Data Sheet

AD8318

USING THE AD8318

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Rev. E | 20 of 24

Figure 44. AD8367 Output Power vs. AD8318 Setpoint Voltage

For the AGC loop to remain locked, the AD8318 must track the

envelope of the VGA output signal and provide the necessary

voltage levels to the AD8367 gain control input. Figure 45 shows

an oscilloscope screen image of the AGC loop depicted in Figure

43. 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 45. Oscilloscope Screen Image 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.

The 45 dB control range is constant for the range of VSET voltages.

The input power levels to the AD8367 must be optimized to achieve

this range. In Figure 46, the minimum and maximum input power

levels are shown vs. setpoint voltage.

Figure 46. Setpoint Voltage vs. Input Power. Optimal signal levels must be

used to achieve the full 45 dB dynamic range capabilities of the AD8367.

In some cases, if VGAIN is >1.0 V it can take an unusually long

time for the AGC loop to recover; that is, the output of the AD8318

remains at an abnormally high value and the gain is set to its

maximum level. A voltage divider is placed between the output of

the AD8318 and the AD8367 GAIN pin to ensure that VGAIN does

not exceed 1.0 V.

In Figure 43, CHP and RHP are configured to reduce oscillation and

distortion due to harmonics at higher gain settings. Some additional

filtering is recommended between the output of the AD8367 and the

input of the AD8318. This helps to decrease the output noise of the

AD8367, which can reduce the dynamic range of the loop at higher

gain settings (smaller VSET).

Response time and the amount of signal integration are controlled

by CFLT. This functionality is analogous to the feedback capacitor

around an integrating amplifier. Though it is possible to use large

capacitors for CFLT, in most applications, values under 1 nF provide

sufficient filtering.

Calibration in controller mode is similar to the method used in

measurement mode. Do a simple 2-point calibration by applying

two known VSET voltages or DAC codes and measuring the output

power from the VGA. Slope and intercept are calculated using

Equation 20 to Equation 22:

Slope = (VSET1 − VSET2)/(POUT1 − POUT2)

(20)

Intercept = POUT1 − VSET1/Slope

(21)

VSET = Slope × (Px − Intercept)

(22)

For more information on AGC applications, refer to the AD8367

data sheet or ADL5330 data sheet.

CHARACTERIZATION SETUP AND METHODS

The general hardware configuration used for the AD8318 character-

ization is shown in Figure 47. The primary setup used for character-

ization is measurement mode. The characterization board is similar

to the customer evaluation board with the exception that the RF