Data Sheet

ADL5506

THEORY OF OPERATION

analog.com

Rev. C | 15 of 22

The ADL5506 is a logarithmic detector (log amp), based on the

principle of successive compression. It is similar in design to the

AD8312 and is fabricated on an advanced BiCMOS process. It

comes in a smaller 0.8 mm × 1.2 mm WLCSP package and offers

4.5 GHz RF bandwidth. Figure 41 shows the main features of the

ADL5506 in block schematic form.

The ADL5506 combines two key functions needed for the measure-

ment of signal level over a moderately wide dynamic range. First,

it provides the amplification needed to respond to small signals in

a chain of four amplifier/limiter cells, each having a small signal

gain of 10 dB and a bandwidth of approximately 4.5 GHz. At the

output of each amplifier stage is a full wave rectifier, essentially a

square law detector cell that converts the RF signal voltages to a

fluctuating current with an average value that increases with signal

level. A further passive detector stage is added preceding the first

stage. Therefore, there are five detectors, each separated by 10

dB, spanning about 50 dB of dynamic range. The overall accuracy

at the extremes of this total range, viewed as the deviation from an

ideal logarithmic response, that is, the log conformance error, can

be judged by referencing Figure 5, Figure 7, and Figure 8, which

show that errors across the central 40 dB are moderate. These

figures show how the conformance to an ideal logarithmic function

varies with temperature, frequency, and supply voltage.

The output of these detector cells is in the form of a differential

current, making their summation a simple matter. It can easily be

shown that such summation closely approximates a logarithmic

function. This result is then converted to a voltage at the VLOG

pin through a high gain stage. This output is connected back to

a voltage-to-current (V-to-I) stage, in such a manner that VLOG is

a logarithmic measure of the RF input voltage with a slope and

intercept controlled by the design. For a fixed termination resistance

at the input of the ADL5506, a given voltage corresponds to a

certain power level.

The external termination added before the ADL5506 determines

the effective power scaling. This often takes the form of a simple

resistor (52.3 Ω provides a net 50 Ω input), but more elaborate

matching networks can be used. This impedance determines the

logarithmic intercept, the input power for which the output crosses

the baseline (VLOG = 0 V) if the function were continuous for all

values of input. Because this is never the case for a practical log

amp, the intercept refers to the value obtained by the minimum

error, straight line fit to the actual graph of VLOG vs. input power.

The quoted values in Table 1 assume a sinusoidal (CW) signal.

Where there is complex modulation, as in CDMA, the calibration of

the power response needs to be adjusted accordingly. Where a true

power (waveform independent) response is needed, consider the

use of an rms responding detector, such as the ADL5504.

However, in terms of the logarithmic slope, the amount by which

the output VLOG changes for each decibel of input change (voltage

or power), is, in principle, independent of waveform or termination

impedance. In practice, it usually falls off at higher frequencies

because of the declining gain of the amplifier stages and other

effects in the detector cells. For the ADL5506, the slope at 30 MHz

is 18.8 mV/dB and falls slightly as frequency increases to about

16.7 mV/dB at 4.5 GHz. These values are sensibly independent of

temperature and almost completely unaffected by supply voltages

of 2.7 V to 5.5 V.

Figure 41. Block Schematic