ADL5513ACPZ-R2 datasheet(19/25 Pages) AD

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Data Sheet

ADL5513

Rev. A | Page 19 of 25

MEASUREMENT MODE

When the VOUT voltage or a portion of the VOUT voltage is fed

back to the VSET pin, the device operates in measurement

mode. As shown in Figure 40, the ADL5513 has an offset

voltage, a positive slope, and a VOUT measurement intercept at

the low end of its input signal range.

3.0

2.5

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

–2.5

–3.0

PIN (dBm)

PIN2

PIN1

VOUT2

VOUT1

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

ERROR 25°C

VOUT 25°C

VOUT

IDEAL

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 × log10(VIN/VINTERCEPT) =

X × VSLOPE/dB × 20 × log10(VIN/VINTERCEPT)

(3)

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))

(4)

P(dBV) = 20 × log10(Vrms/1 Vrms)

(5)

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

(6)

For example, PINTERCEPT for a sinusoidal input signal expressed in

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

PINTERCEPT(dBm) =

PINTERCEPT(dBV) – 10 × log10(Z0 × 1 mW/1 Vrms2) =

−100 dBV − 10 × log10(50 × 10−3) = −87 dBm

(7)

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.

40mV/dB

10kΩ

VOUT

VSET

ADL5513

Figure 41. Increasing the Slope

The required resistor values needed to increase the slope are

calculated from the following equation.

Slope

(8)

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.

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