REV. B

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.

a

AD8313

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

0.1 GHz–2.5 GHz, 70 dB

Logarithmic Detector/Controller

FUNCTIONAL BLOCK DIAGRAM

++

+

+

+

AD8313

VOUT

VSET

COMM

PWDN

GAIN

BIAS

BAND-GAP

REFERENCE

SLOPE

CONTROL

INTERCEPT

CONTROL

EIGHT 8dB 3.5GHz AMPLIFIER STAGES

8dB

8dB

VPOS

INHI

INLO

VPOS

8dB

8dB

NINE DETECTOR CELLS

LP

I

vV

V

vI

FEATURES

Wide Bandwidth: 0.1 GHz to 2.5 GHz Min

High Dynamic Range: 70 dB to

3.0 dB

High Accuracy:

1.0 dB over 65 dB Range (@ 1.9 GHz)

Fast Response: 40 ns Full-Scale Typical

Controller Mode with Error Output

Scaling Stable Over Supply and Temperature

Wide Supply Range: +2.7 V to +5.5 V

Low Power: 40 mW at 3 V

Power-Down Feature: 60

W at 3 V

Complete and Easy to Use

APPLICATIONS

RF Transmitter Power Amplifier Setpoint

Control and Level Monitoring

Logarithmic Amplifier for RSSI Measurement

Cellular Base Stations, Radio Link, Radar

PRODUCT DESCRIPTION

The AD8313 is a complete multistage demodulating logarith-

mic amplifier, capable of accurately converting an RF signal at

its differential input to an equivalent decibel-scaled value at its

dc output. The AD8313 maintains a high degree of log con-

formance for signal frequencies from 0.1 GHz to 2.5 GHz and

is useful over the range of 10 MHz to 3.5 GHz. The nominal

input dynamic range is –65 dBm to 0 dBm (re: 50

Ω), and the

sensitivity can be increased by 6 dB or more with a narrow band

input impedance matching network or balun. Application is

straightforward, requiring only a single supply of 2.7 V–5.5 V

and the addition of a suitable input and supply decoupling.

+25

°C) amounts to only 41 mW. A power-down feature is

provided; the input is taken high to initiate a low current

(20

µA) sleep mode, with a threshold at half the supply voltage.

The AD8313 uses a cascade of eight amplifier/limiter cells,

each having a nominal gain of 8 dB and a –3 dB bandwidth of

3.5 GHz, for a total midband gain of 64 dB. At each amplifier

output, a detector (rectifier) cell is used to convert the RF signal

to baseband form; a ninth detector cell is placed directly at the

input of the AD8313. The current-mode outputs of these cells

are summed to generate a piecewise linear approximation to the

logarithmic function, and converted to a low impedance voltage-

mode output by a transresistance stage, which also acts as a low-

pass filter.

When used as a log amp, the scaling is determined by a separate

feedback interface (a transconductance stage) that sets the slope

to approximately 18 mV/dB; used as a controller, this stage

accepts the setpoint input. The logarithmic intercept is posi-

tioned to nearly –100 dBm, and the output runs from about

0.45 V dc at –73 dBm input to 1.75 V dc at 0 dBm input. The

scale and intercept are supply and temperature stable.

The AD8313 is fabricated on Analog Devices’ advanced

25 GHz silicon bipolar IC process and is available in a 8-lead

µSOIC package. The operating temperature range is –40°C to

+85

°C. An evaluation board is available.

INPUT AMPLITUDE – dBm

2.0

–80

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

–70

–60

–50

–40

–30

–20

–10

0

FREQUENCY = 1.9GHz

5

4

3

2

1

0

–1

–2

–3

–4

–5

Figure 1. Typical Logarithmic Response and Error vs.

Input Amplitude