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a

AD8306

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

5 MHz–400 MHz 100 dB High Precision

Limiting-Logarithmic Amplifier

FUNCTIONAL BLOCK DIAGRAM

FEATURES

Complete, Fully Calibrated Log-Limiting IF Amplifier

100 dB Dynamic Range: –91 dBV to +9 dBV

Stable RSSI Scaling Over Temperature and Supplies:

20 mV/dB Slope, –95 dBm Intercept

0.4 dB RSSI Linearity up to 200 MHz

Programmable Limiter Gain and Output Current

Differential Outputs to 10 mA, 2.4 V p-p

Overall Gain 90 dB, Bandwidth 400 MHz

Constant Phase (Typical

56 ps Delay Skew)

Single Supply of +2.7 V to +6.5 V at 16 mA Typical

500 ns Power-Up Time, <1

A Sleep Current

APPLICATIONS

Receivers for Frequency and Phase Modulation

Very Wide Range IF and RF Power Measurement

Receiver Signal Strength Indication (RSSI)

Low Cost Radar and Sonar Signal Processing

Instrumentation: Network and Spectrum Analyzers

PRODUCT DESCRIPTION

The AD8306 is a complete IF limiting amplifier, providing both

an accurate logarithmic (decibel) measure of the input signal

(the RSSI function) over a dynamic range of 100 dB, and a

programmable limiter output, useful from 5 MHz to 400 MHz.

It is easy to use, requiring few external components. A single

supply voltage of +2.7 V to +6.5 V at 16 mA is needed, corre-

sponding to a power consumption of under 50 mW at 3 V, plus

the limiter bias current, determined by the application and typi-

cally 2 mA, providing a limiter gain of 90 dB when using 200

Ω

loads. A CMOS-compatible control interface can enable the

AD8306 within about 500 ns and disable it to a standby current

of under 1

µA.

The six cascaded amplifier/limiter cells in the main path have a

small signal gain of 12.04 dB (

×4), with a –3 dB bandwidth of

850 MHz, providing a total gain of 72 dB. The programmable

output stage provides a further 18 dB of gain. The input is fully

differential and presents a moderately high impedance (1 k

Ω in

parallel with 2.5 pF). The input-referred noise-spectral-density,

when driven from a terminated 50

Ω, source is 1.28 nV/√Hz,

equivalent to a noise figure of 3 dB. The sensitivity of the

AD8306 can be raised by using an input matching network.

Each of the main gain cells includes a full-wave detector. An

additional four detectors, driven by a broadband attenuator, are

used to extend the top end of the dynamic range by over 48 dB.

The overall dynamic range for this combination extends from

–91 dBV (–78 dBm at the 50

Ω level) to a maximum permissible

value of +9 dBV, using a balanced drive of antiphase inputs each of

2 V in amplitude, which would correspond to a sine wave power

of +22 dBm if the differential input were terminated in 50

Ω.

Through laser trimming, the slope of the RSSI output is closely

controlled to 20 mV/dB, while the intercept is set to –108 dBV

(–95 dBm re 50

Ω). These scaling parameters are determined

by a band-gap voltage reference and are substantially indepen-

dent of temperature and supply. The logarithmic law conform-

ance is typically within

±0.4 dB over the central 80 dB of this

range at any frequency between 10 MHz and 200 MHz, and is

degraded only slightly at 400 MHz.

The RSSI response time is nominally 73 ns (10%–90%). The

averaging time may be increased without limit by the addition of

an external capacitor. The full output of 2.34 V at the maximum

input of +9 dBV can drive any resistive load down to 50

Ω and

this interface remains stable with any value of capacitance on

the output.

The AD8306 is fabricated on an advanced complementary

bipolar process using silicon-on-insulator isolation techniques

and is available in the industrial temperature range of –40

°C to

+85

°C, in a 16-lead narrow body SO package. The AD8306 is

also available for the full military temperature range of –55

°C to

+125

°C, in a 16-lead side-brazed ceramic DIP.

12dB

LIM

DET

12dB

DET

DET

4

DET

LADR ATTEN

INHI

INLO

I–V

BIAS

CTRL

TEN DETECTORS SPACED 12dB

INTERCEPT

TEMP COMP

BAND-GAP

REFERENCE

ENBL

GAIN

BIAS

LMHI

LMLO

LMDR

VLOG

FLTR

SIX STAGES TOTAL GAIN 72dB

TYP GAIN 18dB

SLOPE

BIAS

12dB