LMH6505 datasheet(13/20 Pages) NSC

13 / 20 page

Application Information (Continued)

GAIN ACCURACY

Gain accuracy is defined as the actual gain compared

against the theoretical gain at a certain V

G, the results of

which are expressed in dB. (See Figure 2).

Theoretical gain is given by:

Eq. 3

Where K = 0.940 (nominal) N = 1.01V & V

C =79mV @ 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 difference between the "Typical 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

As Figure 2 shows, gain matching is the limit on gain varia-

tion at a certain V

G, expressed in dB, and is specified as

"±Max" only. There is 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 LMH6505. 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 and the output of the LMH6505 to 4V. Accordingly,

the designer must carefully consider the contributions of

each stage to the overall characteristics. Through gain par-

titioning the designer is provided with an opportunity to op-

timize the frequency response, noise, distortion, settling

time, and loading effects of each amplifier to achieve im-

proved overall performance.

LMH6505 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

LMH6505 is theoretically zero, but in practical circuits it 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

VMAX = 20 dB, the gain control range is 80 dB.

20171051

FIGURE 2. LMH6505 Gain Accuracy & Gain Matching

Defined

20171052

FIGURE 3. Gain Partitioning

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