LMH6504 datasheet(12/19 Pages) NSC

12 / 19 page

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

•

Broad signal and gain control bandwidths

•

Frequency response may be adjusted with R

•

High impedance signal and gain control inputs

Refer to Figure 1 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, AVMAX,of

the LMH6504 is defined by the ratio:K·R

F /RG 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 maxi-

mum set gain.

SETTING THE LMH6504 MAXIMUM GAIN

Eq. 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:

G: determines the input voltage range

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 IRG_MAX spec. This

sets the maximum input voltage:

Eq. 2

As the I

RG_MAX limit is approached (with increasing input

voltage or with lowering of R

G), the device harmonic distor-

tion will increase. Changes in R

F will have a dramatic effect

on the small signal bandwidth. The output amplifier of the

LMH6504 is a current feedback amplifier (CFA) and its band-

width is determined by R

F. As with any CFA, doubling the

feedback resistor will roughly cut the bandwidth of the device

in half. For more about CFA’s, see the basic tutorial, OA-20,

“Current Feedback Myths Debunked”, or a more rigorous

analysis, OA-13, “Current Feedback Amplifier Loop Gain

Analysis and Performance Enhancements”.

OTHER CONFIGURATIONS

1) Single Supply Operation

The LMH6504 can be configured for use in a single supply

environment. Doing so requires the following:

a) Bias pin 4 and R

G to a “virtual half supply” somewhere

close to the middle of V

+ and V- range. The other end of

G is tied to pin 3. The “virtual half supply” needs to be

capable of sinking and sourcing the expected current flow

through R

b) Ensure that V

G can be adjusted from 0V to 2V above the

“virtual half supply”.

c) Bias the input (pin 2) to make sure that it stays within the

range of 1.8V above V

- to 1.8V below V+ (see “Input

voltage Range” specification in the Electrical Character-

istics table). This can be accomplished by either DC

biasing the input and AC coupling the input signal, or

alternatively, by direct coupling if the output of the driving

stage is also biased to half supply.

Arranged this way, the LMH6504 will respond to the current

flowing through R

G. The gain control relationship will be

similar to the split supply arrangement with V

G measured

referenced to pin 4. Keep in mind that the circuit described

above will also center the output voltage to the “virtual half

supply voltage”.

2) Arbitrarily Referenced Input Signal

Having a wide input voltage range on the input (pin 2)

(+/-3.2V typical), the LMH6504 can be configured to control

the gain on signals which are not referenced to ground (e.g.

Half Supply biased circuits, etc.). We will call this node the

“reference node”. In such cases, the other end of R

G (the

side not tied to pin 3) can be tied to this reference node so

that R

G will “look at” the difference between the signal and

this reference only. Keep in mind that the reference node

needs to source and sink the current flowing through R

GAIN ACCURACY

Gain accuracy is defined as the actual gain compared

against the theoretical gain at a certain V

G (results ex-

pressed in dB) (See Figure 2).

Theoretical gain is given by:

20084347

FIGURE 1. LMH6504 Typical Application and Block

Diagram

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