REV. 0

Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.

AD8314

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700

World Wide Web Site: http://www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 1999

100 MHz2500 MHz 45 dB

RF Detector/Controller

FEATURES
Complete RF Detector/Controller Function
Typical Range 58 dBV to 13 dBV

45 dBm to 0 dBm re 50

Frequency Response from 100 MHz to 2.5 GHz
Temperature-Stable Linear-in-dB Response

Accurate to 2.5 GHz

Rapid Response: 70 ns to a 10 dB Step
Low Power: 12 mW at 2.7 V
Power-Down to 20 A

APPLICATIONS
Cellular Handsets (TDMA, CDMA, GSM)
RSSI and TSSI for Wireless Terminal Devices
Transmitter Power Measurement and Control

PRODUCT DESCRIPTION

The AD8314 is a complete low-cost subsystem for the mea-
surement and control of RF signals in the frequency range
0.1 GHz2.5 GHz, with a typical dynamic range of 45 dB,
intended for use in a wide variety of cellular handsets and other
wireless devices. It provides a wider dynamic range and better
accuracy than possible using discrete diode detectors. In particular,
its temperature stability is excellent over the full operating range of
30

C to +85

Its high sensitivity allows control at low power levels, thus
reducing the amount of power that needs to be coupled to the
detector. It is essentially a voltage-responding device, with a
typical signal range of 1.25 mV to 224 mV rms or 58 dBV to
13 dBV. This is equivalent to 45 dBm to 0 dBm re 50

For convenience, the signal is internally ac-coupled, using a 5 pF
capacitor to a load of 3 k

in shunt with 2 pF. This high-pass

coupling, with a corner at 16 MHz, determines the lowest oper-
ating frequency. Thus, the source may be dc-grounded.

The AD8314 provides two voltage outputs. The first, called
V_UP, increases from close to ground to about 1.2 V as the
input signal level increases from 1.25 mV to 224 mV. This output
is intended for use in measurement mode. Consult the Appli-
cations section of this data sheet for information on use in this
mode. A capacitor may be connected between the V_UP and
FLTR pins when it is desirable to increase the time interval over
which averaging of the input waveform occurs.

The second output, V_DN, is an inversion of V_UP, but with
twice the slope and offset by a fixed amount. This output starts
at about 2.25 V (provided the supply voltage is

3.3 V) for

the minimum input and falls to a value close to ground at the
maximum input. This output is intended for analog control
loop applications. A setpoint voltage is applied to VSET and
V_DN is then used to control a VGA or power amplifier. Here
again, an external filter capacitor may be added to extend the
averaging time. Consult the Applications section of this data
sheet for information on use in this mode.

The AD8314 is available in a micro_SOIC package and con-
sumes 4.5 mA from a 2.7 V to 5.5 V supply. When powered
down, the typical sleep current is 20

FUNCTIONAL BLOCK DIAGRAM

10dB

OFFSET
COMP'N

V-I

I-V

RFIN

COMM

(PADDLE)

VPOS

ENBL

V DN

V UP

VSET

FLTR

AD8314

10dB

BAND-GAP

REFERENCE

DET

REV. 0

AD8314SPECIFICATIONS

= 3 V, T

= +25 C, unless otherwise noted)

Parameter

Condition

Min

Typ

Max

Unit

OVERALL FUNCTION

Frequency Range

To Meet All Specifications

0.1

2.5

GHz

Input Voltage Range

Internally AC-Coupled

1.25

224

mV rms

Equivalent Power Range

52.3

External Termination

dBm

Logarithmic Slope

Main Output, V_UP, 100 MHz

18.85

21.3

23.35

mV/dB

Logarithmic Intercept

Main Output, V_UP, 100 MHz

dBV

Equivalent dBm Level

52.3

External Termination

dBm

INPUT INTERFACE

(Pin RFIN)

DC Resistance to COMM

100

Inband Input Resistance

f = 0.1 GHz

Input Capacitance

f = 0.1 GHz

MAIN OUTPUT

(Pin V_UP)

Voltage Range

V_UP Connected to VSET

0.01

1.2

Minimum Output Voltage

No Signal at RFIN, R

10 k

0.01

0.02

0.05

Maximum Output Voltage

10 k

1.9

General Limit

2.7 V

5.5 V

1.1

Available Output Current

Sourcing/Sinking

1/0.5

2/1

Response Time

10%90%, 10 dB Step

Residual RF (at 2f)

f = 0.1 GHz (Worst Condition)

100

INVERTED OUTPUT

(Pin V_DN)

Gain Referred to V_UP

= 2.25 V 2

Minimum Output Voltage

3.3 V

0.01

0.05

0.1

Maximum Output Voltage

3.3 V

2.1

2.2

2.5

Available Output Current

Sourcing/Sinking

4/100

6/200

mA/

Output-Referred Noise

RF Input = 2 GHz, 33 dBV, f

NOISE

= 10 kHz

1.05

Response Time

10%90%, 10 dB Input Step

Full-Scale Settling Time

40 dBm to 0 dBm Input Step, to 95%

150

SETPOINT INPUT

(Pin VSET)

