LMH6505 datasheet(12/20 Pages) NSC

12 / 20 page

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

•

Broad signal and gain control bandwidths

•

Frequency response may be adjusted with R

•

High impedance signal and gain control inputs

The LMH6505 combines a closed loop input buffer (“X1”

Block in Figure 1), 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 LMH6505 is

defined by the ratio:K·R

F/RG where “K” is the gain multiplier

with a nominal value of 0.940. As the gain control input (V

changes over its 0 to 2V range, the gain is adjusted over a

range of about 80 dB relative to the maximum set gain.

SETTING THE LMH6505 MAXIMUM GAIN

Eq. 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:

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 given in the IRG_MAX specification.

This sets the maximum input voltage:

Eq. 2

As the I

RG_MAX limit is approached with increasing the input

voltage or with the lowering of R

G, the device’s harmonic

distortion will increase. Changes in R

F will have a dramatic

effect on the small signal bandwidth. The output amplifier of

the LMH6505 is a current feedback amplifier (CFA) and its

bandwidth 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 LMH6505 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 2V above V

− to 2V below V+. See the Input

Voltage Range specification in the Electrical Characteris-

tics table. This can be accomplished by either DC biasing

the input and AC coupling the input signal, or alterna-

tively, by direct coupling if the output of the driving stage

is also biased to half supply.

Arranged this way, the LMH6505 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

with reference to pin 4. Keep in mind that the circuit de-

scribed 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)

(±3V typical), the LMH6505 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 “ref-

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

G which is

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

20171047

FIGURE 1. LMH6505 Typical Application and Block

Diagram

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