AD8311 datasheet(11/24 Pages) AD

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AD8311

Rev. 0 | Page 11 of 24

THEORY OF OPERATION

The AD8311 is a wideband logarithmic amplifier (log amp)

similar in design to the AD8313, AD8314, and AD8315. Like

the AD8315, it is strictly optimized for use in power control

applications rather than as a measurement device. Figure 25

shows the main features in block schematic form. The output

(Pin 2, VAPC) is intended to be applied directly to the

automatic power-control (APC) pin of a power amplifier

module.

BASIC THEORY

Logarithmic amplifiers provide a type of compression in which

a signal having a large range of amplitudes is converted to one

of a smaller range. The use of the logarithmic function uniquely

results in the output representing the decibel value of the input.

The fundamental mathematical form is

SLP

OUT

log

(1)

where:

VIN is the input voltage.

VZ is called the intercept (voltage) because when VIN = VZ the

argument of the logarithm is unity and thus the result is zero.

VSLP is called the slope (voltage), which is the amount by which

the output changes for a certain change in the ratio (VIN/VZ).

When BASE-10 logarithms are used, denoted by the function

log10, VSLP represents the volts/decade, and since a decade

corresponds to 20 dB, VSLP/20 represents the volts/dB. For the

AD8311, a nominal (low frequency) slope of 24 mV/dB was

chosen, and the intercept VZ was placed at −59 dBm for a sine

wave input (251 µV rms). However, both the slope and the

intercept are dependent on frequency.

Keeping in mind that log amps do not respond to power but

only to voltages and that the calibration of the intercept is

waveform dependent and is only quoted for a sine wave signal,

the equivalent power response can be written as

(

)

OUT

−

(2)

where the input power

PIN and the equivalent intercept PZ are

both expressed in dBm (thus, the quantity in parentheses is

simply a number of decibels), and

VDB is the slope expressed in

mV/dB. For a log amp having a slope VDB of 24 mV/dB and an

intercept at −59 dBm, the output voltage for an input power of –

30 dBm is 0.024 [−30 − (−57)] = 0.696 V.

Further details about the structure and function of log amps can

be found in data sheets for other log amps produced by Analog

Devices. Refer to the data sheets for the AD640 and AD8307,

both of which include a detailed discussion of the basic

principles of operation and explain why the intercept depends

on waveform, an important consideration when complex

modulation is imposed on an RF carrier.

10dB

INTERCEPT

POSITIONING

10dB

DET

(CURRENT-

NULLING

MODE)

RFIN

(WEAK GM STAGE)

(CURRENT-MODE SIGNAL)

⋅ 1.35

HI-Z

LOW NOISE

RAIL-TO-RAIL BUFFER

(CURRENT-MODE

FEEDBACK)

VSET

23mV/dB

250mV TO

1.4V = 50dB

(SMALL INTERNAL

FILTER CAPACITOR

FOR GHz RIPPLE)

FLTR

VAPC

LOW NOISE

BAND GAP

REFERENCE

LOW NOISE

GAIN BIAS

COM

VPOS

(PRECISE GAIN

CONTROL)

(PRECISE SLOPE

CONTROL)

OFFSET

COMPENSATION

Figure 25. Block Schematic

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