CN-0072 Datasheet, PDF (1/3 Pages) Analog Devices – Extending the Dynamic Range of the ADL5513 Logarithmic Detector Using the AD8368 Variable Gain Amplifier

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Circuit Note
CN-0072
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
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 VOUT 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
215Ω
INPUT _10nF
10nH
10kΩ
VPOS1
0Ω
0.1µF
C10
1nF
1nF
5.6pF
INPT
ICOM
MODE
VPSI
VPSI
ENBL
AD8368
VPSI
VPSI
VPSO
VPSO
OUTP
OCOM
10nF
1nF
VPOS2
VPOS VPOS
GND
0Ω
C12
C15
1nF
0.1µF
VPOS3
C7
1000pF
10nF
C12
1nF
0Ω C15
0.1µF
47nF
52.3Ω
BAND-PASS
120MHz
VPOS
VPOS
1
12
2 INHI
VSET 11
ADL5513
3 INLO
COMM 10
4
9
VOUT
1kΩ
TADJ
VOUT
100pF
Z1
1kΩ
VTADJ = 0.89V
0.1µF
VPOS
1kΩ
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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