LMH6504 Datasheet, PDF (13/29 Pages) National Semiconductor (TI) – Wideband, Low Power, Variable Gain Amplifier

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.
INPUT
SIGNAL
GAIN
CONTROL
V_G
RX
V_IN
50:
R_G
R_IN
50:
GND
R_G
100:
5V ^{+ -}
6.8 µF
MULT
X1
+V_CC 0.1 µF
R_F
I-
1 k:
V_O
R_O OUTPUT
-
CFA
+
-V_CC
0.1 µF
50:
-
6.8 µF
+
-5V
Figure 42. LMH6504 Typical Application and Block Diagram
SETTING THE LMH6504 MAXIMUM GAIN
R_F
A_VMAX = _RG · K
(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:
V_IN (MAX) = I_{RG MAX} · R_G
(2)
Copyright © 2003–2013, Texas Instruments Incorporated
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