LMH6611_14 Datasheet, PDF (22/42 Pages) National Semiconductor (TI) – LMH6611/LMH6612 Single Supply 345 MHz Rail-to-Rail Output Amplifiers

LMH6611, LMH6612
SNOSB00K – NOVEMBER 2007 – REVISED OCTOBER 2013
R_S
50:
INVERTING
INPUT
D4
D1
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D3
D2
NON-INVERTING
INPUT
Figure 67. Input Equivalent Circuit During Shutdown
When the LMH6611 is shutdown, there may be current flow through the internal diodes shown, caused by input
potential, if present. This current may flow through the external feedback resistor and result in an apparent output
signal. In most shutdown applications the presence of this output is inconsequential. However, if the output is
“forced” by another device, the other device will need to conduct the current described in order to maintain the
output potential.
To keep the output at or near ground during shutdown when there is no other device to hold the output low, a
switch using a transistor can be used to shunt the output to ground.
SELECTION OF R_F AND EFFECT ON STABILITY AND PEAKING
The peaking of the LMH6611 depends on the value of the R_F. From the graph shown in Figure 68, as the R_F
value increases, the peaking increases.
For A_V = 2, at R_F = 1 kΩ, the −3 dB bandwidth is 113 MHz and peaking is about 0.6 dB whereas at R_F = 665Ω,
the −3 dB bandwidth is about 110 MHz and peaking is 0 dB. R_F and the input capacitance form a pole in the
amplifier’s response. If the time constant is too big, it will cause peaking and ringing.
Except for A_V = 1 when R_F should be 0Ω, across all other gain settings it is recommended that R_F remain
between 500Ω and 1 kΩ to ensure optimum performance.
3
R_F = R_G = 1000:
0
R_F = R_G = 665:
-3 _V⁺ _{= +2.5V}
V^- = -2.5V
V_OUT = 0.2V
R_L = 1 k:
-6
1
10
100
FREQUENCY (MHz)
1000
Figure 68. Closed Loop Gain vs. Frequency and R_F = R_G
22
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