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
+
-
+
-
LMH6504
www.ti.com
SNOSA96D –NOVEMBER 2003–REVISED MARCH 2013
APPLICATION INFORMATION
GENERAL DESCRIPTION
The key features of the LMH6504 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
Refer to Figure 42 below. The LMH6504 combines a closed loop input buffer (“X1” Block), 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 LMH6504 is defined by the ratio: K · R
F
/ R
G
where “K” is the gain multiplier with a
nominal value of 0.965. 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 42. LMH6504 Typical Application and Block Diagram
SETTING THE LMH6504 MAXIMUM GAIN
(1)
Although the LMH6504 is specified at A
VMAX
= 9.7V/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 specified in the I
RG_MAX
spec. This sets the maximum input voltage:
(2)
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