VIN (MAX) = IR

G

MAX

· RG

AVMAX =

RF

RG

· K

RX

5V

0.1 µF

6.8 µF

OUTPUT

6.8 µF

0.1 µF

I-

-VCC

VG

GND

INPUT

SIGNAL

VIN

RG

VO

+VCC

RF

RO

RIN

RG

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 AVMAX down to complete cutoff)

•

Bandwidth independent, resistor programmable gain range (RG)

•

Broad signal and gain control bandwidths

•

Frequency response may be adjusted with RF

•

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, RG. 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, RF. The

maximum gain, AVMAX, of the LMH6504 is defined by the ratio: K · RF / RG where “K” is the gain multiplier with a

nominal value of 0.965. As the gain control input (VG) 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 AVMAX = 9.7V/V, the recommended AVMAX varies between 2 and 100.

Higher gains are possible but usually impractical due to output offsets, noise and distortion. When varying AVMAX

several tradeoffs are made:

RG: determines the input voltage range

RF: determines overall bandwidth

The amount of current which the input buffer can source/sink into RG is limited and is specified in the IRG_MAX

spec. This sets the maximum input voltage:

(2)

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