ADL5513ACPZ-R2 datasheet(13/28 Pages) AD

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ADL5513

Rev. 0 | Page 13 of 28

THEORY OF OPERATION

The ADL5513 is a demodulating logarithmic amplifier, specifi-

cally designed for use in RF measurement and power control

applications 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.

DET

SLOPE

CONTROL

GAIN

BIAS

BAND GAP

REFERENCE

ADL5513

VOUT

VSET

COMM

TADJ

INHI

INLO

VPOS

CLPF

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).

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