CN-0082

Circuit Note

Rev. 0 | Page 2 of 4

amplifier output) until the level at the RF input corresponds to

the applied setpoint voltage. GAIN settles to a value that results

in the correct balance between the input signal level at the

detector and the setpoint voltage.

The basic connections for operating the ADL5331 in an AGC

loop with the AD8319 are shown in Figure 1. The AD8319 is a

1 MHz to 10 GHz precision demodulating logarithmic

amplifier. It offers a detection range of 45 dB with ±0.5 dB

GAIN (gain control) pin of the ADL5331. When the AD8319 is

AD8319 can drive the ADL5331 GAIN pin over its full linear

range of 0 V to 1.4 V. Under very low power RF in conditions,

AD8319 may be driven to its maximum value very close to

the AD8319 and GAIN on the ADL5331. This may have a slight

effect on the overall speed of the loop, for instance, when the

input power to the ADL5331 is stepped.

A coupler/attenuation of 23 dB is used to match the desired

output power range from the VGA to the linear operating

range of the AD8319. In this case, the desired output power

range of the VGA is −15 dBm to +15 dBm. With the given

attenuator/coupler, the range of power to the AD8319 RF input

is −8 dBm to −38 dBm, within the specified range of −3 dBm to

−43dBm for a ±1 dB error.

The detector’s error amplifier uses CLFP, a ground-referenced

capacitor pin, to integrate the error signal (in the form of a

current). A capacitor must be connected to CLFP to set the

loop bandwidth and to ensure loop stability.

Figure 2, Figure 3, and Figure 4 show the transfer function of

the ADL5331 output power vs. the AD5621 DAC code for a

100 MHz sine wave with an input power of 0 dBm, −10 dBm,

and −20 dBm. Note that the power control of the AD8319 has a

negative sense. Decreasing the DAC code, which corresponds

to demanding a higher signal from the ADL5331, tends to

increase GAIN.

In order for the AGC loop to remain in equilibrium, the

AD8319 must track the envelope of the ADL5331 output signal

and provide the necessary voltage levels to the ADL5331’s gain

control input. Figure 5 shows an oscilloscope screenshot of the

AGC loop in Figure 1. A 100 MHz sine wave with 50% AM

modulation is applied to the ADL5331. The output signal from

the ADL5331 is a constant envelope sine wave with amplitude

corresponding to a setpoint voltage at the AD8319 of 1.5 V.

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

changing input envelope.

–40

–30

–20

–10

0

10

20

1024

1524

2024

2524

3024

3524

AD5621 DAC CODE

–5.0

–2.5

0

2.5

5.0

POWER OUT

STRAIGHT LINE

ERROR

Figure 2. ADL5331 Power Out vs. AD5621 DAC Code with

RF Input Signal = 0 dBm

–40

–30

–20

–10

0

10

20

1280

1780

2280

2780

3280

3780

AD5621 DAC CODE

–5.0

–2.5

0

2.5

5.0

POWER OUT

STRAIGHT LINE

ERROR

Figure 3. ADL5331 Power Out vs. AD5621 DAC Code with

RF Input Signal = −10 dBm

–40

–30

–20

–10

0

10

20

1280

1780

2280

2780

3280

3780

AD5621 DAC CODE

–5.0

–2.5

0

2.5

5.0

POWER OUT

STRAIGHT LINE

ERROR

Figure 4. ADL5331 Power Out vs. AD5621 DAC Code with

RF Input Signal = −20 dBm