AD8367
APPLICATIONS
The AD8367 can be configured either as a VGA whose gain
is controlled externally through the GAIN pin or as an AGC
amplifier, using a supply voltage of 2.7 V to 5.5 V. The supply
to the VPSO and VPSI pins should be decoupled using a low
inductance, 0.1 μF surface-mount, ceramic capacitor as close
as possible to the device. Additional supply decoupling can
be provided by a small series resistor. A 10 nF capacitor from
Pin DECL to Pin OCOM is recommended to decouple the
output reference voltage.
Minimum-loss, L-pad networks are used on the evaluation
board (see Figure 45) to allow easy interfacing to standard
50 Ω test equipment. Each pad introduces an 11.5 dB power
loss (5.5 dB voltage loss).
1
3
0.3333
INPUT AND OUTPUT MATCHING
The AD8367 is designed to operate in a 200 Ω impedance
system. The output amplifier is a low output impedance voltage
buffer with a 50 Ω damping resistor to desensitize it from load
reactance and parasitics. The quoted performance includes the
voltage division between the 50 Ω resistor and the 200 Ω load.
The AD8367 can be reactively matched to an impedance other
than 200 Ω by using traditional step-up and step down
R
SOURCE
1
0.3333
3
Z
IN
SERIES L
SHUNT C
matching networks or high quality transformers. Table 4 lists
–0.3333
–3
the 50 Ω S-parameters for the AD8367 at a VGAIN = 750 mV.
Figure 32 illustrates an example where the AD8367 is matched
to 50 Ω at 140 MHz. As shown in the Smith Chart, the input
matching network shifts the input impedance from ZIN to 50 Ω
with an insertion loss of <2 dB over a 5 MHz bandwidth. For
the output network, the 50 Ω load is made to present 200 Ω to
the AD8367 output. Table 5 provides the component values
required for 50 Ω matching at several frequencies of interest.
–1
= 140MHz, Z = 197 – j34.2, R = 50Ω
SOURCE
f
C
IN
XS
IN
100nH
AD8367
XP
8.2pF
IN
C
0.1μF
XS
OUT
13pF
AC
R
SOURCE
50Ω
Z
Z
OUT
IN
When added loss and noise can be tolerated, a resistive pad can
be used to provide broadband, near-matched impedances at the
device terminals and the terminations.
V
S
Z
IN
Z
LOAD
R
LOAD
XP
50Ω
OUT
120nH
Figure 32. Reactive Matching Example for f = 140 MHz
Table 4. S-Parameters for 200 Ω System for VS = 5 V and VGAIN = 0.75 V
Frequency (MHz)
S11
S21
S12
S22
10
0.04 ∠ −43.8°
0.09 ∠ −81.5°
0.17 ∠ −103.4°
0.21 ∠ −111.7°
0.26 ∠ −103.8°
41.1 ∠ 178.8°
43.6 ∠ 163.4°
48.0 ∠ 141.4°
47.5 ∠ 124.0°
48.3 ∠ 107.6°
0.0003 ∠ 76.1°
0.0002 ∠ 63.7°
0.0009 ∠ 130.8°
0.0017 ∠ 96.8°
0.0018 ∠ 113.5°
0.56 ∠ −179.3°
0.55 ∠ +176.1°
0.56 ∠ +170.2°
0.54 ∠ +166.5°
0.48 ∠ +164.6°
70
140
190
240
Table 5. Reactive Matching Components for a 50 Ω System where RSOURCE = 50 Ω, RLOAD = 50 Ω
Frequency (MHz)
XSIN
XPIN (pF)
120
15
8.2
2.7
XSOUT (pF)
XPOUT
10
70
140
190
240
1.5 μH
220 nH
100 nH
82 nH
68 nH
180
27
13
10
7
1.8 μH
270 nH
120 nH
100 nH
82 nH
1.5
Rev. A | Page 14 of 24
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