ADL5519ACPZ-R2 datasheet(16/27 Pages) AD

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ADL5519

Preliminary Technical Data

Rev. PrB | Page 16 of 27

In this configuration, all four measurements, OUT[A, B, P,

N], are made available simultaneously. A differential

output can be taken from OUTP − OUTN, and VLVL can

be used to adjust the common-mode level for an ADC

connection.

MEASUREMENT MODE

The ADL5519 requires a single supply of 3.0 V to 5 V. The

supply is connected to the three supply pins, VPSA, VPSB,

and VPSR. Each pin should be decoupled using the two

capacitors with values equal or similar to those shown in

Figure 19. These capacitors must provide a low impedance

over the full frequency range of the input, and they should

be placed as close as possible to the positive supply pins.

Two different capacitors are used in parallel to provide a

broadband ac short to ground.

The device is placed in measurement mode by connecting

OUTA and/or OUTB to VSTA and/or VSTB, respectively. As

seen in Figure 18, the ADL5519 has an offset voltage, a

negative slope, and a VOUT[A,B] measurement intercept at the

high end of its input signal range.

Figure 18. Typical Output Voltage vs. Input Signal, Single Channel

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 form

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

(1)

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

(2)

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 (Figure 18).

VINTERCEPT is +2 dBV for a sinusoidal input signal.

An offset voltage, VOFFSET, of 0.35 V is internally added to

the detector signal, so that the minimum value for VOUT is

X × VOFFSET. So for X = 1, minimum VOUT is 0.35 V.

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

When base-10 logarithms are used, VSLOPE/DECADE represents the

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

VSLOPE/dB represents the slope in volts/dB.

As noted in Equation 1 and Equation 2, the VOUT voltage has a

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

the gain of many power amplifiers in a negative feedback

configuration. Because both the slope and intercept vary slightly

with frequency, it is recommended to refer to the Specifications

section for application-specific values for slope and intercept.

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-

teristic 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))

(3)

P(dBV) = 20 × log10(Vrms/1 Vrms)

(4)

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

(5)

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) =

(6)

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

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

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

+6 dBm

Further information on the intercept variation dependence upon

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

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

their nominal input dynamic range of +10 dBm to −50 dBm,

the output swings from 0.5 V to 1.75 V. The voltages OUTA and

OUTB are also internally applied to a difference amplifier with

a gain of two. So as the dB difference between INA and INB

ranges from approximately −30 dB to +30 dB, the difference

voltage on OUTP and OUTN swings from 0.5 V to 1.75 V.

Input differences larger than ±30 dB can be measured as long as

the absolute input level at INA and INB are within their nominal

ranges of +10 dBm to −50 dBm. However, measurement of large

differences between INA and INB are affected by on-chip signal

leakage. The common-mode level of OUTP and OUTN is set by

the voltage applied to VLVL. These output can be easily biased

up to a common-mode voltage of 2.5 V by connecting VREF to

VLVL. As the gain range is swept, OUTP swings from

approximately 0.5 V to 1.75 V and OUTN swings from 1.75 V

to 0.5 V.

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