Voltage Range

Corresponding to Central 40 dB

0.15

1.2

Input Resistance

Logarithmic Scale Factor

f = 0.900 GHz

20.7

mV/dB

f = 1.900 GHz

19.7

mV/dB

ENABLE INTERFACE

(Pin ENBL)

Logic Level to Enable Power

HI Condition, 30

+85

1.6

POS

Input Current when HI

2.7 V at ENBL, 30

+85

300

Logic Level to Disable Power

LO Condition, 30

+85

0.5

0.8

POWER INTERFACE

(Pin VPOS)

Supply Voltage

2.7

3.0

5.5

Quiescent Current

3.0

4.5

5.7

Over Temperature

+85

2.7

4.4

6.6

Total Supply Current when Disabled

Over Temperature

+85

NOTES

Mean and Standard Deviation specifications are available in Table I.

Increased output possible when using an attenuator between V_UP and VSET to raise the slope.

Refer to Figure 19 for details.

Specifications subject to change without notice.

AD8314

REV. 0

ABSOLUTE MAXIMUM RATINGS

Supply Voltage VPOS . . . . . . . . . . . . . . . . . . . . . . . . . . .5.5 V
V_UP, V_DN, VSET, ENBL . . . . . . . . . . . . . . . . 0 V, VPOS
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 V rms
Equivalent Power . . . . . . . . . . . . . . . . . . . . . . . . . . . +17 dBm
Internal Power Dissipation . . . . . . . . . . . . . . . . . . . . 200 mW

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

Maximum Junction Temperature . . . . . . . . . . . . . . . . . 125

Operating Temperature Range . . . . . . . . . . . 30

C to +85

Storage Temperature Range . . . . . . . . . . . . 65

C to +150

Lead Temperature Range (Soldering 60 sec) . . . . . . . . . 300

*Stresses above those listed under Absolute Maximum Ratings may cause permanent

damage to the device. This is a stress rating only; functional operation of the device
at these or any other conditions above those indicated in the operational section
of this specification is not implied. Exposure to absolute maximum rating conditions
for extended periods may affect device reliability.

CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although
the AD8314 features proprietary ESD protection circuitry, permanent damage may occur on
devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are
recommended to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

Pin Function Descriptions

Pin

Name

Function

RFIN

RF Input.

ENBL

Connect pin to V

for normal operation.

Connect pin to ground for disable mode.

VSET

Setpoint input for operation in controller
mode. To operate in detector mode connect
VSET to V_UP

FLTR

Connection for an external capacitor to slow
the response of the output. Capacitor is con-
nected between FLTR and V_UP.

COMM

Device Common (Ground).

V_UP

Logarithmic output. Output voltage increases
with increasing input amplitude.

V_DN

Inversion of V_UP, governed by the following
equation: V_DN = 2.25 V 2

VPOS

Positive supply voltage (V

), 2.7 V to 5.5 V.

PIN CONFIGURATION

TOP VIEW

(Not to Scale)

RFIN

ENBL

VSET

VPOS

V DN

V UP

COMM

FLTR

AD8314

ORDERING GUIDE

Model

Temperature Range

Package Description

Package Option

AD8314ARM

C to +85

Tube, 8-Lead micro_SOIC

RM-8

AD8314ARM-REEL

13" Tape and Reel

AD8314ARM-REEL7

7" Tape and Reel

AD8314-EVAL

Evaluation Board

*Device branded as J5A.

AD8314

REV. 0

INPUT AMPLITUDE dBV

1.2

75

65

 Volts

55

45

35

25

15

1.0

0.8

0.6

0.4

0.2

(52dBm)

(2dBm)

2.5GHz

1.9GHz

0.9GHz

0.1GHz

Figure 1. V

vs. Input Amplitude

INPUT AMPLITUDE dBV

1.2

70

60

(47dBm)

 Volts

50

40

30

20

10

(+3dBm)

1.0

0.8

0.6

0.4

0.2

30 C

+85 C

+25 C

30 C

SLOPE AND INTERCEPT
NORMALIZED AT +25 C AND
APPLIED TO 30 C AND +85 C

ERROR dB

Figure 2. V

and Log Conformance vs. Input Amplitude at

0.1 GHz; 30

C, +25

C, and +85

INPUT AMPLITUDE dBV

1.2

70

60

(47dBm)

 Volts

50

40

30

20

10

(+3dBm)

1.0

0.8

0.6

0.4

0.2

30 C

+85 C

+25 C

SLOPE AND INTERCEPT
NORMALIZED AT +25 C AND
APPLIED TO 30 C AND +85 C

ERROR dB

Figure 3. V

and Log Conformance vs. Input Amplitude

at 0.9 GHz; 30

C, +25

C, and +85

Typical Performance Characteristics

INPUT AMPLITUDE dBV

70

60

ERROR dB

50

40

30

20

10

2.5GHz

1.9GHz

0.9GHz

(47dBm)

(+3dBm)

0.1GHz

Figure 4. Log Conformance vs. Input Amplitude

INPUT AMPLITUDE dBV

1.2

70

60

(47dBm)

