lmh6504mmx National Semiconductor Corporation, lmh6504mmx Datasheet - Page 15

• Manufacturer Part Number: lmh6504mmx
• Description: Wideband, Low Power, Variable Gain Amplifier
• Manufacturer: National Semiconductor Corporation

Application Information
trim pot R
eliminate the output stage offsets. Next set V
adjust the trim pot R
This will eliminate the input stage offsets.
DIGITAL GAIN CONTROL
Digitally variable gain control can be easily realized by driv-
ing the LMH6504’s gain control input with a digital-to-analog
converter (DAC). Figure 7 illustrates such an application.
This circuit employs National Semiconductor’s eight-bit
DAC0830, the LMC8101 MOS input op-amp (Rail-to-Rail
Input/Output), and the LMH6504 VGA. With V
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 6. OFFSET ADJUST CIRCUIT
4
to null the offset voltage at the output. This will
FIGURE 7. Digital Gain Control
1
to null the offset voltage at the output.
(Continued)
REF
G
to 2V and
set to 2V,
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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
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
100Ω. When the gain is adjusted to -15 dB (i.e. 35 dB down
from A
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
input amplitudes for gains 40 dB down from A
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
distortion performance and its limitations on dynamic range,
would be to raise the value of R
speed amplifier, by increasing the load resistance, and
therefore decreasing the demanded load current, the distor-
tion performance will be improved in most cases. With an
increased R
same A
may be possible to insert a series RC combination across R
in order to counteract the negative effect on BW when a
large R
AUTOMATIC GAIN CONTROL (AGC) #1
Fast Response AGC Loop
The AGC circuit shown in Figure 8 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
fast AGC loops: Precision rectifiers work best with large
output signals. Accuracy is improved by blocking DC offsets,
as shown in Figure 8.
VMAX
F
VMAX
is used.
PP
), the input amplitude would be 1.41 V
G
in the specified configuration, R
, R
and this will decrease the overall bandwidth. It
F
will also have to be increased to keep the
PP
G
. Another way of addressing
may exceed that which is
G
9
. Just like any other high-
. Some notes on building
VMAX
F
= 1k, R
www.national.com
PP
would be
and we
PP
, the
G
=
F