2

VIN

VO

RG

100:

+

-

LMC8101

LMH6505

3

1

6

7

4

RF

1 k:

Io1

Io2

DIGITAL

INPUT

VREF

RFB

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 RG 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 VPP in the specified configuration, RF = 1 kΩ, RG = 100Ω. When the gain is

adjusted to

−15 dB (i.e. 35 dB down from AVMAX), the input amplitude would be 1.41 VPP 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 VPP, the

input amplitudes for gains 40 dB down from AVMAX 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 mVPP. Another way of addressing distortion

performance and its limitations on dynamic range, would be to raise the value of RG. 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 RG, RF will also have to be increased

to keep the same AVMAX and this will decrease the overall bandwidth. It may be possible to insert a series RC

combination across RF in order to counteract the negative effect on BW when a large RF 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 R9. 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.

Copyright © 2005–2013, Texas Instruments Incorporated

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