ADL5519ACPZ-R2 datasheet(28/40 Pages) AD

28 / 40 page

ADL5519

Rev. 0 | Page 28 of 40

APPLICATIONS INFORMATION

MEASUREMENT MODE

The ADL5519 is placed in measurement mode by connecting

OUTA, OUTB to VSTA, VSTB, respectively. The part has an offset

voltage, a negative slope, and a VOUTA, VOUTB measurement inter-

cept at the high end of its input signal range.

The output voltage vs. input signal voltage of the ADL5519 is

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

function is of the following form:

VOUT = x × VSLOPE/DEC × log10(VIN/VINTERCEPT) =

(13)

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

(14)

where:

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

VSLOPE/DEC is nominally −440 mV/decade or −22 mV/dB.

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

the VOUT vs. VIN curve.

VINTERCEPT is 2 dBV for a sinusoidal input signal.

An offset voltage, VOFFSET, of 0.45 V is internally added to

the detector signal so that the minimum value for VOUT is

x × VOFFSET. If x = 1, the minimum VOUT value is 0.45 V.

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

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

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

VSLOPE/dB represents the slope in V/dB.

As noted in Equation 13 and Equation 14, the VOUT voltage has

a negative slope. This is also the correct slope polarity to control

the gain of many VGAs in a negative feedback configuration.

Because both the slope and intercept vary slightly with frequency,

see the Specifications section for application-specific values for

slope and intercept.

Although demodulating log amps respond to input signal

voltage and 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))

(15)

P (dBV) = 20 × log10(Vrms/1 Vrms)

(16)

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

(17)

For example, PINTERCEPT, for a sinusoidal input signal expressed

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

PINTERCEPT (dBm) =

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

2 dBV − 10 × log10(50 × 10−3) = 15 dBm

For a square wave input signal in a 200 Ω system

PINTERCEPT (dBm) =

−1 dBV − 10 × log10[(200 Ω × 1 mW/1Vrms2)] = +6 dBm

More information about the intercept variation dependence upon

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

As the input signals to Channel A and Channel B are swept over

their nominal input dynamic range of −5 dBm to −55 dBm, the

output swings from 0.5 V to 1.6 V. The voltages of OUTA, OUTB

are also internally applied to a difference amplifier with a gain

of 1. When the input power is swept, OUTP swings from approxi-

mately 0.5 V to 1.75 V, and OUTN swings from 1.75 V to 0.5 V.

The VLVL pin sets the common-mode voltage for OUTP, OUTN.

An output common-mode voltage of ≤1.15 V can be set using

a resistor divider between the VREF and VLVL pins. Measurement

of large differences between INHA, INHB can be affected by

on-chip signal leakage.

CONTROLLER MODE

In addition to being a measurement device, the ADL5519 can

also be configured to set and control signal levels. Each of the two

log detectors can be separately configured to set and control the

output power level of a VGA or variable voltage attenuator (VVA).

See the Controller Mode section of the AD8317 datasheet for more

information on running a single channel in controller mode.

Alternatively, the two log detectors can be configured to measure

and control the gain of an amplifier or signal chain. The channel

difference outputs can be used to control a feedback loop to the

ADL5519 RF inputs. A capacitor connected between FBKA and

OUTP forms an integrator, keeping in mind that the on-chip 1 kΩ

feedback resistor forms a 0. (The value of the on-chip resistors can

vary as much as ±20% with manufacturing process variation.)

If Channel A is driven and Channel B has a feedback loop from

OUTP through a VGA, OUTP integrates to a voltage value

such that

OUTB = (OUTA + VLVL)/2

(18)

The output value from OUTN may or may not be useful. It is

given by

OUTN = 0 V

(19)

for VLVL < OUTA/3.

Otherwise,

OUTN = (3 × VLVL − OUTA)/2

(20)

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