LMH6504 datasheet(13/19 Pages) NSC

13 / 19 page

Application Information (Continued)

Eq. 3

Where K = 0.965 (nominal) N = 0.96V & V

C = 80mV @ room

temperature

ForaV

G range, the value specified in the tables represents

the worst case accuracy over the entire range. The "Typical"

value would be the worst case difference between the "Typi-

cal gain" and the "Theoretical gain". The "Max" value would

be the worst case difference between the actual gain and the

"Theoretical gain" for the entire population.

GAIN MATCHING

Gain matching as the limit on gain variation at a certain V

(expressed in dB) (see Figure 2) and is specified as "Max"

only (no "Typical"). For a V

G range, the value specified

represents the worst case matching over the entire range.

The "Max" value would be the worst case difference between

the actual gain and the typical gain for the entire population.

GAIN PARTITIONING

If high levels of gain are needed, gain partitioning should be

considered:

The maximum gain range for this circuit is given by the

following equation:

Eq. 4

The LMH6624 is a low noise wideband voltage feedback

amplifier. Setting R

2 at 909

Ω and R

1 at 100

Ω produces a

gain of 20 dB. Setting R

F at 1000

Ω as recommended and R

at 50

Ω, produces a gain of about 26 dB in the LMH6504. The

total gain of this circuit is therefore approximately 46 dB. It is

important to understand that when partitioning to obtain high

levels of gain, very small signal levels will drive the amplifiers

to full scale output. For example, with 46 dB of gain, a 20 mV

signal at the input will drive the output of the LMH6624 to 200

mV, the output of the LMH6504 to 4V. Accordingly, the

designer must carefully consider the contributions of each

stage to the overall characteristics. Through gain partitioning

the designer is provided with an opportunity to optimize the

frequency response, noise, distortion, settling time, and

loading effects of each amplifier to achieve improved overall

performance.

LMH6504 GAIN CONTROL RANGE AND MINIMUM GAIN

Before discussing Gain Control Range, it is important to

understand the issues which limit it. The minimum gain of the

LMH6504, theoretically, is zero, but in practical circuits is

limited by the amount of feedthrough, here defined as the

gain when V

G = 0V. Capacitive coupling through the board

and package as well as coupling through the supplies will

determine the amount of feedthrough. Even at DC, the input

signal will not be completely rejected. At high frequencies

feedthrough will get worse because of its capacitive nature.

At frequencies below 10 MHz, the feed through will be less

than −60 dB and therefore, it can be said that with A

VMAX =

20 dB, the gain control range is 80 dB.

20084351

FIGURE 2. LMH6504 Gain Accuracy & Gain Matching

Defined

20084352

FIGURE 3. Gain Partitioning

LMH6504 Datasheet(PDF) 13 Page - National Semiconductor (TI)

LMH6504 Datasheet(HTML) 13 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Datasheet Download

Link URL