AD8317

Rev. 0 | Page 14 of 20

Although demodulating log amps respond to input signal voltage,

not input signal power, it is customary to discuss the amplitude

of high frequency signals in terms of power. In this case, the charac-

voltages to their corresponding power levels. The following

equations are used to perform this conversion:

(3)

(4)

(5)

terms of dBm (decibels referred to 1 mW), in a 50 Ω system is

(6)

For a square wave input signal in a 200 Ω system,

+6 dBm

Further information on the intercept variation dependence upon

waveform can be found in the AD8313 and AD8307 data sheets.

SETTING THE OUTPUT SLOPE

IN MEASUREMENT MODE

To operate in measurement mode, VOUT must be connected to

VSET. Connecting VOUT directly to VSET yields the nominal

logarithmic slope of approximately −22 mV/dB. The output

swing corresponding to the specified input range is then approxi-

mately 0.35 V to 1.7 V. The slope and output swing can be

increased by placing a resistor divider between VOUT and

VSET (that is, one resistor from VOUT to VSET and one

resistor from VSET to ground). The input impedance of VSET

is approximately 40 kΩ. Slope-setting resistors should be kept

below 20 kΩ to prevent this input impedance from affecting

the resulting slope. If two equal resistors are used (for example,

10 kΩ/10 kΩ), the slope doubles to approximately −44 mV/dB.

VOUT

AD8317

–44mV/dB

VSET

10k

Ω

10k

Ω

Figure 28. Increasing the Slope

CONTROLLER MODE

The AD8317 provides a controller mode feature at the VOUT

AD8317 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

variable gain amplifier (VGA), and the detector’s RF input is

connected to the output of the VGA (usually using a directional

coupler and some additional attenuation). Based on the defined

is in measurement mode, the AD8317 adjusts the voltage on

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

operates in controller mode, there is no defined relationship

the correct input signal level appearing at INHI/INLO.

For this output power control loop to be stable, a ground-

referenced capacitor must be connected to the CLPF pin. This

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

Further details on control loop dynamics can be found in the

AD8315 data sheet.

RFIN

VGA/VVA

GAIN

CONTROL

VOLTAGE

DIRECTIONAL

COUPLER

ATTENUATOR

INHI

VSET

INLO

CLPF

VOUT

AD8317

52.3

Ω

47nF

47nF

DAC

Figure 29. AD8317 Controller Mode

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 AD8317 in an automatic

gain control (AGC) loop with the ADL5330 are shown in

Figure 30. The ADL5330 is a 10 MHz to 3 GHz variable gain

amplifier. It offers a large gain control range of 60 dB with

±0.5 dB gain stability. This configuration is similar to Figure 29.