CN-0050

Circuit Note

Rev. B | Page 2 of 3

The basic connections for operating the ADL5330 in an AGC

loop with the AD8318 are shown in Figure 1. The AD8318 is a

1 MHz to 8 GHz precision demodulating logarithmic amplifier.

It offers a large detection range of 60 dB with ±0.5 dB

temperature stability. The gain control pin of the ADL5330 is

controlled by the output pin of the AD8318. This voltage,

VOUT, has a range of 0 V to near VPSx. To avoid overdrive

recovery issues, the AD8318 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 23 dB is used to match the desired

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

linear operating range of the AD8318 (at approximately −5 dBm

at 900 MHz).

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

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

current). A capacitor must be connected to CLPF to set the loop

bandwidth and to ensure loop stability.

–4

4

3

2

1

0

–1

–2

–3

SETPOINT VOLTAGE (V)

2.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

30

20

10

0

–10

–20

–30

–40

–50

Figure 2. ADL5330 Output Power vs. AD8318 Setpoint Voltage,

PIN = −1.5 dBm

Figure 2 shows the transfer function of the output power vs. the

VSET voltage over temperature for a 900 MHz sine wave with

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

AD8318 has a negative sense. Decreasing VSET, which corres-

ponds to demanding a higher signal from the ADL5330, tends

to increase GAIN.

The AGC loop is capable of controlling signals just under the

full 60 dB gain control range of the ADL5330. The performance

over temperature is most accurate over the highest power range,

where it is generally most critical. Across the top 40 dB range

of output power, the linear conformance error is well within

±0.5 dB over temperature.

The broadband noise added by the logarithmic amplifier is

negligible.

AD8318 OUTPUT

CH3 250mV

Ω

M2.00ms

1

2

3

T

0.00000s

T

T

ADL5330 OUTPUT

AM MODULATED INPUT

Figure 3. Oscilloscope Showing an AM Modulated Input Signal

For the AGC loop to remain in equilibrium, the AD8318 must

track the envelope of the ADL5330 output signal and provide

the necessary voltage levels to the ADL5330 gain control input.

Figure 3 shows an oscilloscope screen shot of the AGC loop

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

modulation is applied to the ADL5330. The output signal from

the ADL5330 is a constant envelope sine wave with amplitude

corresponding to a setpoint voltage at the AD8318 of 1.5 V.

Also shown is the gain control response of the AD8318 to the

changing input envelope.

Ω

M10.0µs

A CH1

2.60V

2

1

T

20.2000µs

T

ADL5330 OUTPUT

T

Figure 4. Oscilloscope Showing the ADL5330 Output

Figure 4 shows the response of the AGC RF output to a pulse on

VSET. As VSET decreases to 1 V, the AGC loop responds with

an RF burst. Response time and the amount of signal

integration are controlled by the capacitance at the AD8318

CLPF pin—a function analogous to the feedback capacitor

around an integrating amplifier. An increase in the capacitance

results in slower response time.

The circuit must be constructed on a multilayer printed circuit

board with a large area ground plane. Proper layout, grounding,

and decoupling techniques must be used to achieve optimum

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

and the ADL5330 and ADL8318 evaluation board layouts).