AD8315 datasheet(12/24 Pages) AD

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AD8315

Rev. C | Page 12 of 24

THEORY OF OPERATION

The AD8315 is a wideband logarithmic amplifier (log amp)

similar in design to the AD8313 and AD8314. However, it is

strictly optimized for use in power control applications rather

than as a measurement device. Figure 31 shows the main

features in block schematic form. The output (Pin 7, 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 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)

Here 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, and 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 AD8315, a nominal (low

frequency) slope of 24 mV/dB was chosen, and the intercept VZ

was placed at the equivalent of −70 dBV for a sine wave input

(316 μV rms). This corresponds to a power level of −57 dBm

when the net resistive part of the input impedance of the log

amp is 50 Ω. However, both the slope and the intercept are

dependent on frequency (see Figure 15 and Figure 18).

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

VOUT

= VDB (PIN − PZ)

(2)

where:

, the input power, and PZ, the equivalent intercept, are both

expressed in dBm (thus, the quantity in parentheses is simply a

number of decibels).

VDB

is the slope expressed as so many mV/dB.

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

at −57 dBm, the output voltage for an input power of –30 dBm

is 0.024 [−30 − (−57)] = 0.648 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, Inc. Refer to the AD640 data sheet and AD8307 data

sheet, 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.

VPOS

ENBL

RFIN

COMM

(PADDLE)

DET

10dB

DET

10dB

DET

10dB

OFFSET

COMP’N

INTERCEPT

POSITIONING

LOW NOISE

GAIN BIAS

LOW NOISE

BAND GAP

REFERENCE

OUTPUT

ENABLE

DELAY

×1.35

VAPC

HI-Z

LOW NOISE (25nV/√Hz)

RAIL-TO-RAIL BUFFER

FLTR

VSET

V-I

23mV/dB

250mV TO

1.4V = 50dB

(CURRENT-

NULLING

MODE)

(WEAK GM STAGE)

(CURRENT-MODE SIGNAL)

(CURRENT-MODE

FEEDBACK)

(SMALL INTERNAL

FILTER CAPACITOR

FOR GHz RIPPLE)

(ELIMINATES

GLITCH)

(PRECISE SLOPE

CONTROL)

(PRECISE GAIN

CONTROL)

Figure 31. Block Schematic

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