Data Sheet

AD8319

Rev. D | Page 15 of 19

For the AGC loop to remain in equilibrium, the AD8319 must

track the envelope of the output signal of the ADL5330 and

provide the necessary voltage levels to the gain control input

of the ADL5330. Figure 32 shows an oscilloscope screenshot of

the AGC loop depicted in Figure 30. A 100 MHz sine wave with

50% AM modulation is applied to the ADL5330. The output signal

from the VGA is a constant envelope sine wave with amplitude

corresponding to a setpoint voltage at the AD8319 of 1.3 V.

The gain control response of the AD8319 to the changing input

envelope is also shown.

CH1 200mV

A Ch2

1.03V

M2.00ms

T

0.00000 s

1

AM MODULATED INPUT

AD8319 OUTPUT

2

3

ADL5330 OUTPUT

Figure 32. Oscilloscope Screenshot Showing an AM Modulated Input Signal

and the Response from the AD8319

Figure 33 shows the response of the AGC RF output to a pulse

responds with an RF burst. In this configuration, the input signal to

the ADL5330 is a 1 GHz sine wave at a power level of −15 dBm.

A Ch1

2.60V

T

179.800µs

AD8319 VSET PULSE

ADL5330 OUTPUT

3

1

M10.µs

Ch1 2.00V

T

Figure 33. Oscilloscope Screenshot Showing the

Response Time of the AGC Loop

Response time and the amount of signal integration are

feedback capacitor around an integrating amplifier. While it

values under 1 nF provide sufficient filtering.

Calibration in controller mode is similar to the method used

in measurement mode. A simple two-point calibration can be

measuring the output power from the VGA. Slope and intercept

can then be calculated by:

(8)

(9)

(10)

More information on the use of the ADL5330 in AGC applications

can be found in the ADL5330 data sheet.

OUTPUT FILTERING

For applications in which maximum video bandwidth and,

consequently, fast rise time are desired, it is essential that the

CLPF pin be left unconnected and free of any stray capacitance.

The nominal output video bandwidth of 50 MHz can be reduced

pin, as shown in Figure 34. This is generally done to reduce output

ripple (at twice the input frequency for a symmetric input

waveform such as sinusoidal signals).

+4

VOUT

CLPF

AD8319

3.5pF

1.5kΩ

Figure 34. Lowering the Postdemodulation Bandwidth



pF

3.5

kΩ

1.5

2π

1









Bandwidth

Video

C

FLT

(11)

The video bandwidth should typically be set to a frequency

equal to approximately one-tenth the minimum input frequency.

This ensures that the output ripple of the demodulated log

output, which is at twice the input frequency, is well filtered.

In many log amp applications, it may be necessary to lower the

corner frequency of the postdemodulation filtering to achieve

low output ripple while maintaining a rapid response time to

changes in signal level. An example of a four-pole active filter

is shown in the AD8307 data sheet.