CN-0072: Extending the Dynamic Range of the ADL5513 Logarithmic Detector Using the AD8368 Variable Gain Amplifier PDF

Circuit Note
CN-0072
Devices Connected/Referenced
1 MHz to 4 GHz , 80 dB Logarithmic
ADL5513
Detector/Controller
800 MHz Linear-in-dB Variable Gain
AD8368
Amplifier (VGA)
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Extending the Dynamic Range of the ADL5513 Logarithmic Detector
Using the AD8368 Variable Gain Amplifier
CIRCUIT FUNCTION AND BENEFITS
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.
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
VPOS
VPOS
1nF
VPOS1 VPOS2 VPOS3
5.6pF
GND
NC 13
VOUT
1kΩ
3 INLO
VPOS
4
COMM 10
9
8 NC
BAND-PASS
120MHz
VOUT
ADL5513
7 NC
1nF
12
VSET 11
2 INHI
52.3Ω
10nF
CLPF 14
1
NC 15
VPOS
47nF
NC 16
0Ω C15
0.1µF
5 NC
OCOM
C12
1nF
6 NC
ENBL
10nF
0.1µF
VPSI
OUTP
DETI
VPSO
VPSI
C7
1000pF
100pF
VPOS3
OCOM
C10
1nF
C15
0.1µF
C12
1nF
VPSO
AD8368
VPSI
DECL
0.1µF
DECL
VPSI
VPSI
GAIN
0Ω
ICOM
INPT
MODE
VPOS1
VPOS
VPOS2
0Ω
ICOM
HPFL
10nH
10kΩ
DECL
ICOM
215Ω
DETO
INPUT 10nF
ICOM
VPOS
TADJ
1kΩ
VTADJ = 0.89V
100pF
Z1
0.1µF
1kΩ
08175-001
VPOS
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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CN-0072
Circuit Note
Due to the amplification of out-of-band noise by the AD8368, a
band-pass filter is used between the AD8368 and ADL5513 to
increase the low-signal sensitivity. The VGA amplifies low
power signals and attenuates high power signals to fit them in
the detectable range of the ADL5513. If an amplifier with higher
gain and lower noise figure is used, better than 90 dB sensitivity
can be achieved for use in a receive-signal-strength-indicator
(RSSI) application.
Figure 2 shows the data results of the extended dynamic range
at 120 MHz and the corresponding error in VOUT.
Due to the high frequencies and wide dynamic ranges involved,
the circuit must be constructed on a multilayer PC board that
has at least one layer for the ground plane. Each IC power pin
must be decoupled with a 0.1 µF low inductance ceramic
capacitor as shown in Figure 1. Some power pins have an
additional smaller value decoupling capacitor (low ESL) for
extremely high frequencies. The “0 Ω” resistors on the
schematic represent optional small resistor values (less than
10 Ω) or ferrite beads that may be added for more decoupling.
The data sheets for the ADL5513 and the AD8368 show the
optimal layouts of the respective evaluation boards.
On the underside of both the ADL5513 and the AD8368 chip
scale package there is an exposed paddle. This paddle is
internally connected to the ground of the chip. Solder the
paddle to the low impedance ground plane on the PCB to
ensure specified electrical performance and to provide thermal
relief. It is also recommended that the ground planes on all
layers under the paddle be stitched together with vias to reduce
thermal impedance.
3.0
1.625
2.5
1.500
2.0
1.375
1.5
1.250
1.0
1.125
0.5
1.000
0
0.875
–0.5
0.750
0.500
–1.5
–2.0
ERROR +25°C
ERROR –40°C
ERROR +85°C
0.375
0.250
–90
–1.0
VOUT +25°C
VOUT –40°C
VOUT +85°C
0.625
–80
–70
–60
–50
–40 –30 –20
PIN (dBm)
ERROR (dB)
1.750
–2.5
–10
0
10
–3.0
20
08175-002
The ADL5513 logarithmic detector dynamic range can be
extended by adding a separate VGA whose gain control input is
derived directly from VOUT. This extends the dynamic range by
the gain control range of the VGA. For the overall measurement
to remain linear in dB, the VGA must provide a linear-in-dB
(exponential) gain control function. The VGA gain must
decrease with an increase in its gain bias in the same way as the
ADL5513. Alternatively, an inverting op amp with suitable level
shifting can be used. It is convenient to select a VGA that needs
only a single 5.0 V supply and is capable of generating a singleended output. All of these conditions are met by the AD8368.
Figure 1 shows the simplified schematic. Using the inverse gain
mode (MODE pin low) of the AD8368, its gain decreases on a
slope of 37.5 mV/dB to a minimum value of −12 dB for a gain
voltage (VGAIN) of 1.0 V. The voltage, VGAIN, required by the
AD8368 is 50% of the output of the ADL5513. To scale this
voltage, it is necessary to insert a voltage divider at the output of
the ADL5513. Over the 1.5 V range from the output of the
ADL5513, the gain of the AD8368 varies by (0.5 × 1.5 V)÷
(37.5 mV/dB), or 20 dB. Combined with the 75 dB gain span (at
120 MHz) of the ADL5513, this results in a 1.5 V change in
VOUT for a 95 dB change at the input to the AD8368.
VOUT (V)
CIRCUIT DESCRIPTION
Figure 2. Output and Conformance for the AD8368/ADL5513 Extended
Dynamic Range Circuit at 120 MHz
COMMON VARIATIONS
The application circuit described here can be modified for any
IF frequency within the operating range of the AD8368 and
ADL5513 by selecting the appropriate band-pass filter. For
operation above 800 MHz, the ADL5330 can be used in place of
the AD8368. The ADL5330 VGA can be used at frequencies
between 10 MHz and 3 GHz. Using the ADL5330 also requires
an inverter between the ADL5330 gain pin and the gain control
voltage from the ADL5513. A single-supply operational
amplifier such as the AD8061 can be configured for use as an
inverter in this application.
LEARN MORE
MT-073 Tutorial, High Speed Variable Gain Amplifiers (VGAs).
Analog Devices.
MT-077 Tutorial, Log Amp Basics. Analog Devices.
MT-078 Tutorial, High Frequency Log Amps. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
Data Sheets
AD8061 Data Sheet.
AD8368 Data Sheet.
ADL5330 Data Sheet.
ADL5513 Data Sheet.
REVISION HISTORY
7/09—Revision 0: Initial Version
Rev. 0 | Page 2 of 3
Circuit Note
CN-0072
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CN08175-0-7/09(0)
Rev. 0 | Page 3 of 3
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