Circuit Note

CN-0082

Rev. 0 | Page 3 of 4

AD8319 OUTPUT

CH3 250mV

Ω

M2.00ms

A CH4

1.80V

1

2

3

T

0.00000s

T

T

ADL5331 OUTPUT

AM MODULATED INPUT

Figure 5. Oscilloscope Screenshot Showing an

AM Modulated Input Signal

Figure 6 shows the response of the AGC RF output to a pulse

an RF burst. Response time and the amount of signal

integration are controlled by the capacitance at the AD8319

CLFP pin—a function analogous to the feedback capacitor

around an integrating amplifier. An increase in the capacitance

results in a slower response time.

The circuit must be constructed on a multilayer PC board with

a large area ground plane. Proper layout, grounding, and

decoupling techniques must be used to achieve optimum

performance (see Tutorial MT-031 and Tutorial MT-101 and

the ADL5331and AD8319 evaluation board layouts).

On the underside of the ADL5331 and AD8319 chip scale

packages, there is an exposed compressed paddle. This paddle is

internally connected to the chip’s ground. Solder the paddle to

the low impedance ground plane on the printed circuit board to

ensure the specified electrical performance and to provide

thermal relief. It is also recommended that the ground planes

on all layers under the paddle be stitched together with vias to

reduce thermal impedance.

COMMON VARIATIONS

This circuit can be used to implement a constant power out

function (fixed setpoint with variable input power) or a variable

power out function (variable setpoint with fixed or variable

input power). If a higher output power control range is desired,

the AD8318 log amp (60 dB power detection range) can be used

in place of the AD8319. For a constant output power function,

the lower dynamic range of the AD8319 will be adequate since

the loop will always servo the detector’s input power to a

constant level.

The ADL5331 VGA, which is optimized for transmit

applications, can also be replaced by the AD8368 VGA. The

AD8368 is optimized for low frequency receive applications up

to 800 MHz and provides 34 dB of linear-in-dB voltage-

controlled variable gain.

CH3 200mV

Ω

W

M 4.0µs 12.5MS/s 80.0ns/pt

A CH1

150mV

2

t1: 4.48µs

t2: 2.4µs

Δt: –2.08µs

1/Δt: –480.8kHz

MEAN(C1) 440.3mV

CURS2 POS

2.4µs

CURS1 POS

4.48µs

AMPL(C1) 3.36V

AMPL(C2) 900mV

Figure 6. Oscilloscope Screenshot Showing the ADL5331 Output

LEARN MORE

MT-031 Tutorial, Grounding Data Converters and Solving the

Mystery of"AGND" and "DGND." Analog Devices.

MT-073 Tutorial, High Speed Variable Gain Amplifiers. Analog

Devices.

MT-077 Tutorial, Log Amp Basics. Analog Devices.

MT-078 Tutorial, High Speed Log Amps. Analog Devices.

MT-101 Tutorial, Decoupling Techniques. Analog Devices.

Whitlow, Dana. Design and Operation of Automatic Gain

Control Loops for Receivers in Modern Communications

Systems. ChapterVIII, Analog Devices Wireless

Seminar (2006).

Data Sheets and Evaluation Boards

AD5621 Data Sheet.

AD8318 Data Sheet.

AD8319 Data Sheet.

AD8319 Evaluation Board.

AD8368 Data Sheet.

ADL5331 Data Sheet.

ADL5331 Evaluation Board.

REVISION HISTORY

7/09—Revision 0: Initial Version