Datasheet
2
V
IN
V
O
R
G
100:
+
-
LMC8101
LMH6504
3
1
6
7
4
R
F
1 k:
I
o1
I
o2
DIGITAL
INPUT
V
REF
R
FB
DAC0830
2
V
O
V
G
R
G
R
2
10 k:
LMH6504
3
1
6
7
4
R
F
V
IN
+5V
-5V
R
1
10 k:
R
4
10 k:
R
3
10 k:
0.1 µF
0.1 µF
+5V
-5V
LMH6504
SNOSA96D –NOVEMBER 2003–REVISED MARCH 2013
www.ti.com
Figure 47. OFFSET ADJUST CIRCUIT
DIGITAL GAIN CONTROL
Digitally variable gain control can be easily realized by driving the LMH6504’s gain control input with a digital-to-
analog converter (DAC). Figure 48 illustrates such an application. This circuit employs Texas Instruments' eight-
bit DAC0830, the LMC8101 MOS input op-amp (Rail-to-Rail Input/Output), and the LMH6504 VGA. With V
REF
set
to 2V, the circuit provides up to 80 dB of gain control in 256 steps with up to 0.05% full scale resolution. The
maximum gain of this circuit is 20 dB.
Figure 48. Digital Gain Control
USING THE LMH6504 IN AGC APPLICATIONS
In AGC applications, the control loop forces the LMH6504 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, Distortion 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
= 1k, 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/ 6720/ 6722 family or 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.
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