 Volts

50

40

30

20

10

(+3dBm)

1.0

0.8

0.6

0.4

0.2

30 C

+85 C

+25 C

SLOPE AND INTERCEPT
NORMALIZED AT +25 C AND
APPLIED TO 30 C AND +85 C

ERROR dB

Figure 5. V

and Log Conformance vs. Input Amplitude

at 1.9 GHz; 30

C, +25

C, and +85

INPUT AMPLITUDE dBV

1.2

70

60

(47dBm)

 Volts

50

40

30

20

10

(+3dBm)

1.0

0.8

0.6

0.4

0.2

30 C

+85 C

+25 C

SLOPE AND INTERCEPT
NORMALIZED AT +25 C AND
APPLIED TO 30 C AND +85 C

ERROR dB

+85 C

Figure 6. V

and Log Conformance vs. Input Amplitude

at 2.5 GHz; 30

C, +25

C, and +85

AD8314

REV. 0

FREQUENCY GHz

0.5

SLOPE mV/dB

1.0

30 C

+85 C

+25 C

1.5

2.0

2.5

Figure 7. Slope vs. Frequency; 30

C, +25

C, and +85

 Volts

2.5

SLOPE mV/dB

2.5GHz

1.9GHz

0.9GHz

0.1GHz

3.0

3.5

4.0

4.5

5.0

5.5

Figure 8. V

Slope vs. Supply Voltage

FREQUENCY GHz

0.5

1.0

1.5

2.0

2.5

500

1000

1500

2000

2500

3000

3500

RESISTANCE 

200

400

600

800

1000

1200

1400

|| - jX
|| - j748
|| - j106
|| - j80
|| - j141

3030

760
301

FREQUENCY (GHz)
0.1
0.9
1.9
2.5

REACTANCE 

Figure 9. Input Impedance

FREQUENCY GHz

0.5

1.0

75

30 C

+85 C

+25 C

1.5

2.0

2.5

70

65

60

55

INTERCEPT dBV

Figure 10. V

Intercept vs. Frequency: 30

C, +25

C, and

+85

 Volts

67

2.5

INTERCEPT dBV

2.5GHz

1.9GHz

0.9GHz

0.1GHz

3.0

3.5

4.0

4.5

5.0

5.5

66

65

64

63

62

61

Figure 11. V

Intercept vs. Supply Voltage

ENBL

 Volts

0.2

SUPPLY CURRENT mA

INCREASING
V

ENBL

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2 2.4

2.6

DECREASING

ENBL

Figure 12. Supply Current vs. ENBL Voltage, V

= 3 V

AD8314

REV. 0

1 s PER
HORIZONTAL
DIVISION

ENBL

5V PER VERTICAL DIVISION

GND

ENBL

GND

500mV/VERT. DIV.

AVERAGE: 128 SAMPLES

Figure 13. ENBL Response Time

ENBL

RFIN

AD8314

RF OUT

TEK

TDS784C

SCOPE

TRIG
OUT

HP8116A

PULSE

GENERATOR

10MHz REF OUTPUT

EXT TRIG

NC = NO CONNECT

0.1 F

VSET

FLTR

V DN

VPOS

COMM

V UP

TEK P6204

FET PROBE

TEK P6204

FET PROBE

3.0V

PULSE OUT

TRIG

52.3

33dBV

HP8648B

SIGNAL

GENERATOR

Figure 14. Test Setup for ENBL Response Time

FREQUENCY Hz

AMPLITUDE dB

PHASE De

rees

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

100

10k

100k

10M

Figure 15. AC Response from VSET to V_DN

200mV PER
VERTICAL
DIVISION

100ns PER
HORIZONTAL
DIVISION

RF INPUT

AVERAGE: 128 SAMPLES

PULSED RF
0.1GHz, 13dBV

GND

500mV/

VERT. DIV.

1V/VERT. DIV.

Figure 16. V

and V

Response Time, 40 dBm to 0 dBm

ENBL

RFIN

AD8314

RF OUT

TEK

TDS784C

SCOPE

TRIG
OUT

PICOSECOND

PULSE LABS

PULSE

GENERATOR

HP8648B

SIGNAL

GENERATOR

PULSE

MODULATION

MODE

10MHz REF OUTPUT

EXT TRIG

NC = NO CONNECT

0.1 F

VSET

FLTR

V DN

VPOS

COMM

V UP

TEK P6204

FET PROBE

TEK P6204

FET PROBE

3.0V

TRIG

52.3

OUT

PULSE MODE IN

3dB

3.0V

SPLITTER

TEK P6204

FET PROBE

3dB

Figure 17. Test Setup for Pulse Response

NOISE SPECTRAL DENSITY 

V/ Hz

FREQUENCY Hz

10.0

0.1

100

1.0

10k

100k

10M

60dBm

40dBm

30dBm

20dBm

RF INPUT
70dBm

50dBm

Figure 18. V

Noise Spectral Density

AD8314

REV. 0

 Volts

2.3

1.7

2.7

 V

2.2

2.1

2.0

1.9

1.8

2.8

2.9

3.0

3.1

3.2

3.3

3.4

3.5

0mA

2mA

4mA

6mA

Figure 19. Maximum V

Voltage vs. V

by Load Current

1 s PER
HORIZONTAL
DIVISION

VPOS AND ENABLE

2V PER
VERTICAL
DIVISION

500mV/VERT. DIV.

