Circuit Note
Circuit Designs Using Analog Devices Products
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Devices Connected/Referenced
1 MHz to 4 GHz , 80 dB Logarithmic
Detector/Controller
800 MHz Linear-in-dB Variable Gain
Amplifier (VGA)
CN-0072
Extending the Dynamic Range of the ADL5513 Logarithmic Detector
Using the AD8368 Variable Gain Amplifier
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
(VGA) at the input of the ADL5513. The V
OUT
signal from the
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.
VPOS
VPOS
VPOS1 VPOS2 VPOS3
1nF
5.6pF
GND
VPOS
ICOM
ICOM
ICOM
DECL
VPOS
0.1µF
INPUT 10nF
215Ω
INPT
DECL
VPSI
VPOS2
0Ω
VPSI
VPSI
10nH
10kΩ
VPOS1
0Ω
0.1µF
C10
1nF
ICOM
MODE
VPSI
VPSI
C12
1nF
C15
0.1µF
VPOS3
C7
1000pF
AD8368
VPSO
VPSO
OUTP
CLPF 14
NC 16
NC 15
NC 13
10nF
C12
1nF
0Ω C15
0.1µF
100pF
VPOS
47nF
52.3Ω
1
DETO
GAIN
HPFL
OCOM
ENBL
OCOM
12
VOUT
1kΩ
VOUT
DECL
DETI
2 INHI
VSET 11
ADL5513
3 INLO
VPOS
COMM 10
9
4
10nF
1nF
BAND-PASS
120MHz
6 NC
5 NC
7 NC
8 NC
TADJ
1kΩ
VTADJ = 0.89V
100pF
Z1
0.1µF
VPOS
1kΩ
08175-001
Figure 1.ADL5513 Logarithmic Detector and AD8368 VGA Providing 95 dB Dynamic Range at 120 MHz (Simplified Schematic)
Note: VPOS = +5 V
Rev. 0
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each circuit, and their function and performance have been tested and verified in a lab environment
at room temperature. However, you are solely responsible for testing the circuit and determining its
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