Circuit Note

CN-0072

Circuit Designs Using Analog Devices Products

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Devices Connected/Referenced

ADL5513

1 MHz to 4 GHz , 80 dB Logarithmic

Detector/Controller

AD8368

800 MHz Linear-in-dB Variable Gain

Amplifier (VGA)

Extending the Dynamic Range of the ADL5513 Logarithmic Detector

Using the AD8368 Variable Gain Amplifier

Rev. 0

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CIRCUIT FUNCTION AND BENEFITS

This circuit provides a solution for increasing the dynamic

range of the ADL5513 logarithmic detector. This is

accomplished by adding an AD8368 variable gain amplifier

ADL5513 is attenuated and fed back to the gain control input of

the AD8368, providing precise control of the power at the input

of the ADL5513. Using this approach, the dynamic range of the

circuit is increased to 95 dB with excellent temperature stability

(see Figure 2). The output voltage is linear in dB with respect to

the input signal.

INPT

ENBL

VPSI

OCOM

ICOM

MODE

VPSI

VPSI

VPSO

VPSO

VPSI

OUTP

AD8368

VPOS1

10k

Ω

215

Ω

0

Ω

0

Ω

10nH

10nF

10nF

1nF

0.1µF

C10

1nF

5.6pF

1nF

C12

1nF

C15

0.1µF

INPUT 10nF

VPOS3

0

Ω

C12

1nF

C15

0.1µF

VPOS2

BAND-PASS

120MHz

52.3

Ω

1

VPOS

2 INHI

3 INLO

4

VPOS

11

VSET

12

TADJ

VOUT

VOUT

10

COMM

9

ADL5513

Z1

VPOS

GND

1k

Ω

47nF

100pF

C7

1000pF

0.1µF

VTADJ = 0.89V

VPOS

VPOS

1k

Ω

1k

Ω

VPOS

VPOS1

VPOS

VPOS2 VPOS3

Figure 1.ADL5513 Logarithmic Detector and AD8368 VGA Providing 95 dB Dynamic Range at 120 MHz (Simplified Schematic)

Note: VPOS = +5 V