AVERAGE: 128 SAMPLES

GND

Figure 20. Power-On and -Off Response, Measurement
Mode

HP8648B

SIGNAL

GENERATOR

ENBL

RFIN

AD8314

RF OUT

TEK

TDS784C

SCOPE

TRIG
OUT

HP8116A

PULSE

GENERATOR

10MHz REF OUTPUT

EXT TRIG

NC = NO CONNECT

VSET

FLTR

V DN

VPOS

COMM

V UP

TEK P6204

FET PROBE

TEK P6204

FET PROBE

TRIG

52.3

PULSE
OUT

49.9

AD811

732

33dBV

Figure 21. Test Setup for Power-On and -Off Response

 Volts

2.3

1.7

2.7

 V

2.2

2.1

2.0

1.9

1.8

2.8

2.9

3.0

3.1

3.2

3.3

3.4

3.5

SHADING INDICATES

3 SIGMA

Figure 22. Maximum V

Voltage vs. V

with 3 mA Load

100ns PER
HORIZONTAL
DIVISION

200mV PER
VERTICAL
DIVISION

AVERAGE: 128 SAMPLES

2V PER
VERTICAL
DIVISION

VPOS AND ENABLE

GND

Figure 23. Power-On Response, V

, Controller Mode with

VSET Held Low

ENBL

RFIN

AD8314

RF OUT

TEK

TDS784C

SCOPE

TRIG
OUT

HP8112A

PULSE

GENERATOR

10MHz REF OUTPUT

EXT TRIG

NC = NO CONNECT

VSET

FLTR

V DN

VPOS

COMM

V UP

TEK P6204

FET PROBE

TRIG

52.3

+0.2

AD811

PULSE
OUT

49.9

732

HP8648B

SIGNAL

GENERATOR

Figure 24. Test Setup for Power-On Response at V_DN
Output, Controller Mode with VSET Pin Held Low

AD8314

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

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

0.7

0.6

2.5

19.2

0.4

62.1

0.7

16.4

1.7

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 (VI) 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

ENBL

V DN

V UP

VSET

FLTR

AD8314

10dB

BAND-GAP

REFERENCE

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

. When this imped-

ance is altered to 200

, the same voltage clearly represents a

power level that is four times smaller (P = V

/R), that is, 6 dBm.

Note that dBV may be converted to dBm for the special case of a
50

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

will provide a net 50

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
actual graph of V_UP versus P

(more generally, V

). Again,

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

AD8314

REV. 0

frequencies, due to the declining gain of the amplifier stages and
other effects in the detector cells. For the AD8314, the slope
at low frequencies is nominally 21.3 mV/dB, falling almost linearly
with frequency to about 19.2 mV/dB at 2.5 GHz. These values
are sensibly independent of temperature (see Figure 7) and
almost totally unaffected by the supply voltage from 2.7 V to
5.5 V (Figure 8).

Inverted Output

The second provision is the inclusion of an inverting amplifier
to the output, for use in controller applications. Most power
amplifiers require a gain-control bias that must decrease from a
large positive value toward ground level as the power output is
required to decrease. This control voltage, which appears at the pin
V_DN, is not only of the opposite polarity to V_UP, but also
needs to have an offset added in order to determine its most posi-
tive value when the power level (assumed to be monitored through
a directional coupler at the output of the PA) is minimal.

The starting value of V_DN is nominally 2.25 V, and it falls
on a slope of twice that of V_UP, in other words, 43 mV/dB.
Figure 26 shows how this is achieved: the reference voltage
that determines the maximum output is derived from the on-
chip voltage reference, and is substantially independent of the
supply voltage or temperature. However, the full output cannot
be attained for supply voltages under 3.3 V; Figure 19 shows
this dependency. The relationship between V_UP and V_DN is
shown in Figure 27.

VI

BAND-GAP

REFERENCE

VSET

FLTR

IV

1.125V

= 2.25V 2.0 V_UP

CURRENTS FROM

DETECTORS

AD8314

V_UP

V_DN

Figure 26. Output Interfaces

INPUT AMPLITUDE dBV

60

VOLTS

2.5

2.0

1.5

1.0

0.5

OUTPUT FOR
PA CONTROL

50

40

30

20

10

OUTPUT FOR
MEASUREMENT

V_UP

V_DN

Figure 27. Showing V_UP and V_DN Relationship

APPLICATIONS
Basic Connections

Figure 28 shows connections for the basic measurement mode.
A supply voltage of 2.7 V to 5.5 V is required. The supply to
the VPOS pin should be decoupled with a low inductance 0.1

surface mount ceramic capacitor. A series resistor of about 10

may be added; this resistor will slightly reduce the supply voltage to
the AD8314 (maximum current into the VPOS pin is approxi-
mately 9 mA when V_DN is delivering 5 mA). Its use should be
avoided in applications where the power supply voltage is very
low (i.e., 2.7 V). A series inductor will provide similar power
supply filtering with minimal drop in supply voltage.

