ADL5519ACPZ-R2 datasheet(21/27 Pages) AD

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Preliminary Technical Data

ADL5519

Rev. PrB | Page 21 of 27

Once slope and intercept have been calculated, an equation can

be written that will allow calculation of the input power based

on the output voltage of the detector.

PIN (unknown) = (VOUT1(measured)/Slope) + Intercept

The log conformance error of the calculated power is given by

Error (dB) = (VOUT(MEASURED) − VOUT(IDEAL))/Slope

Figure 24 includes a plot of the error at 25°C, the temperature at

which the log amp is calibrated. Note that the error is not zero.

This is because the log amp does not perfectly follow the ideal

VOUT vs. PIN equation, even within its operating region. The

error at the calibration points (−43 dBm and −23 dBm in this

case) will, however, be equal to zero by definition.

Figure 24 also includes error plots for the output voltage at

−40°C and +85 °C. These error plots are calculated using the

slope and intercept at 25°C. This is consistent with calibration

in a mass-production environment, where calibration at

temperature is not practical.

ALTERING THE SLOPE

None of the changes to operating conditions discussed so far

affect the logarithmic slope, VSLOPE, in Equation 7. The slope can

readily be altered by controlling the fraction of OUT[A, B] that

is fed back to the setpoint interface at the VST[A, B] pin. When

the full signal from OUT[A, B] is applied to VST[A, B], the

slope assumes its nominal value of -22 mV/dB. It can be

increased by including a voltage divider between these pins, as

shown in Figure 25. Moderately low resistance values should be

used to minimize scaling errors due to the approximately 40 kΩ

input resistance at the VST[A, B] pin. Keep in mind that this

resistor string also loads the output, and it eventually reduces

the load-driving capabilities if very low values are used.

Equation 17 can be used to calculate the resistor values.

R1 = R2' (SD/-22 − 1)

(17)

where:

SD is the desired slope, expressed in mV/dB.

R2' is the value of R2 in parallel with 40 kΩ.

For example, using R1 = 1.65 kΩ and R2 = 1.69 kΩ (R2' =

1.62 kΩ), the nominal slope is increased to -44 mV/dB.

Operating at a high slope is useful when it is desired to measure

a particular section of the input range in greater detail.

When the slope is raised by some factor, the loop capacitor,

CLP[A, B], should be raised by the same factor to ensure

stability and to preserve a chosen averaging time. The slope can

be lowered by placing a voltage divider after the output pin,

following standard practice.

OUT[A,B]

ADL5519

VST[A,B]

OUT

Figure 25. External Network to Raise Slope

OUTPUT FILTERING

Accurate power detection for signals with RF bursts is achieved

when the ADL5519 is able to respond quickly to the change in

RF power. For applications in which maximum video

bandwidth and, consequently, fast rise time are desired, it is

essential that the CLP[A,B] pin be left unconnected and free of

any stray capacitance.

The nominal output video bandwidth of 50 MHz can be

reduced by connecting a ground-referenced capacitor (CFLT) to

the CLPF pin, as shown in Figure 26. This is generally done to

reduce output ripple (at twice the input frequency for a

symmetric input waveform such as sinusoidal signals).

OUT[A,B]

CLP[A,B]

ADL5519

3.5pF

ILOG[A,B]

CFLT

1.5k

Figure 26. Lowering the Postdemodulation Bandwidth

CFLT is selected using the following equation:

()

−

Bandwidth

Video

FLT

(10)

The video bandwidth should typically be set to a frequency

equal to about one-tenth the minimum input frequency. This

ensures that the output ripple of the demodulated log output,

which is at twice the input frequency, is well filtered.

BASIS FOR ERROR CALCULATIONS

The slope and intercept are derived using the coefficients of a

linear regression performed on data collected in its central

operating range. Error is stated in two forms: (1) error from

linear response to CW waveform and (2) output delta from

25°C performance.

The error from linear response to CW waveform is the decibel

difference in output from the ideal output defined by the

conversions gain and output reference. This is a measure of the

linearity of the device response to both CW and modulated

waveforms. The error in dB is calculated by

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