AD8318

Rev. B | Page 19 of 24

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. For an example of a 4-pole active filter,

see the AD8307 data sheet.

CONTROLLER MODE

The AD8318 provides a controller mode feature at the VOUT

AD8318 to control subsystems, such as power amplifiers (PAs),

variable gain amplifiers (VGAs), or variable voltage attenuators

(VVAs) that have output power that increases monotonically

with respect to their gain control signal.

To operate in controller mode, the link between VSET and

VOUT is broken. A setpoint voltage is applied to the VSET

input; VOUT is connected to the gain control terminal of the

VGA, and the detector RF input is connected to the output of

the VGA (usually using a directional coupler and some

additional attenuation). Based on the defined relationship

measurement mode, the AD8318 adjusts the voltage on VOUT

(VOUT is now an error amplifier output) until the level at the

When the AD8318 operates in controller mode, there is no

to a value that results in the correct input signal level appearing

at INHI/INLO.

In order for this output power control loop to be stable, a

ground-referenced capacitor is connected to the CLPF pin.

current) to set the loop bandwidth and ensure loop stability. For

further details on control loop dynamics, refer to the AD8315

data sheet.

RFIN

VGA/VVA

GAIN

CONTROL

VOLTAGE

DIRECTIONAL

COUPLER

ATTENUATOR

INHI

VSET

INLO

CLPF

VOUT

AD8318

52.3Ω

1nF

1nF

DAC

Figure 42. AD8318 Controller Mode

voltage of the VGA must have a positive sense. A positive

control voltage to the VGA increases the gain of the device.

The basic connections for operating the AD8318 as an analog

controller with the AD8367 are shown in Figure 43. The

AD8367 is a low frequency to 500 MHz VGA with 45 dB of

dynamic range. This configuration is very similar to the one

shown in Figure 42. For applications working at high input

frequencies, such as cellular bands or WLAN, or those

requiring large gain control ranges, the AD8318 can control

the 10 MHz to 3 GHz ADL5330 RF VGA. For further details

and an application schematic, refer to the ADL5330 data sheet.

The voltage applied to the GAIN pin controls the gain of the

of 20 mV/dB and runs from 50 mV at −2.5 dB of gain up to

1.0 V at +42.5 dB.

The incoming RF signal to the AD8367 has a varying amplitude

level. Receiving and demodulating it with the lowest possible

error requires that the signal levels be optimized for the highest

signal-to-noise ratio (SNR) feeding into the analog-to-digital

converters (ADC). This is done by using an automatic gain

control (AGC) loop. In Figure 43, the voltage output of the

AD8318 modifies the gain of the AD8367 until the incoming

RF signal produces an output voltage that is equal to the

+3V

INPT

VOUT

VPOS GND

GAIN

AD8367

VGA

HPFL

RF INPUT SIGNAL

RF OUTPUT SIGNAL

+5V

VSET

CLPF

INHI

VPOS

GND

AD8318

VOUT

INLO

R1

1kΩ

1nF

1nF

0.1µF

100pF

100MHz

BANDPASS

FILTER

174Ω

R2

261Ω

100Ω

100pF

DAC

SETPOINT

VOLTAGE

57.6Ω

Figure 43. AD8318 Operating in Controller Mode to Provide Automatic Gain

Control Functionality in Combination with the AD8367

The AGC loop is capable of controlling signals over ~45 dB

dynamic range. The output of the AD8367 is designed to drive

loads ≥ 200 Ω. As a result, it is not necessary to use the 53.6 Ω

resistor at the input of the AD8318; the nominal input imped-

ance of 2 kΩ is sufficient.

If the AD8367 output drives a 50 Ω load, such as an oscilloscope

or spectrum analyzer, use a simple resistive divider network.

The divider used in Figure 43 has an insertion loss of 11.5 dB.

voltage for a 100 MHz sine wave at −40 dBm into the AD8367.