ENBL

RFIN

AD8314

VSET

FLTR

V DN

VPOS

COMM

V UP

0.1 F

OPTIONAL
(SEE TEXT)

52.3

INPUT

Figure 28. Basic Connections for Operation in
Measurement Mode

The ENBL pin is here connected to VPOS. The AD8313 may
be disabled by pulling this pin to ground when the chip current
is reduced to about 20

A from its normal value of 4.5 mA.

The logic threshold is around +V

/2 and the enable function

occurs in about 1.5

s. Note, however, further settling time is

generally needed at low input levels.

The AD8314 has an internal input coupling capacitor. This
eliminates the need for external ac-coupling. A broadband input
match is achieved in this example by connecting a 52.3

resis-

tor between RFIN and ground. This resistance combines with
the internal input impedance of approximately 3 k

to give

an overall broadband input resistance of 50

. Several other

coupling methods are possible; these are described in the Input
Coupling section.

The measurement mode is selected by connecting VSET to V_UP,
which establishes a feedback path and sets the logarithmic slope
to its nominal value. The peak voltage range of the measurement
extends from 58 dBV to 13 dBV at 0.9 GHz, and only slightly
less at higher frequencies up to 2.5 GHz. Thus, using the 50

termination, the equivalent power range is 45 dBm to 0 dBm.
At a slope of 21.5 mV/dB, this would amount to an output span
of 967 mV. Figure 29 shows the transfer function for V_UP at a
supply voltage of 3 V, and input frequency of 0.9 GHz.

V_DN, which will generally not be used when the AD8314 is
used in the measurement mode, is essentially an inverted version
of V_UP. The voltage on V_UP and V_DN are related by the
equation.

= 2.25 V 2 V

While V_DN can deliver up to 6 mA, the load resistance on V_UP
should not be lower than 10 k

in order that the full-scale output

of 1 V can be generated with the limited available current of
200

A max. Figure 29 shows the logarithmic conformance

under the same conditions.

AD8314

REV. 0

INPUT AMPLITUDE dBV

1.2

70

60

(47dBm)

 Volts

50

40

30

20

10

(+3dBm)

1.0

0.8

0.6

0.4

0.2

= 3V

= 52.3

1dB DYNAMIC RANGE

ERROR dB

3dB DYNAMIC RANGE

INTERCEPT

Figure 29. V

and Log Conformance Error vs. Input

Level vs. Input Level at 900 MHz

Transfer Function in Terms of Slope and Intercept

The transfer function of the AD8314 is characterized in terms of
its Slope and Intercept. The logarithmic slope is defined as the
change in the RSSI output voltage for a 1 dB change at the input.
For the AD8314, slope is nominally 21.5 mV/dB. So a 10 dB
change at the input results in a change at the output of approxi-
mately 215 mV. The plot of Log-Conformance (Figure 29) shows
the range over which the device maintains its constant slope. The
dynamic range can be defined as the range over which the error
remains within a certain band, usually

1 dB or

3 dB. In

Figure 29, for example, the

1 dB dynamic range is approxi-

mately 50 dB (from 13 dBV to 63 dBV).

The intercept is the point at which the extrapolated linear
response would intersect the horizontal axis (Figure 29). Using
the slope and intercept, the output voltage can be calculated for
any input level within the specified input range using the equation:

= V

SLOPE

 P

)

where V

is the demodulated and filtered RSSI output, V

SLOPE

is the logarithmic slope, expressed in V/dB, P

is the input sig-

nal, expressed in decibels relative to some reference level (either
dBm or dBV in this case) and P

is the logarithmic intercept,

expressed in decibels relative to the same reference level.

For example, at an input level of 40 dBV (27 dBm), the
output voltage will be

OUT

= 0.020 V/dB

(40 dBV (63 dBV )) = 0.46 V

dBV vs. dBm

The most widely used convention in RF systems is to specify power
in dBm, that is, decibels above 1 mW in 50

. Specification of

log amp input levels in terms of power is strictly a concession to
popular convention; they do not respond to power (tacitly "power
absorbed at the input"), but to the input voltage. The use of dBV,
defined as decibels with respect to a 1 V rms sine wave, is more pre-
cise, although this is still not unambiguous because waveform is
also involved in the response of a log amp, which, for a complex
input (such as a CDMA signal), will not follow the rms value
exactly. Since most users specify RF signals in terms of power--
more specifically, in dBm/50

--we use both dBV and dBm in

specifying the performance of the AD8314, showing equivalent
dBm levels for the special case of a 50

environment. Values in

dBV are converted to dBm re 50

by adding 13.

