LMH6505 Datasheet, PDF (13/20 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB, Variable Gain Amplifier

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:
GAIN PARTITIONING
If high levels of gain are needed, gain partitioning should be
considered:
Eq. 3
Where K = 0.940 (nominal) N = 1.01V & V_C = 79 mV @ room
temperature
For a V_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.
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FIGURE 2. LMH6505 Gain Accuracy & Gain Matching
Defined
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FIGURE 3. Gain Partitioning
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 R₂ at 909Ω and R₁ at 100Ω produces a
gain of 20 dB. Setting R_F at 1000Ω as recommended and R_G
at 50Ω, 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
A_VMAX = 20 dB, the gain control range is 80 dB.
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