LMH6523 Datasheet, PDF (24/31 Pages) Texas Instruments – High Performance Quad DVGA

LMH6523
SNOSC88 – DECEMBER 2012
www.ti.com
There is no evaluation board available for the LMH6523, however the LMH6522 is pin compatible, so the
LMH6522 evaluation board should be used as a suggested layout design.
The LMH6522EVAL evaluation board was designed for both signal integrity and thermal dissipation. The
LMH6522EVAL has eight layers of copper. The inner copper layers are two ounce copper and are as solid as
design constraints allow. The exterior copper layers are one ounce copper in order to allow fine geometry
etching. The benefit of this board design is significant. The JEDEC standard 4 layer test board gives a θ_JA of
23°C/W. The LMH6522EVAL eight layer board gives a measured θ_JA of 15°C/W (ambient temperature 25°C, no
forced air). With the typical power dissipation of 2.3W this is a temperature difference of 18 degrees in junction
temperature between the standard 4 layer board and the enhanced 8 layer evaluation board. In a system design
the location and power dissipation of other heat sources may change the results observed compared with the
LMH6522EVAL board.
Applying a heat sink to the package will also help to remove heat from the device. The ATS-54150K-C2–R0 heat
sink, manufactured by Advanced Thermal Solutions, provided good results in lab testing. Using both a heat sink
and a good board thermal design will provide the best cooling results. If a heat sink will not fit in the system
design, the external case can be used as a heat sink.
Package information is available on the TI web site.
http://www.ti.com/packaging/
INTERFACING TO AN ADC
The LMH6523 was designed to be used with high speed ADCs such as the ADC16DV160. As shown in the
PERFORMANCE CURVE, AC coupling provides the best flexibility especially for IF sub-sampling applications.
The inputs of the LMH6523 will self bias to the optimum voltage for normal operation. The internal bias voltage
for the inputs is approximately mid rail which is 2.5V with the typical 5V power supply condition. In most
applications the LMH6523 input will need to be AC coupled.
The output pins require a DC path to ground that will carry the ~36 mA of bias current required to power the
output transistors. The output common mode voltage should be established very near to ground. This means that
using RF chokes or RF inductors is the easiest way to bias the LMH6523 output pins. Inductor values of 1μH to
400nH are recommended. High Q inductors will provide the best performance. If low frequency operation is
desired, particular care must be given to the inductor selection because inductors that offer good performance at
very low frequencies often have very low self resonant frequencies. If very broadband operation is desired the
use of conical inductors such as the BCL–802JL from Coilcraft may be considered. These inductors offer very
broadband response, at the expense of large physical size and a high DC resistance of 3.4 Ohms.
ADC Noise Filter
Below are schematics and a table of values for second order Butterworth response filters for some common IF
frequencies. These filters, shown in Figure 56, offer a good compromise between bandwidth, noise rejection and
cost. This filter topology is the same as is used on the ADC14V155KDRB High IF Receiver reference design
board. This filter topology works best with the 12, 14 and 16 bit analog to digital converters shown in the table.
Table 10. Filter Component Values⁽¹⁾
Center Frequency
Bandwidth
R1, R2
L1, L2
C1, C2
C3
L5
R3, R4
75 MHz
40 MHz
90Ω
390 nH
10 pF
22 pF
220 nH
100Ω
150 MHz
60 MHz
90Ω
370 nH
3 pF
19 pF
62 nH
100Ω
180 MHz
75 MHz
90Ω
300 nH
2.7 pF
15 pF
54 nH
100Ω
250 MHz
100 MHz
90Ω
225 nH
1.9 pF
11 pF
36 nH
100Ω
(1) Resistor values are approximate, but have been reduced due to the internal 10 Ω of output resistance
per pin.
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