Datasheet
2
OUTPUT
V
G
3
1
6
7
4
R
T
R
I
R
S
R
G
C
O
R
O
Z
O
Z
O
R
F
SIGNAL
INPUT
LMH6505
+
-
R
F
R
G
· K
-
A
VMAX
=
2
V
O
V
G
R
G
25:
LMH6505
3
1
6
7
4
R
F
V
IN
LMH6505
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SNOSAT4E –DECEMBER 2005–REVISED APRIL 2013
AVOIDING OVERDRIVE OF THE LMH6505 GAIN CONTROL INPUT
There is an additional requirement for the LMH6505 Gain Control Input (V
G
): V
G
must not exceed +2.3V (with
±5V supplies). The gain control circuitry may saturate and the gain may actually be reduced. In applications
where V
G
is being driven from a DAC, this can easily be addressed in the software. If there is a linear loop
driving V
G
, such as an AGC loop, other methods of limiting the input voltage should be implemented. One simple
solution is to place a 2.2:1 resistive divider on the V
G
input. If the device driving this divider is operating off of
±5V supplies as well, its output will not exceed 5V and through the divider V
G
can not exceed 2.3V.
IMPROVING THE LMH6505 LARGE SIGNAL PERFORMANCE
Figure 45 illustrates an inverting gain scheme for the LMH6505.
Figure 45. Inverting Amplifier
The input signal is applied through the R
G
resistor. The V
IN
pin should be grounded through a 25Ω resistor. The
maximum gain range of this configuration is given in the following equation:
(5)
The inverting slew rate of the LMH6505 is much higher than that of the non-inverting slew rate. This ≈ 2X
performance improvement comes about because in the non-inverting configuration the slew rate of the overall
amplifier is limited by the input buffer. In the inverting circuit, the input buffer remains at a fixed voltage and does
not affect slew rate.
TRANSMISSION LINE MATCHING
One method for matching the characteristic impedance of a transmission line is to place the appropriate resistor
at the input or output of the amplifier. Figure 46 shows a typical circuit configuration for matching transmission
lines.
Figure 46. Transmission Line Matching
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