LMH6505 Datasheet, PDF (16/20 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB, Variable Gain Amplifier

Application Information (Continued)
FIGURE 8. Digital Gain Control
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USING THE LMH6505 IN AGC APPLICATIONS
In AGC applications, the control loop forces the LMH6505 to
have a fixed output amplitude. The input amplitude will vary
over a wide range and this can be the issue that limits
dynamic range. At high input amplitudes, the distortion due
to the input buffer driving R_G may exceed that which is
produced by the output amplifier driving the load. In the plot,
THD vs. Gain, total harmonic distortion (THD) is plotted over
a gain range of nearly 35 dB for a fixed output amplitude of
0.25 V_PP in the specified configuration, R_F = 1 kΩ,
R_G = 100Ω. When the gain is adjusted to −15 dB (i.e. 35 dB
down from A_VMAX), the input amplitude would be 1.41 V_PP
and we can see the distortion is at its worst at this gain. If the
output amplitude of the AGC were to be raised above 0.25
V_PP, the input amplitudes for gains 40 dB down from A_VMAX
would be even higher and the distortion would degrade
further. It is for this reason that we recommend lower output
amplitudes if wide gain ranges are desired. Using a post-
amp like the LMH6714/LMH6720/LMH6722 family or the
LMH6702 would be the best way to preserve dynamic range
and yield output amplitudes much higher than 100 mV_PP.
Another way of addressing distortion performance and its
limitations on dynamic range, would be to raise the value of
R_G. Just like any other high-speed amplifier, by increasing
the load resistance, and therefore decreasing the demanded
load current, the distortion performance will be improved in
most cases. With an increased R_G, R_F will also have to be
increased to keep the same A_VMAX and this will decrease the
overall bandwidth. It may be possible to insert a series RC
combination across R_F in order to counteract the negative
effect on BW when a large R_F is used.
AUTOMATIC GAIN CONTROL (AGC) #1
Fast Response AGC Loop
The AGC circuit shown in Figure 9 will correct a 6 dB input
amplitude step in 100 ns. The circuit includes a two op amp
precision rectifier amplitude detector (U1 and U2), and an
integrator (U3) to provide high loop gain at low frequencies.
The output amplitude is set by R₉. The following are some
suggestions for building fast AGC loops: Precision rectifiers
work best with large output signals. Accuracy is improved by
blocking DC offsets, as shown in Figure 9.
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