AD811
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.
APPLICATIONS
General Design Considerations
The AD811 is a current feedback amplifier optimized for use in
high performance video and data acquisition applications. Since
it uses a current feedback architecture, its closed-loop –3 dB
bandwidth is dependent on the magnitude of the feedback resis-
tor. The desired closed-loop gain and bandwidth are obtained
by varying the feedback resistor (RFB) to tune the bandwidth,
and varying the gain resistor (RG) to get the correct gain. Table I
contains recommended resistor values for a variety of useful
closed-loop gains and supply voltages.
Choice of Feedback and Gain Resistors
Because of the above-mentioned relationship between the 3 dB
bandwidth and the feedback resistor, the fine scale gain flatness
will, to some extent, vary with feedback resistor tolerance. It is,
therefore, 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 characteriza-
tion for this data sheet. It is possible that values other than those
indicated will be optimal for other resistor types.
Table I. –3 dB Bandwidth vs. Closed-Loop Gain and
Resistance Values
VS = ؎15 V
Closed-Loop
Gain
Printed Circuit Board Layout Considerations
–3 dB BW
(MHz)
As to be 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 to be 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.
RFB
RG
+1
+2
+10
–1
–10
750 Ω
649 Ω
511 Ω
590 Ω
511 Ω
140
120
100
115
95
649 Ω
56.2 Ω
590 Ω
51.1 Ω
VS = ؎5 V
Closed-Loop
Gain
–3 dB BW
(MHz)
RFB
RG
Quality of Coaxial Cable
+1
+2
+10
–1
619 Ω
562 Ω
442 Ω
562 Ω
442 Ω
80
80
65
75
65
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 was ideal,
then the resulting flatness would not be affected by the length of
the cable. While outstanding results can be achieved using inex-
pensive cables, it should be noted that some variation in flatness
due to varying cable lengths may be experienced.
562 Ω
48.7 Ω
562 Ω
44.2 Ω
–10
VS = ؎10 V
Closed-Loop
Gain
–3 dB BW
(MHz)
RFB
RG
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) will be required to provide the best
settling time and lowest distortion. Although the recommended
0.1 µF power supply bypass capacitors will be sufficient in many
applications, more elaborate bypassing (such as using two paral-
leled capacitors) may be required in some cases.
+1
+2
+10
–1
–10
649 Ω
590 Ω
499 Ω
590 Ω
499 Ω
105
105
80
105
80
590 Ω
49.9 Ω
590 Ω
49.9 Ω
Figures 11 and 12 illustrate the relationship between the feed-
back resistor and the frequency and time domain response char-
acteristics for a closed-loop gain of +2. (The response at other
gains will be similar.)
The 3 dB bandwidth is somewhat dependent on the power supply
voltage. As the supply voltage is decreased for example, the
magnitude of 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.
REV. D
–9–
ADI [ ADI ]
