Data Sheet

ADL5513

APPLICATIONS INFORMATION

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Rev. B | 19 of 25

Figure 40. Typical Output Voltage vs. Input Signal

The output voltage vs. input signal voltage of the ADL5513 is linear-

in-dB over a multidecade range. The equation for this function is

VOUT=X×VSLOPE/DEC×log10VINVINTERCEPT =

X×VSLOPE/dB×20×log10VINVINTERCEPT

(9)

where:

X is the feedback factor in VSET = VOUT/X.

VSLOPE/DEC is nominally 400 mV/decade or 20 mV/dB.

VINTERCEPT is the x-axis intercept of the linear-in-dB portion of the

VOUT vs. PIN curve (see Figure 40).

VINTERCEPT is −100 dBV for a sinusoidal input signal.

An offset voltage, VOFFSET, of 0.47 V is internally added to the

detector signal, so that the minimum value for VOUT is X × VOFFSET;

therefore, for X = 1, the minimum VOUT is 0.47 V.

The slope is very stable vs. process and temperature variation.

When Base 10 logarithms are used, VSLOPE/DEC represents the

volts per decade. A decade corresponds to 20 dB; VSLOPE/DEC/20 =

VSLOPE/dB represents the slope in volts per decibel (V/dB).

As shown in Figure 40, VOUT voltage has a positive slope.

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

characteristic impedance of the system, Z0, must be known to

convert voltages to their corresponding power levels. The following

equations are used to perform this conversion:

P(dBm) = 10 × log10(Vrms2/(Z0 × 1 mW))

(10)

P(dBV) = 20 × log10(Vrms/1 Vrms)

(11)

P(dBm) = P(dBV) − 10 × log10(Z0 × 1 mW/1 Vrms2)

(12)

For example, PINTERCEPT for a sinusoidal input signal expressed in

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

PINTERCEPTdBm =

PINTERCEPTdBV −10×log10Z0×1 mW/1Vrms2 =

‐100 dBV−10×log1050×10−3 =‐87 dBm

(13)

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 is connected to VSET.

Connecting VOUT directly to VSET yields the nominal logarithmic

slope of approximately 20 mV/dB. The output swing corresponding

to the specified input range is then approximately 0.47 V to 2.0

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 40

mV/dB.

Figure 41. Increasing the Slope

The required resistor values needed to increase the slope are

calculated from the following equation.

R

1R2

+1=Slope2Slope1

(14)

where:

R1 is the resistor from VOUT to VSET.

R2 is the resistor from VSET to ground.

Slope1 is the nominal slope of the ADL5513.

Slope2 is the new slope.

It is important to remember when increasing the slope of the

ADL5513 that R1 and R2 must be properly sized so the output

current drive capability is not exceeded. The dynamic range of the

ADL5513 may be limited if the maximum output voltage is achieved

before the maximum input power is reached. In cases where VPOS

is 5 V, the maximum output voltage is 4.7 V.

The slope of the ADL5513 can be reduced by connecting VSET to

VOUT and adding a voltage divider on the output.

CONTROLLER MODE

The ADL5513 provides a controller mode feature at Pin VOUT.

Using VSET for the setpoint voltage, it is possible for the ADL5513 to

control subsystems, such as power amplifiers (PAs), variable gain