800 MHz, Linear-in-dB

VGA with AGC Detector

AD8368

Rev. A

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FEATURES

Analog variable gain range: −12 dB to +22 dB

Linear-in-dB scaling: 37.5 dB/V

Integrated rms detector

P1dB: 16 dBm @ 140 MHz

Output IP3: 33 dBm @ 140 MHz

Noise figure at maximum gain: 9.5 dB @ 140 MHz

Input and output impedances: 50 Ω

Single-supply voltage from 4.5 V to 5.5 V

RoHS-compliant, 24-lead LFCSP

APPLICATIONS

Complete IF AGC amplifiers

Gain trimming and leveling

Cellular base stations

Point-to-point radio links

RF instrumentation

FUNCTIONAL BLOCK DIAGRAM

9

VPSO

11

VPSI

12

VPSI

22

VPSI

23

VPSI

13

VPSI

24

ENBL

8

OUTP

3

HPFL

4

DECL

14

DECL

15

DECL

20

ICOM

18

ICOM

17

ICOM

19

INPT

7

OCOM

1

GAIN

6

OCOM

16

ICOM

10

VPSO

21

MODE

2

DETO

5

DETI

REF

–

+

AD8368

ATTENUATOR LADDER

DECL

50Ω

0dB –2dB –4dB

–36dB

GAIN INTERPOLATOR

FIXED-GAIN

AMPLIFIER

OUTPUT

BUFFER

Figure 1.

GENERAL DESCRIPTION

The AD8368 is a variable gain amplifier (VGA) with analog

linear-in-dB gain control that can be used from low frequencies

to 800 MHz. Its excellent gain range, conformance, and flatness

are attributed to the Analog Devices, Inc. X-AMP® architecture,

an innovative technique for implementing high performance

variable gain control.

The gain range of −12 dB to +22 dB is scaled accurately to

37.5 dB/V with excellent conformance error. The AD8368 has

a 3 dB bandwidth of 800 MHz that is nominally independent

of gain setting. At 140 MHz, the OIP3 is 33 dBm at maximum

gain. The output noise floor is −143 dBm/Hz, which corresponds

to a 9.5 dB noise figure at maximum gain. The single-ended

input and output impedances are nominally 50 Ω.

The gain of the AD8368 can be configured to be an increasing

or decreasing function of the gain control voltage depending

on whether the MODE pin is pulled to the positive supply or

to ground, respectively. When MODE is pulled high, the

AD8368 operates as a typical VGA with increasing gain.

By connecting MODE to ground and using the on-board rms

detector, the AD8368 can be configured as a complete AGC

system with RSSI. The output power is accurately leveled to

the internal default setpoint of 63 mV rms (−11 dBm referenced

to 50 Ω), independent of the waveform crest factor. Because

the uncommitted detector input is available at DETI, the AGC

loop can level the signal at the AD8368 output or at any other

point in the signal chain over a maximum input power range

of 34 dB. Furthermore, the setpoint level can be raised by dividing

down the output signal before applying it to the detector.

The AD8368 operates from a supply voltage of 4.5 V to 5.5 V

and consumes 60 mA of current. It can be fully powered down

to <3 mA by grounding the ENBL pin. The AD8368 is fabricated

using the Analog Devices proprietary SiGe SOI complementary

bipolar IC process. It is available in a 24-lead LFCSP and

operates over the industrial temperature range of −40°C to

+85°C. Application boards are available upon request.