and maintaining gain flatness of better than 0.1 dB at
frequencies above 10 MHz requires careful consideration of
several issues.
Choice of Feedback and Gain Resistors
Because of the previously mentioned relationship between the
3 dB bandwidth and the feedback resistor, the fine scale gain
flatness varies, to some extent, with feedback resistor tolerance.
Therefore, it is recommended that resistors with a 1% tolerance
be used if it is desired to maintain flatness over a wide range of
production lots. In addition, resistors of different construction
have different associated parasitic capacitance and inductance.
Metal film resistors were used for the bulk of the character-
ization for this data sheet. It is possible that values other than
those indicated are optimal for other resistor types.
Printed Circuit Board Layout Considerations
As is expected for a wideband amplifier, PC board parasitics can
affect the overall closed-loop performance. Of concern are stray
capacitances at the output and the inverting input nodes. If a
ground plane is used on the same side of the board as the signal
traces, a space (3/16" is plenty) should be left around the signal
lines to minimize coupling. Additionally, signal lines connecting
the feedback and gain resistors should be short enough so that
their associated inductance does not cause high frequency gain
errors. Line lengths less than 1/4" are recommended.
Quality of Coaxial Cable
Optimum flatness when driving a coax cable is possible only
when the driven cable is terminated at each end with a resistor
matching its characteristic impedance. If the coax is ideal, then
the resulting flatness is not affected by the length of the cable.
While outstanding results can be achieved using inexpensive
cables, note that some variation in flatness due to varying cable
lengths may occur.
Power Supply Bypassing
Adequate power supply bypassing can be critical when optimiz-
ing the performance of a high frequency circuit. Inductance in
the power supply leads can form resonant circuits that produce
peaking in the amplifier’s response. In addition, if large current
transients must be delivered to the load, then bypass capacitors
(typically greater than 1 µF) are required to provide the best
settling time and lowest distortion. Although the recommended
0.1 µF power supply bypass capacitors are sufficient in many
applications, more elaborate bypassing (such as using two
paralleled capacitors) may be required in some cases.
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