Filter Capacitor

The video bandwidth of both V_UP and V_DN is approximately
3.5 MHz. In CW applications where the input frequency is much
higher than this, no further filtering of the demodulated signal
will be required. Where there is a low-frequency modulation of
the carrier amplitude, however, the low-pass corner must be
reduced by the addition of an external filter capacitor, C

(see

Figure 28). The video bandwidth is related to C

by the equation

Video Bandwidth

4 4

(

)

Operating in Controller Mode

Figure 30 shows the basic connections for operation in the con-
troller mode and Figure 31 shows a block diagram of a typical
controller mode application. The feedback from V_UP to VSET is
broken and the desired setpoint voltage is applied to VSET from
the controlling source (often this will be a DAC). V

will rail

high (2.2 V on a 3.3 V supply, 1.9 V on a 2.7 V supply) when
the applied power is less than the value corresponding to the set-
point voltage. When the input power slightly exceeds this value,
V

would, in the absence of the loop via the power amplifier

gain pin, decrease rapidly toward ground. In the closed loop,
however, the reduction in V

causes the power amplifier to re-

duce its output. This restores a balance between the actual power
level sensed at the input of the AD8314 and the demanded value
determined by the setpoint. This assumes that the gain control
sense of the variable gain element is positive, that is, an increas-
ing voltage from V_DN will tend to increase gain. The output
swing and current sourcing capability of V_DN are shown in
Figures 19 and 20.

ENBL

RFIN

AD8314

VSET

FLTR

V DN

VPOS

COMM

V UP

VDN

INPUT

VSET

0.1 F

52.3

Figure 30. Basic Connections for Operation in Controller
Mode

DAC

FLTR

V UP

VSET

AD8314

DIRECTIONAL
COUPLER

POWER

AMPLIFIER

RF INPUT

GAIN
CONTROL
VOLTAGE

RFIN

V DN

52.3

Figure 31. Typical Controller Mode Application

AD8314

REV. 0

The relationship between the input level and the setpoint voltage
follows from the nominal transfer function of the device (V

vs.

Input Amplitude, see Figure 1). For example, a voltage of 1 V
on VSET is demanding a power level of 0 dBm at RFIN. The cor-
responding power level at the output of the power amplifier will be
greater than this amount due to the attenuation through the direc-
tional coupler.

When connected in a PA control loop, as shown in Figure 31,
the voltage V

is not explicitly used, but is implicated in again

setting up the required averaging time, by choice of C

. However,

now the effective loop response time is a much more complicated
function of the PA's gain-control characteristics, which are very
nonlinear. A complete solution requires specific knowledge of
the power amplifier.

The transient response of this control loop is determined by the
filter capacitor, C

. When this is large, the loop will be uncon-

ditionally stable (by virtue of the "dominant pole" generated
by this capacitor), but the response will be sluggish. The minimum
value ensuring stability should be used, requiring full attention
to the particulars of the power amplifier control function. Because
this is invariably nonlinear, the choice must be made for the
worst-case condition, which usually corresponds to the smallest
output from the PA, where the gain function is steepest. In practice,
an improvement in loop dynamics can often be achieved by adding
a response zero, formed by a resistor in series with C

Power-On and Enable Glitch

As already mentioned, the AD8314 can be put into a low power
mode by pulling the ENBL pin to ground. This reduces the quies-
cent current from 4.5 mA to 20

A. Alternatively, the supply can

be turned off completely to eliminate the quiescent current. Figures
13 and 23 show the behavior of the V_DN output under these
two conditions (in Figure 23, ENBL is tied to VPOS). The glitch
that results in both cases can be reduced by loading the V_DN
output.

Input Coupling Options

The internal 5 pF coupling capacitor of the AD8314, along with
the low frequency input impedance of 2 k

give a high-pass input

corner frequency of approximately 16 MHz. This sets the mini-
mum operating frequency. Figure 32 shows three options for
input coupling. A broadband resistive match can be implemented
by connecting a shunt resistor to ground at RFIN (Figure 32a).
This 52.3

resistor (other values can also be used to select dif-

ferent overall input impedances) resistor combines with the
input impedance of the AD8314 (3 k

2 pF) to give a broad-

band input impedance of 50

. While the input resistance and

capacitance (C

and R

) will vary by approximately

20% from

device to device, the dominance of the external shunt resistor
means that the variation in the overall input impedance will
be close to the tolerance of the external resistor.

At frequencies above 2 GHz, the input impedance drops below
250

(see Figure 9), so it is appropriate to use a larger value of

shunt resistor. This value is calculated by plotting the input
impedance (resistance and capacitance) on a Smith Chart and
choosing the best value of shunt resistor to bring the input imped-
ance closest to the center of the chart. At 2.5 GHz, a shunt
resistor of 165

is recommended.

A reactive match can also be implemented as shown in Figure
32b. This is not recommended at low frequencies as device toler-
ances will dramatically vary the quality of the match because of

the large input resistance. For low frequencies, Option a or
Option c (see below) are recommended.

