LMH6504MM datasheet(16/19 Pages) NSC

16 / 19 page

Application Information (Continued)

Signal frequencies must not reach the gain control port of the

LMH6504, or the output signal will be distorted (modulated

by itself). A fast settling AGC needs additional filtering be-

yond the integrator stage to block signal frequencies. This is

provided in Figure 8 by a simple R-C filter (R

10 and C3);

better distortion performance can be achieved with a more

complex filter. These filters should be scaled with the input

signal frequency. Loops with slower response time (longer

integration time constants) may not need the R

10 –C3 filter.

Checking the loop stability can be done by monitoring the V

voltage while applying a step change in input signal ampli-

tude. Changing the input signal amplitude can be easily

done with an arbitrary waveform generator.

AUTOMATIC GAIN CONTROL (AGC) #2

Figure 9 illustrates an automatic gain control circuit that

employs two LMH6504’s. In this circuit, U1 receives the input

signal and produces an output signal of constant amplitude.

U2 is configured to provide negative feedback. U2 generates

a rectified gain control signal that works against an adjust-

able bias level which may be set by the potentiometer and

B.CI integrates the bias and negative feedback. The result-

ant gain control signal is applied to the U1 gain control input

G. The bias adjustment allows the U1 output to be set at an

arbitrary level less than the maximum output specification of

the amplifier. Rectification is accomplished in U2 by driving

both the amplifier input and the gain control input with the U1

output signal. The voltage divider that is formed by R

1 and

2, sets the rectifier gain.

20084359

FIGURE 8. Automatic Gain Control Circuit #1

LMH6504MM Datasheet(PDF) 16 Page - National Semiconductor (TI)