AD8319

Rev. 0 | Page 13 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

known to convert 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 −22 mV/dB. The output swing corresponding

to the specified input range is then 0.35 V to 1.5 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 imped-

ance of VSET is 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 −44 mV/dB.

VOUT

AD8319

–44mV/dB

VSET

10k

Ω

10k

Ω

Figure 28. Increasing the Slope

CONTROLLER MODE

The AD8319 provides a controller mode feature at the VOUT

AD8319 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

device is in measurement mode, the AD8319 adjusts the voltage

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

AD8319 operates in controller mode, there is no defined relation-

results in 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

AD8319

52.3

Ω

47nF

47nF

DAC

Figure 29. AD8319 Controller Mode

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

voltage to the VGA increases the gain of the device.