Texas Instruments LMK00725EVM User manual

LMK00725EVM User’s Guide
User's Guide
Literature Number: SNOU126
September 2013

Contents
1 Introduction ........................................................................................................................ 4
2 Features ............................................................................................................................. 4
3 Setup ................................................................................................................................. 4
3.1 Input/ Output Connector Description .................................................................................. 4
4 Operation ........................................................................................................................... 5
5 PCB Layout ........................................................................................................................ 7
6 Schematic ......................................................................................................................... 11
7 Bill of Materials ................................................................................................................. 12
2Table of Contents SNOU126–September 2013
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List of Figures
1 LMK00725 Input Structure and Default Input Termination ............................................................ 5
2 Top Layer .................................................................................................................... 7
3 Inner Layer 2 (Ground Plane, Inverted).................................................................................. 8
4 Inner Layer 3 (Supply Plane, Inverted)................................................................................... 9
5 Bottom Layer............................................................................................................... 10
6 LMK00725EVM Schematic............................................................................................... 11
List of Tables
1 Device and Package Configurations ..................................................................................... 4
2 LMK00725EVM Bill of Materials ........................................................................................ 12
3
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User's Guide
SNOU126–September 2013
LMK00725EVM User’s Guide
1 Introduction
The LMK00725 is a low skew, high performance clock fanout buffer, which distributes up to five 3.3 V
LVPECL outputs. The clocks are derived from one of two selectable inputs, which can accept differential
or single-ended input signals.
This evaluation module (EVM) is designed to demonstrate the functionality and electrical performance of
the LMK00725 device. For optimum performance, the board is equipped with 50-ohm SMA connectors
and 50-ohm controlled impedance traces.
Table 1. Device and Package Configurations
DESIGNATOR IC PACKAGE
U1 LMK00725 PW-20 (TSSOP 20 pin)
2 Features
• Easy to use evaluation board to fan-out up to five LVPECL clocks with low phase noise/jitter
• Accepts differential or single-ended/LVCMOS input clock
• Device control pins configurable through jumpers
• Board power at 3.3-V for VCC
3 Setup
This section describes the jumpers and connectors on the EVM as well and how to properly connect, set
up and use the LMK00725EVM.
With this EVM, one could distribute one of two input clocks to up to 5 LVPECL outputs. Therefore, a
minimum of one clock source is needed and appropriate test equipment to observe or measure the
outputs.
3.1 Input/ Output Connector Description
Connectors:
•CLK0 and nCLK0 SMA connectors are used to interface an external AC-coupled clock input to the first
of the two differential input pairs (CLK0, nCLK0) of the device.
•CLK1 and nCLK1 SMA connectors are used to interface an external AC-coupled clock input to the
second of the two differential input pairs (CLK1, nCLK1) of the device..
•Q1,nQ1 and Q3,nQ3 SMA connectors are used to distribute two of the five differential clock outputs.
The other three differential clock outputs are not connected to the traces, so their SMA connectors are
not populated by default.
•PWR is a 3-pin header used to 3.3-V supply power to the board/device. VCC and GND pins are
labeled on the top side of the board. The center pin is not connected (N/C).
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Jumpers:
•CLK-SEL selects between one of two inputs.
– 0 (position 1-2) = Select CLK0, nCLK0 inputs
– 1 (position 2-3) = Select CLK1, nCLK1 inputs
•CLK-EN selects between U1 clock enabled or disabled modes.
– 0 (position 1-2) = Clock Disabled
– 1 (position 2-3) = Clock Enabled (normal operation)
4 Operation
Power:
Before, applying any clock inputs, supply the board with 3.3 V and ground at VCC and GND pins of the
PWR header. Make sure the supply current being drawn is less than 115 mA.
Inputs:
Figure 1 shows the LMK00725 input structure. The internal 51 kΩpull-up and pull-down resistors work
with the external 50 Ωtermination resistors, which bias the device inputs to mid-rail. Therefore, AC-
coupled clock sources from 0.15Vpp to 1.3Vpp (50 Ωterminated) can be tied to either of the two
differential clock inputs. With the default input termination shown in Figure 1, the input SMAs expect a 10
0Ωdifferential clock source. Note that with the default input configuration, the differential input has only
very small offset voltage (~3.2 mV) so that when the selected clock inputs are left open/floating, the
outputs could have the tendency to chatter.
With DC-coupled clock sources, use a “DC-block” at the input SMAs to ensure DUT input voltage range
compliance. Alternatively, adjust the clock source DC bias (if available) to make sure the LMK00725 input
voltage range is not violated.
Figure 1. LMK00725 Input Structure and Default Input Termination
The clock inputs can accommodate a differential input or single-ended input signal with the proper
external input termination using the various component options on the board. Refer to the datasheet for
input interface application circuits.
To achieve the best possible additive jitter and noise floor performance, it is recommended to drive the
CLK/nCLK pair using an input signal with fast slew rate of 3 V/ns (differential) or higher. Driving the input
with a lower slew rate can degrade the additive jitter and noise floor performance. For this reason, a
differential input signal (e.g. LVPECL), is recommended because it typically provides higher slew rate and
common-mode noise rejection compared to a single-ended input (LVCMOS/LVTTL or sine-wave, for
example).
5
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Outputs:
By default, two of the five LVPECL output pairs are connected to the traces and SMA connectors via
series 0-Ωresistors. The remaining three LVPECL pairs are not connected to the traces (0-Ωresistors not
installed) and thus do not have SMA connectors installed. The routed outputs have 180-Ωbias resistors to
ground and are AC-coupled to the SMA connectors to allow direct connections to RF instruments via SMA
cables for test and evaluation. The AC-coupled outputs should be terminated with single-ended 50 Ωor
100 Ωdifferential loads.
To minimize reflections and signal integrity issues, it is recommended to terminate any driven output trace
with a 50-ΩSMA load termination, or otherwise disconnect any unused output from the trace by removing
the 0-Ωseries resistor (R30 to R39).
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5 PCB Layout
Figure 2. Top Layer
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Figure 3. Inner Layer 2 (Ground Plane, Inverted)
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Figure 4. Inner Layer 3 (Supply Plane, Inverted)
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Figure 5. Bottom Layer
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