LMH6505 Datasheet, PDF (12/18 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB, Variable Gain Amplifier

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 LMH6505 will respond to the current
flowing through R_G. The gain control relationship will be sim-
ilar to the split supply arrangement with V_G measured with
reference 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)
(±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). This node will be called the “reference
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_G.
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:
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FIGURE 2. LMH6505 Gain Accuracy & Gain Matching
Defined
GAIN PARTITIONING
If high levels of gain are needed, gain partitioning should be
considered:
(3)
Where K = 0.940 (nominal) N = 1.01V & V_C = 79 mV at 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 variation
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.
FIGURE 3. Gain Partitioning
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The maximum gain range for this circuit is given by the fol-
lowing equation:
(4)
The LMH6624 is a low noise wideband voltage feedback am-
plifier. 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 partitioning
the designer is provided with an opportunity to optimize the
frequency response, noise, distortion, settling time, and load-
ing effects of each amplifier to achieve improved overall per-
formance.
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