AN1418: Use of ISL55016 as a Single-Ended to Differential Converter

Use of ISL55016 as a Single-Ended
to Differential Converter
®
Application Note
September 16, 2008
AN1418.0
Introduction
Impact of Balun on Layout
The ISL55016 Silicon Bipolar amplifier can match a 75Ω
single-ended source to a 100Ω differential load. This feature
makes the ISL55016 ideal for a radios where the tuner chip
has a differential input, such as Satellite TV. Here we will
introduce the application circuit and discuss the advantages
of this part. Specifically, the differential nature of the output
removes the need for a balun, which improves both board
area and frequency response.
In contrast with the ISL55016 is its predecessor, the
ISL55012. The amplifier will be followed with a Balun
structure to transfer the single-ended signal to a differential
tuner. The ISL55016 can be used to substitute the Balun
Structure and LNA to save PCB space and components
(Figure 3). The differential capabilities of the ISL55016 allow
a reduction in excess of 50%.
The ISL55012 is a predecessor to the ISL55016. It has similar
functions with the exception of the output, which is limited to
single-ended operation. We will discuss using the ISL55016 to
substitute for the ISL55012 with a balun structure.
GND
GND
ISL55012
ISL55012
RF
RF IN
IN
C
C
A balun is a passive structure which converts between
balanced and unbalanced electrical signals (Figure 1). Some
normal balun structures include classical transformers,
transmission line baluns and 180° power divider/combiners.
RF IN
RF
IN
BALUN
DIFFERENTIAL SIGNAL
FIGURE 1. BLOCK DIAGRAM OF A BALUN
Typical Application Circuit
C
C
C
C
C
BALUN
Balun
CC
VDD
DIFFERENTIAL
Differential
OUT
Out
Vdd
C
C
C
C
CC
L
R
R
L
ISL55016
ISL55016
VDD
Vdd
Figure 4 shows gain measured for the ISL55016 and
ISL55012 with a transmission line balun on a customer
board as shown in Figure 3. One of the design goals of the
ISL55016 is to provide gain no less than the ISL55012 at
0.5GHz to 2GHz. Also the ISL55016 can be used at low
frequency down to 50MHz, where the ISL55012 can not
work well.
19
The ISL55016 is a true differential amplifier and doesn’t
need a balun. Figure 2 shows the typical application circuit of
ISL55016.
The ISL55016 can be used as differential-in and differential-out
as well since Pin 2 and Pin 3 are equivalent, balanced inputs.
+5V
C6
C3
1
6
R1
ISL55016
2
5
4
3
C2
L1
R2
L2
C5
RF OUT 2
15
13
ISL55016
11
9
7
5
500M
ISL55012+TRANSMISSION
LINE BALUN
1.0G
1.5G
2.0G
FREQUENCY (Hz)
2.5G
3.0G
FIGURE 4. GAIN OF ISL55012 WITH A TRANSMISSION LINE
BALUN vs ISL55016
The ISL55016 shows a little higher gain than the ISL55012 with
balun structure. In this way, Figure 4 shows the realization of
the ISL55016 as a single-ended to differential converter.
Trade-off Between Power and OIP2
RF OUT 1
C4
FIGURE 2. APPLICATION CIRCUIT
1
17
dB
The ISL55016 can be used as a single-ended input. Pin 2
and Pin 3 are equivalent. Either one can be used as the
input; the other one needs to be connected to an AC
Ground. The input is internally matched to 75Ω single-ended
and the output is matched to 50Ω single-ended or 100Ω
differentially.
C1
RF IN 2
C
C
C DIFFERENTIAL
Differential
OUT
Out
C
C
C
FIGURE 3. COMPARISON OF THE LAYOUTS OF HE ISL55012
WITH A BALUN AND THE ISL55016 (RELATIVE
SIZE)
SINGLE-ENDED
SIGNAL
RF IN 1
C
C
R
R
C
LL
Vdd
VDD
VDD
Vdd
C
C
L
L
R
R
GND
GND
C
C
Balun
L
L
L
C
Once the signal is truly balanced, even order distortion drops
to zero. The ISL55016 converts the signal to differential early
in its signal path thus reducing second order distortion and
boosting OIP2. Nevertheless, the ISL55016 does generate
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 1418
some even order distortion because of the asymmetric
loading of its input and improving the device linearity will
improve OIP2. The values of R1 and R2 (Figure 1) have
been characterized for two options, 27Ω and 0Ω. Decreasing
the R1 and R2 value will increase the voltage across the
output transistor leading to an increase in the dissipation
power. At the same time, it will increase the amplitude of the
compression and OIP2, OIP3. This allows a trade-off
between the power dissipation and the distortion.
Thermal Management
Matching at the Input and Output
Choices of Components
In PCB layout design, a matching network is needed,
especially at the input. The Figure 5 shows the matching
network used for the ISL55016 Evaluation Board. 12mm of
100Ω trace and 8mm of 50Ω trace are used to form the input
matching network while 4mm of 100Ωr trace forms the
output matching network on the FR4 material.
ISL55016 is designed for wide bandwidth Applications,
50MHz-3GHz. The decoupling and RF choke components
should be chosen carefully for different frequency
applications. Tables 1 and 2 list the component information
for the Evaluation Board.
Vdd
VDD
FREQUENCY
BAND
1 0100Ω
0ohm
L
L22
C
C22
L1
L1
1 0100Ω
0ohm
C1
C1
5 050Ω
ohm
RRF
F IIN
N
TABLE 1. LIST OF COMPONENTS (50MHz~300MHz)
R FOUT
OUT
5 050Ω
o h m RF
CC3
3
10
0ohm
100Ω
The power dissipation of ISL55016 is about 500mW. The
PCB layout shown in Figure 5 provides the good thermal
connectivity between the device back plate and the main
ground plane, hence the copper and vias holes are needed
on either side of the chip. So via holes have two functions to
perform, providing a good RF ground for C2, C3 and back
plate and providing a low thermal conductivity to the ground
plane which dispenses the heat.
RRF
F OUT
OUT
7 5 o75Ω
hm
5 050Ω
ohm
2200pF
0603
C4, C5
2200pF
0603
L1, L2
2.2µH
Multilayer Ferrite/0603
C3
1nF
0603
C6
0.1µF
1206
R1, R2
27Ω/0Ω
0402
TABLE 2. LIST OF COMPONENTS (300MHz~3GHz)
Figure 6 shows the improvement in input matching (S11)
provided by this network of traces. At 2GHz, the input match
is improved 3dB and remains below a -10dB level.
0
-5
-10
dB
DESCRIPTION/DIMENSIONS
C1, C2
FIGURE 5. PCB LAYOUT OF MATCHING NETWORK
EVAL BOARD WITH
MATCHING NETWORK
-15
VALUE
FREQUENCY
BAND
VALUE
DESCRIPTION/DIMENSIONS
C1, C2
100pF
0603
C4, C5
100pF
0603
L1, L2
100nH
Surface Mount/0402
C3
100pF
0603
C6
0.1µF
1206
R1, R2
27Ω/0Ω
0402
-20
PROBE STATION
-25
-30
0
0.5G
1.0G
1.5G
2.0G
FREQUENCY (Hz)
2.5G
3.0G
FIGURE 6. |S11| vs FREQUENCY
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verify that the Application Note or Technical Brief is current before proceeding.
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2
AN1418.0
September 16, 2008
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