In Figure 32b, the matching components are drawn as general
reactances. Depending on the frequency, the input impedance at
that frequency and the availability of standard value components,
either a capacitor or an inductor will be used. As in the previous
case, the input impedance at a particular frequency is plotted on
a Smith Chart and matching components are chosen (shunt
or series L, shunt or series C) to move the impedance to the
center of the chart. Table II gives standard component values
for some popular frequencies. Matching components for other
frequencies can be calculated using the input resistance and
reactance data over frequency which is given in Figure 9. Note
that the reactance is plotted as though it appears in parallel with
the input impedance (which it does because the reactance is prima-
rily due to input capacitance).

The impedance matching characteristics of a reactive matching
network provide voltage gain ahead of the AD8314; this increases
the device sensitivity (see Table II). The voltage gain is calculated
using the equation:

Voltage Gain

log

where R2 is the input impedance of the AD8314 and R1 is the
source impedance to which the AD8314 is being matched. Note
that this gain will only be achieved for a perfect match. Component
tolerances and the use of standard values will tend to reduce
the gain.

SHUNT

52.3

AD8314

SOURCE

RFIN

BIAS

a. Broadband Resistive

SOURCE

AD8314

RFIN

BIAS

b. Narrowband Reactive

AD8314

RFIN

BIAS

ATTN

STRIPLINE

c. Series Attenuation

Figure 32. Input Coupling Options

Figure 32c shows a third method for coupling the input signal
into the AD8314, applicable in applications where the input signal
is larger than the input range of the log amp. A series resistor,
connected to the RF source, combines with the input impedance

AD8314

REV. 0

of the AD8314 to resistively divide the input signal being applied
to the input. This has the advantage of very little power being
"tapped off" in RF power transmission applications.

Table II. Recommended Components for X1 and X2 in
Figure 32b

Frequency

Voltage Gain

(GHz)

(dB)

0.1

Short

52.3

0.9

33 nH

39 nH

11.8

1.9

10 nH

15 nH

7.8

2.5

1.5 pF

3.9 nH

2.55

Increasing the Logarithmic Slope in Measurement Mode

The nominal logarithmic slope of 21.5 mV/dB (see Figure 7 for
the variation of slope with frequency) can be increased to an
arbitrarily high value by attenuating the signal between V_UP
and VSET as shown in Figure 33. The ratio R1/R2 is set using
the equation

R1/R2 = (New Slope/Original Slope) 1

In the example shown, two 5 k

resistors combine to change the

slope at 1900 MHz from 20 mV/dB to 40 mV/dB. The slope can
be increased to higher levels. This will, however, reduce the usable
dynamic range of the device.

AD8314

R1
5k

V_UP

VSET

40mV/dB
@ 1900MHz

R2
5k

Figure 33. Increasing the Output Slope

Effect of Waveform Type on Intercept

Although specified for input levels in dBm (dB relative to 1 mW),
the AD8314 fundamentally responds to voltage and not to power.
A direct consequence of this characteristic is that input signals of
equal rms power but differing crest factors will produce different
results at the log amp's output.

The effect of differing signal waveforms is to shift the effective
value of the intercept upwards or downwards. Graphically, this
looks like a vertical shift in the log amp's transfer function. The
logarithmic slope, however, is not affected. For example, consider
the case of the AD8314 being alternately fed by an unmodulated
sine wave and by a single CDMA channel of the same rms power.
The AD8314's output voltage will differ by the equivalent of
3.55 dB (70 mV) over the complete dynamic range of the device
(the output for a CDMA input being lower).

Table III shows the correction factors that should be applied to
measure the rms signal strength of a various signal types. A
sine wave input is used as a reference. To measure the rms power
of a square wave, for example, the mV equivalent of the dB value
given in the table (20 mV/dB times 3.01 dB) should be subtracted
from the output voltage of the AD8314.

Table III. Shift in AD8314 Output for Signals with Differing
Crest Factors

Correction Factor
(Add to Measured

Signal Type

Input Level)

Sine Wave

0 dB

Square Wave

3.01 dB

GSM Channel (All Time Slots On)

0.55 dB

CDMA Channel (Forward Link,

3.55 dB

9 Channels On)

CDMA Channel (Reverse Link)

0.5 dB

PDC Channel (All Time Slots On)

0.58 dB

Mobile Handset Power Control Examples

Figure 34 shows a complete power amplifier control circuit for
a dual mode handset. This circuit is applicable to any dual
mode handset using TDMA or CDMA technologies. The
PF08107B (Hitachi) is driven by a nominal power level of
+3 dBm. Some of the output power from the PA is coupled off
using an LDC15D190A0007A (Murata) directional coupler.
This has a coupling factor of approximately 19 dB for its lower
frequency band (897.5

17.5 MHz) and 14 dB for its upper band

(1747.5

37.5 MHz) and an insertion loss of 0.38 dB and 0.45 dB

respectively. Because the PF08107B transmits a maximum power
level of +35 dBm, additional attenuation of 15 dB is required
before the coupled signal is applied to the AD8314.

