LMH6504 Datasheet, PDF (18/29 Pages) National Semiconductor (TI) – Wideband, Low Power, Variable Gain Amplifier

LMH6504
SNOSA96D – NOVEMBER 2003 – REVISED MARCH 2013
V_G
www.ti.com
V_IN
+5V
R₁
10 k:
R₂
10 k:
0.1 µF
-5V
1
2
6
LMH6504
3
7 ^RF
4
R_G
R₃
10 k:
0.1 µF
V_O
+5V
R₄
10 k:
-5V
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.
DIGITAL
INPUT
V_REF
R_FB
I_o1
DAC0830
I_o2
-
LMC8101
+
V_IN
R_G
100:
21
6 ^VO
LMH6504
3
7
4
R_F
1 k:
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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