Datasheet
V
IN
(MAX) = I
R
G
MAX
· R
G
A
VMAX
=
R
F
R
G
· K
RX
5V
0.1 µF
6.8 µF
OUTPUT
6.8 µF
0.1 µF
I-
-V
CC
V
G
GND
INPUT
SIGNAL
V
IN
R
G
V
O
+V
CC
R
F
R
O
R
IN
R
G
100:
50:
50:
50:
-5V
1 k:
X1
CFA
+
-
GAIN
CONTROL
MULT
+
-
+
-
LMH6505
www.ti.com
SNOSAT4E –DECEMBER 2005–REVISED APRIL 2013
APPLICATION INFORMATION
GENERAL DESCRIPTION
The key features of the LMH6505 are:
• Low power
• Broad voltage controlled gain and attenuation range (from A
VMAX
down to complete cutoff)
• Bandwidth independent, resistor programmable gain range (R
G
)
• Broad signal and gain control bandwidths
• Frequency response may be adjusted with R
F
• High impedance signal and gain control inputs
The LMH6505 combines a closed loop input buffer (“X1” Block in Figure 41), a voltage controlled variable gain
cell (“MULT” Block) and an output amplifier (“CFA” Block). The input buffer is a transconductance stage whose
gain is set by the gain setting resistor, R
G
. The output amplifier is a current feedback op amp and is configured
as a transimpedance stage whose gain is set by, and is equal to, the feedback resistor, R
F
. The maximum gain,
A
VMAX
, of the LMH6505 is defined by the ratio: K · R
F
/R
G
where “K” is the gain multiplier with a nominal value of
0.940. As the gain control input (V
G
) changes over its 0 to 2V range, the gain is adjusted over a range of about
80 dB relative to the maximum set gain.
Figure 41. LMH6505 Typical Application and Block Diagram
SETTING THE LMH6505 MAXIMUM GAIN
(1)
Although the LMH6505 is specified at A
VMAX
= 9.4 V/V, the recommended A
VMAX
varies between 2 and 100.
Higher gains are possible but usually impractical due to output offsets, noise and distortion. When varying A
VMAX
several tradeoffs are made:
R
G
: determines the input voltage range
R
F
: determines overall bandwidth
The amount of current which the input buffer can source/sink into R
G
is limited and is given in the I
RG_MAX
specification. This sets the maximum input voltage:
(2)
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