AD8318 datasheet(19/24 Pages) AD

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AD8318

Rev. 0 | Page 19 of 24

RFIN

VGA/VVA

GAIN

CONTROL

VOLTAGE

DIRECTIONAL

COUPLER

ATTENUATOR

INHI

VSET

INLO

CLPF

VOUT

AD8318

52.3

Ω

1nF

CFLT

1nF

DAC

Figure 39. AD8318 Controller Mode

Decreasing VSET, which corresponds to demanding a higher

signal from the VGA, will tend to increase VOUT. The gain

control voltage of the VGA must have a positive sense that is

increasing gain control voltage increases gain.

The basic connections for operating the AD8318 as an analog

controller with the AD8367 are shown in Figure 40. 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 39.

The gain of the AD8367 is controlled by the voltage applied to

the GAIN pin. This voltage, VGAIN, is scaled linear-in-dB with a

slope 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 can be accomplished by using an

automatic gain control (AGC) loop. In Figure 40 the voltage

output of the AD8318 is used to modify the gain of the AD8367

until the incoming RF signal produces an output voltage that is

equal to the setpoint voltage VSET.

+3V

INPT

VOUT

VPOS

GND

GAIN

AD8367

VGA

HPLF

RF INPUT SIGNAL

RF OUTPUT SIGNAL

+5V

VSET

CLPF

INHI

VPOS

GND

AD8318

VOUT

INLO

Ω

1nF

0.1

µF

CFLT

100pF

100MHz

BANDPASS

FILTER

174

Ω

261

Ω

RHP

100

Ω

CHP

100pF

DAC

+VSET

SETPOINT

VOLTAGE

57.6

Ω

Figure 40. AD8318 Operating in Controller Mode to Provide Automatic Gain

Control Functionality in Combination with the AD8367

This 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 impedance of 2 kΩ is sufficient. If the AD8367’s

output is to be driving a 50 Ω load, such as an oscilloscope

or spectrum analyzer, a simple resistive divider network can

be used. Note that the divider used in Figure 40 has an

insertion loss of 11.5 dB.

Figure 41 shows the transfer function of output power

versus VSET voltage for a 100 MHz sine wave at −40 dBm

into the AD8367.

–60

–55

–50

–45

–40

–35

–30

–25

–20

–15

–10

–5

1.2

–1.2

–1.0

–0.8

–0.6

–0.4

–0.2

0.2

0.4

0.6

0.8

1.0

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

VSET (V)

Figure 41. AD8367 Output Power vs. AD8318 Setpoint Voltage

In order for the AGC loop to remain locked, the AD8318

must track the envelope of the VGA’s output signal and

provide the necessary voltage levels to the AD8367’s gain

control input. Figure 42 shows an oscilloscope screenshot of

the AGC loop depicted in Figure 40. A 50 MHz sine wave

with 50% AM modulation is applied to the AD8367. The

output signal from the VGA is a constant envelope sine

wave with an amplitude corresponding to a setpoint voltage

at the AD8318 of 1.0 V.

Figure 42. Oscilloscope Screenshot Showing an AM Modulated Input

Signal to the AD8367. The AD8318 tracks the envelope of this input

signal and applies the appropriate voltage to ensure a constant output

from the AD8367.

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