ENBL

RFIN

AD8314

VSET

FLTR

VPOS

COMM

V UP

2.7V

VSET

0V1.1V

PF081807B

(HITACHI)

PIN BAND 1
+3dBm

PIN BAND 2
+3dBm

1000pF

0dBm

MAX

ATTN

15dB

V DN

220pF

POUT

BAND 2

+32dBm MAX

POUT BAND 1
+35dBm MAX

4.7 F

ANTENNA

49.9

7
8
5

1
4
3

LDC15D190A0007A

BAND

SELECT

0V/2V

3.5V

CTL

APC

0.1 F

52.3

Figure 34. A Dual Mode Power Amplifier Control Circuit

AD8314

REV. 0

The setpoint voltage, in the range 0 V to 1.1 V, is applied to the
VSET pin of the AD8314. This will typically be supplied by a
Digital-to-Analog Converter (DAC). This voltage is compared
to the input level to the AD8314. Any imbalance is between VSET
and the RF input level is corrected by V_DN, which drives the
V

APC

(gain control) of the power amplifier. V_DN reaches a

maximum value of approximately 1.9 V on a 2.7 V supply (this
will be higher for higher supply voltages) while delivering approxi-
mately 3 mA to the V

APC

input.

A filter capacitor (C

) must be used to stabilize the loop. The

choice of C

will depend to a large degree on the gain control

dynamics of the power amplifier, something that is frequently
poorly characterized, so some trial and error may be necessary.
In this example, a 220 pF capacitor gives the loop sufficient
speed to follow the GSM and DCS1800 time slot ramping profiles,
while still having a stable, critically-damped response.

Figure 35 shows the relationship between the setpoint voltage,
V

SET

and output power, at 0.9 GHz. The overall gain control

function is linear in dB for a dynamic range of over 40 dB.

Figure 36 shows a similar circuit for a single band handset power
amplifier. The BGY241 (Phillips) is driven by a nominal power
level of 0 dBm. A 20 dB directional coupler, DC09-73 (Alpha) is
used to couple the signal in this case. Figure 37 shows the rela-
tionship between the control voltage and the output power at
0.9 GHz.

In both of these examples, noise on the V_DN pin can be reduced
by placing a simple RC low-pass filter between V

and the gain

control pin of the power amplifier. However, the value of the
resistor should be kept low to minimize the voltage drop across
it due to the dc current flowing into the gain control input.

VSET Volts

30

POUT dBm

0.2

0.4

0.6

0.8

1.0

1.2

20

10

Figure 35. POUT vs. VSET at 0.9 GHz for Dual Mode
Handset Power Amplifier Application

ENBL

RFIN

AD8314

VSET

FLTR

VPOS

COMM

V UP

2.7V

SET

0V1.1V

RF INPUT

0dBm

MAX

ATTN

15dB

V DN

220pF

+35dBm

MAX

47 F

ANTENNA

BGY241

+15dBm

2.2 F

680pF

0dBm

DC09-73

3.5V

0.1 F

52.3

Figure 36. A Single Mode Power Amplifier Control Circuit

VSET Volts

30

POUT dBm

0.2

0.4

0.6

0.8

1.0

20

10

40

50

Figure 37. POUT vs. VSET at 0.9 GHz for Single Mode
Handset

AD8314

REV. 0

Table IV. Evaluation Boards Configuration Options

Component

Function

Default Condition

TP1, TP2

Supply and Ground Vector Pins

Not Applicable

SW1

Device Enable: When in position A, the ENBL

SW1 = A

pin is connected to +V

and the AD8314 is in

operating mode. In Position B, the ENBL pin is
grounded, putting the device in power-down mode.

R1, R2

Input Interface: The 52.3

resistor in position

R2 = 52.3

(Size 0603)

R2 combines with the AD8314's internal input

R1 = 0

(Size 0402)

impedance to give a broadband input impedance
of around 50

. A reactive match can be imple-

mented by replacing R2 with an inductor and
R1 (0

) with a capacitor. Note that the AD8314's

RF input is internally ac-coupled.

R3, R4, C2, R5, R6, C3

Output Interface: R4, C2, R6, and C3 can be

R4 = C2 = R6 = C3 = Open (Size 0603)

used to check the response of V_UP and V_DN

R3 = R5 = 0

(Size 0603)

to capacitive and resistive loading. R3/R4 and
R5/R6 can be used to reduce the slope of V_UP
and V_DN.

C1, R9

Power Supply Decoupling: The nominal supply

C1 = 0.1

F (Size 0603)

decoupling consists of a 0.1

F capacitor (C1). A

R9 = 0

(Size 0603)

series inductor or small resistor can be placed in
R9 for additional decoupling.

Filter Capacitor: The response time of V_UP

C4 = Open (Size 0603)

and V_DN can be modified by placing a capacitor
between FLTR (pin 4) and V_UP.

R7, R8

Slope Adjust: By installing resistors in R7 and R8,

R7 = 0

(Size 0603)

the nominal slope of 20 mV/dB can be increased.

R8 = Open (Size 0603)

See Slope Adjust discussion for more details.

LK1

Measurement/Controller Mode: LK1 shorts

LK1 = Installed

V_UP to VSET, placing the AD8314 in
measurement mode. Removing LK1 places
the AD8314 in controller mode.

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