Data Sheet

ADL5513

THEORY OF OPERATION

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Rev. B | 13 of 25

The ADL5513 is a demodulating logarithmic amplifier, specifically

designed for use in RF measurement and power control applica-

tions at frequencies up to 4 GHz. A block diagram is shown in

Figure 24. Sharing much of its design with the AD8313 logarithmic

detector/controller, the ADL5513 maintains tight intercept variability

vs. temperature over a 80 dB range. Additional enhancements over

the AD8313, such as a reduced RF burst response time of 20 ns

and board space requirements of only 3 mm × 3 mm, add to the low

cost and high performance benefits found in the ADL5513.

Figure 24. Block Diagram

A fully differential design, using a proprietary, high speed SiGe

process, extends high frequency performance. The maximum input

with ±1 dB log conformance error is typically 10 dBm (referred

to 50 Ω). The noise spectral density of −70 dBm sets the lower

limit of the dynamic range. The common pin, COMM, provides a

quality low impedance connection to the printed circuit board (PCB)

ground. The package paddle, which is internally connected to the

COMM pin, should also be grounded to the PCB to reduce thermal

impedance from the die to the PCB.

The logarithmic function is approximated in a piecewise fashion by

cascaded gain stages. (For a more comprehensive explanation of

the logarithm approximation, see the AD8307 data sheet.) Using

precision biasing, the gain is stabilized over temperature and supply

variations. The overall dc gain is high, due to the cascaded nature

of the gain stages.

The RF signal voltages are converted to a fluctuating differential

current having an average value that increases with signal level.

After the detector currents are summed and filtered, the following

function is formed at the summing node:

ID × log10(VIN/VINTERCEPT)

(1)

where:

ID is the internally set detector current.

VIN is the input signal voltage.

VINTERCEPT is the intercept voltage (that is, when VIN = VINTERCEPT,

the output voltage is 0 V, if it were capable of going to 0).