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Application Note 2268
Loren Siebert
August 15, 2012
LMH6882 Evaluation Board
SUMMARY
The LMH6882EVAL evaluation board is designed to aid in the characterization of Texas Instruments High Speed LMH6882
Differential Amplifier.
In order to use the full bandwidth of the LMH6882 the LMH6882EVAL board has been built with a high performance dielectric
on the top layer (ROGERS RO4350B). This high-performance dielectric provides well matched impedance and low loss to
frequencies beyond 1GHz. All signal paths are routed on the top layer. The remainder of the board is conventional FR4.
http://www.rogerscorp.com/documents/726/acm/RO4000-Laminates---Data-sheet.aspx
Use the evaluation board as a guide for high frequency layout and as a tool to aid in device testing and characterization.
30199301
FIGURE 1. LMH6882EVAL
General Description
The LMH6882 DVGA has differential inputs and differential outputs. To aid evaluation with 50Ω single ended test equipment
the LMH6882EVAL evaluation board is shipped with input and output transformers installed. The signal path uses the INPD
and OUT+ marked connectors. The INMD and OUT− signal paths are grounded. The evaluation board uses edge mounted
SMA connectors. When using the differential inputs a termination resistor can be placed on the pads used by the transformer
primary coils. For DC-coupled operation the appropriate capacitors can be replaced by 0Ω resistors. If DC-coupled operation
is desired it is important to provide for proper bias voltages on the input and output pins (see the LMH6882 datasheet for more
details). The LMH6882EVAL evaluation board is designed to be used with transformers or baluns. There are DC-blocking
capacitors on both the input and output signal traces. The input pins of the LMH6882 will self-bias to approximately mid supply
(2.5V). Capacitors between the amplifier and the output transformer will prevent undesirable currents from flowing through the
transformer primary coil. Many transformers will show increased distortion products when there is a DC current flowing through
the primary coil. Transformers T1 through T4 can provide both impedance matching as well as single ended to differential
conversion. The board is shipped with 2:1 impedance ratio transformers that will match 50Ω equipment with the 100Ω input
impedance of the LMH6882 DVGA . Do not connect the transformer secondary winding directly to ground. The LMH6882 has
a self-biased input common mode voltage of approximately 2.5V.
SPI™ is a trademark of Motorola, Inc.
301993 SNAU129 Copyright © 1999-2012, Texas Instruments Incorporated
Verzeichnis
- ・ Blockdiagramm on Seite 8
- ・ Beschreibung der Funktionen on Seite 1
- ・ Anwendungsbereich on Seite 11
