LMH6505 Datasheet, PDF (15/18 Pages) National Semiconductor (TI) – Wideband, Low Power, Linear-in-dB, Variable Gain Amplifier

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 am-
plifier, by increasing the load resistance, and therefore de-
creasing the demanded load current, the distortion perfor-
mance 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)
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 in-
tegrator (U3) to provide high loop gain at low frequencies. The
output amplitude is set by R₉. The following are some sug-
gestions for building fast AGC loops: Precision rectifiers work
best with large output signals. Accuracy is improved by block-
ing DC offsets, as shown in Figure 9.
FIGURE 9. Automatic Gain Control Circuit
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