AD8319

Rev. 0 | Page 14 of 20

The basic connections for operating the AD8319 in an automatic

gain control (AGC) loop with the ADL5330 are shown in

Figure 30. The ADL5330 is a 10 MHz to 3 GHz variable gain

amplifier. It offers a large gain control range of 60 dB with

±0.5 dB gain stability. This configuration is similar to Figure 29.

The gain of the ADL5330 is controlled by the output pin of the

To avoid overdrive recovery issues, the AD8319 output voltage

can be scaled down using a resistive divider to interface with the

0 V to 1.4 V gain control range of the ADL5330.

A coupler/attenuation of 21 dB is used to match the desired

maximum output power from the VGA to the top end of the

linear operating range of the AD8319 (approximately −5 dBm

at 900 MHz).

INLO

INHI

GAIN

OPLO

OPHI

DIRECTIONAL

COUPLER

ATTENUATOR

VPOS

COMM

ADL5330

+5V

+5V

+5V

COMM

VOUT

VPOS

VSET

INHI

INLO

CLPF

AD8319

LOG AMP

DAC

RF OUTPUT

SIGNAL

4.12k

Ω

10k

Ω

SETPOINT

VOLTAGE

1nF

47nF

47nF

120nH

120nH

100pF

100pF

100pF

100pF

TADJ

18k

Ω

52.3

Ω

RF INPUT

SIGNAL

Figure 30. AD8319 Operating in Controller Mode to Provide Automatic Gain

Control Functionality in Combination with the ADL5330

Figure 31 shows the transfer function of the output power vs.

an input power of −1.5 dBm. Note that the power control of the

to demanding a higher signal from the ADL5330, increases gain.

The AGC loop is capable of controlling signals of ~40 dB. This

range limitation is due to the dynamic range of the AD8319.

Using a wider dynamic range detector such as the AD8317,

AD8318, or AD8362 will allow for the full 60dB range of the

ADL5330 to be utilized. The performance over temperature is

most accurate over the highest power range, where it is gener-

ally most critical. Across the top 40 dB range of output power,

the linear conformance error is well within ±0.5 dB over

temperature.

–50

–40

–30

–10

0

10

20

30

–20

–4

–3

0

1

2

3

4

–1

–2

0.2

0.4

0.6

0.8

1.0

1.2

1.4

SETPOINT VOLTAGE (V)

1.3

1.1

0.3

0.5

0.7

0.9

1.5 1.6

Figure 31. ADL5330 Output Power vs. AD8319 Setpoint Voltage,

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

track the envelope of the ADL5330’s output signal and provide

the necessary voltage levels to the ADL5330’s gain control input.

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.

Also shown is the gain control response of the AD8319 to the

changing input envelope.

CH1 200mV

A Ch2

1.03V

M2.00ms

T

0.00000 s

1

AM MODULATED INPUT

AD8319 OUTPUT

Ω

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.