LMH6504MM datasheet(15/19 Pages) NSC

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Application Information (Continued)

trim pot R

4 to null the offset voltage at the output. This will

eliminate the output stage offsets. Next set V

G to 2V and

adjust the trim pot R

1 to null the offset voltage at the output.

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

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.

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, RF = 1k, RG =

100

Ω. When the gain is adjusted to -15 dB (i.e. 35 dB down

from A

VMAX), 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 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 distor-

tion performance will be improved in most cases. With an

increased R

G,RF 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

in order to counteract the negative effect on BW when a

large R

F is used.

AUTOMATIC GAIN CONTROL (AGC) #1

Fast Response AGC Loop

The AGC circuit shown in Figure 8 will correcta6dB 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

9. Some notes on building

fast AGC loops: Precision rectifiers work best with large

output signals. Accuracy is improved by blocking DC offsets,

as shown in Figure 8.

20084357

FIGURE 6. OFFSET ADJUST CIRCUIT

20084358

FIGURE 7. Digital Gain Control

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