an1187

ISL59420/21EVAL1 Evaluation Board
User’s Guide
®
Application Note
August 10, 2005
AN1187.1
Introduction
High Frequency Layout Considerations
The ISL59420/21EVAL1 evaluation board contains all the
circuitry needed to characterize critical performance
parameters of the ISL59420, ISL59421 single 2:1 MUXamplifier, over a variety of applications.
At frequencies of 500MHz and higher, circuit board layout
may limit performance. The following layout guidelines are
implemented on the evaluation board:
The ISL59420, ISL59421 are single-output, gain-selectable
2:1 MUX-amps. The unity-gain bandwidths are 400MHz for
the ISL59420 and 900MHz for the ISL59421. Each contain a
common set of logic inputs for channel selection (S) threestate output control (HIZ) and an enable control input that
powers down the device (EN).
The evaluation board circuit and layout is optimized for
either 50Ω or 75Ω terminations, and implements a basic
single 2:1 video MUX-amp. The board is supplied with 75Ω
input signal terminations and a 75Ω back-termination
resistor on each of the 3 outputs, making it suitable for
driving video cable. The user has the option of replacing the
75Ω resistors with 50Ω resistors for other applications. The
control lines contain 50Ω resistors to match the 50Ω output
impedance of high speed pulse generators. Control line
termination resistors are recommended for rise and fall times
under 10ns to minimize unwanted transients. If DC is used
for the control logic, the resistors may be removed; or the
applied DC voltage can be reduced to 2.5V to reduce the
dissipation in the termination resistor.
The layout contains component options to include an output
series resistor (RS) followed by a parallel resistor (RL)
capacitor (CL) network to ground. This option allows the user
to select several different output configurations. Examples
are shown in Figures 2A, 2B, and 2C. The evaluation board
is supplied with the 75Ω back termination resistors shown in
Figure 2C.
• No series connected vias are used in signal I/O lines, as
they can add unwanted inductance.
• Signal trace lengths are minimized to reduce transmission
line effects and the need for strip-line tuning of the signal
traces.
• High frequency decoupling caps are placed as close to the
device power supply pin as possible - without series vias
between the capacitor and the device pin.
Power Sequencing
Proper power supply sequencing is -V first, then +V. In
addition, the +V and -V supply pin voltage rate-of-rise must
be limited to ±1V/µs or less. The evaluation board contains
parallel-connected low VON Schottky diodes on each supply
terminal to minimize the risk of latch up due to incorrect
sequencing. In addition, extra 10µF decoupling capacitors
are added to each supply to aid in reducing the applied
voltage rate-of-rise.
Reference Documents
• ISL59420 Data Sheet, FN7459
• ISL59421 Data Sheet, FN7458
S0
EN0
IN0
Amplifier Performance and Output
Configurations
DECODE
EN1
The ISL59420, ISL59421 output amplifier is externally gainselectable with the non-inverting input directly coupled to the
2:1 MUX output. The inverting input is pinned out to the
evaluation board. Resistor RF is set to the value shown in
Figure 2D, and in conjunction with the amplifier internal
capacitance, provides optimum frequency response with
minimal gain peaking. The output amplifier is ideally suited
for driving high impedance high speed selectable-gain
buffers when gain compensation is needed. GBW decreases
slightly at the lower output load impedances typical of backterminated cable driving applications. Reference data sheets
for additional performance data.
1
• Signal I/O lines are the same lengths and widths to match
propagation delay and trace parasitics.
IN1
+
OUT
AMPLIFIER BIAS
HIZ
ENABLE
FIGURE 1. ISL59420, ISL59421 FUNCTIONAL BLOCK
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2005. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
Application Note 1187
TABLE 1. LOGIC TABLE
S
HIZ
ENABLE
OUT
0
0
0
IN0
1
0
0
IN1
-
-
1
Power-down
-
1
-
High Z
VIN
50Ω
OR
75Ω
VIN
RS, 0Ω
+
-
*Cb1
~0.5pF
RF
*Cb2
~3pF
CL
1.5pF
RL
500Ω
* Cb1, Cb2 are approximate PCB trace capacitances.
TEST
EQUIPMENT
ISL59420, ISL59421
50Ω
OR
75Ω
+
RF
RS
50Ω OR 75Ω
Cb1
~0.5pF
50Ω
OR
75Ω
+
RF
RS
475Ω
Cb1
RL
~0.5pF
50Ω
OR
75Ω
50Ω
OR
75Ω
* Cb1 is approximate PCB trace capacitance.
FIGURE 2A. TEST CIRCUIT WITH OPTIMAL OUTPUT LOAD
VIN
TEST
EQUIPMENT
ISL59420, ISL59421
ISL59420, ISL59421
50Ω
OR
75Ω
FIGURE 2B. TEST CIRCUIT FOR 50Ω OR 75Ω TERMINATIONS
PART #
RF VALUE
ISL59420
200Ω
ISL59421
357Ω
* Cb1 is approximate PCB trace capacitance.
FIGURE 2C. BACK-TERMINATED TEST CIRCUIT FOR CABLE
APPLICATION
FIGURE 2D. RF TABLE FOR FIGURE 2 CIRCUITS
ISL59420/21EVAL1 Top View
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AN1187.1
August 10, 2005
Application Note 1187
ISL59420/21EVAL1 Schematic Diagram
S
1S
JSO
R19
49.9Ω
IN0
R8-75Ω
EN
JSO
IN1
R14-49.9Ω
RF
ISL59420
ISL59421
2 GND
IN- 10
OUT
OUT 9
3 IN0
V+ 8
4 EN
V- 7
5 IN1
HIZ 6
R9-75Ω
RS - 75Ω
R22
C11
JHIZ
HIZ
R12 - 49.9Ω
D1
C2
C1
C3
10µF 0.1µF 10nF
C7
1nF
+
D2
GND. V-
+
C8
C5
C4
C6
10µF 0.1µF 10nF 1nF
V+
ISL59420/21EVAL1 Components Parts List
DEVICE #
DESCRIPTION
COMMENTS
C7, C8
CAP, SMD, 0603, 1000pF, 25V, 10%, X7R
Power Supply Decoupling
C1, C4
CAP, SMD, 0603, 0.01µF, 25V, 10%, X7R
Power Supply Decoupling
C2, C5
CAP, SMD, 0603, 0.1µF, 25V, 10%, X7R
Power Supply Decoupling
C3, C6
CAP, SMD, 0805, 10µF, 6.3V, 10%, X5R
Power Supply Decoupling
D1, D2
Diode-Schottky, 2 Pin, 45V, 7.5A
MBR0550T (Motorola) Reverse Polarity Protection
Resistor, SMD, 0603, 75Ω, 1/10W, 1%
Signal Input/output Termination
R12, R14, R19
Resistor, SMD, 0603, 49.9Ω, 1/16W, 1%
Logic Input Termination
RF - ISL59420
Resistor, SMD, 0603, 200Ω, 1/10W, 1%
Feedback Resistor
RF - ISL59421
Resistor, SMD, 0603, 357Ω, 1/10W, 1%
Feedback Resistor
C11
CAP, SMD, 0603
Optional, not populated
R22
Resistor, SMD, 0603
Optional, not populated
U1
ISL59420/21 - 400/900 MHz Multiplexing Amplifier,
10PIN, QSOP
Device Under Test
R8, R9, RS
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to
verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
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