AD8314

–8–

REV. 0

Table I. Typical Specifications at Selected Frequencies at 25 C (Mean and Sigma)

1 dB Dynamic Range

* – dBV

Slope – mV/dB

Intercept – dBV

High Point

Low Point

Frequency – GHz

0.1

21.3

0.4

–62.2

0.4

–11.8

0.3

–59

0.5

0.9

20.7

0.4

–63.6

0.4

–13.8

0.3

–61.4

0.4

1.9

19.7

0.4

–66.3

0.4

–19

0.7

–64

0.6

2.5

19.2

0.4

–62.1

0.7

–16.4

1.7

–61

1.3

*Refer to Figure 29.

GENERAL DESCRIPTION

The AD8314 is a logarithmic amplifier (log amp) similar in

design to the AD8313; further details about the structure and

function may be found in the AD8313 data sheet and other log

amps produced by Analog Devices. Figure 25 shows the main

features of the AD8314 in block schematic form.

The AD8314 combines two key functions needed for the mea-

surement 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

3.5 GHz. At the output of each of these amplifier stages is a

full-wave rectifier, essentially a square-law detector cell, that

converts the RF signal voltages to a fluctuating current having

an average value that increases with signal level. A further passive

detector stage is added ahead of the first stage. Thus, there are

five detectors, each separated by 10 dB, spanning some 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 law-conformance error, can be judged by

reference to Figure 4, which shows that errors across the central

40 dB are moderate. Other curves show how the conformance

to an ideal logarithmic function varies with supply voltage,

temperature and frequency.

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 logarith-

mic function. This result is then converted to a voltage, at pin

V_UP, through a high-gain stage. In measurement modes, this

output is connected back to a voltage-to-current (V–I) stage, in

such a manner that V_UP 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 AD8314, a given

voltage corresponds to a certain power level.

10dB

OFFSET

COMP'N

V-I

I-V

RFIN

COMM

(PADDLE)

VPOS

X2

ENBL

V DN

V UP

VSET

FLTR

AD8314

10dB

10dB

10dB

BAND-GAP

REFERENCE

DET

DET

DET

DET

DET

Figure 25. Block Schematic

However, in using this part, it must be understood that log amps

do not fundamentally respond to power. It is for this reason that

we use dBV (decibels above 1 V rms) rather than the commonly

used metric of dBm. While the dBV scaling is fixed, independent

of termination impedance, the corresponding power level is not.

For example, 224 mV rms is always –13 dBV (with one further

condition of an assumed sinusoidal waveform; see the Applications

section for more information about the effect of waveform on

logarithmic intercept), and it corresponds to a power of 0 dBm

when the net impedance at the input is 50

Ω. When this imped-

ance is altered to 200

Ω, the same voltage clearly represents a

power level that is four times smaller (P = V

Note that dBV may be converted to dBm for the special case of a

50

Ω system by simply adding 13 dB (0 dBV is equivalent to

+13 dBm).

Thus, the external termination added ahead of the AD8314

determines the effective power scaling. This will often take the

form of a simple resistor (52.3

Ω will provide a net 50 Ω input)

but more elaborate matching networks may be used. This im-

pedance determines the logarithmic intercept, the input power

for which the output would cross the baseline (V_UP = zero) if

the function were continuous for all values of input. Since 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

keep in mind that the quoted values 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,

the use of an rms-responding detector, such as the AD8361,

should be considered.

However, the logarithmic slope, the amount by which the output

V_UP 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 somewhat at higher