Datasheet
2
V
IN
V
O
R
G
100:
+
-
LMC8101
LMH6505
3
1
6
7
4
R
F
1 k:
I
o1
I
o2
DIGITAL
INPUT
V
REF
R
FB
DAC0830
LMH6505
www.ti.com
SNOSAT4E –DECEMBER 2005–REVISED APRIL 2013
Figure 48. Digital Gain Control
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)
Fast Response AGC Loop
The AGC circuit shown in Figure 49 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 R
9
. 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 49.
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