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

θJA of

23°C/W. The LMH6522EVAL eight layer board gives a measured

θJA 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 1

μH 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

75 MHz

150 MHz

180 MHz

250 MHz

Bandwidth

40 MHz

60 MHz

75 MHz

100 MHz

R1, R2

90

Ω

90

Ω

90

Ω

90

Ω

L1, L2

390 nH

370 nH

300 nH

225 nH

C1, C2

10 pF

3 pF

2.7 pF

1.9 pF

C3

22 pF

19 pF

15 pF

11 pF

L5

220 nH

62 nH

54 nH

36 nH

R3, R4

100

Ω

100

Ω

100

Ω

100

Ω

(1)

Resistor values are approximate, but have been reduced due to the internal 10

Ω of output resistance

per pin.

24

Submit Documentation Feedback

Copyright © 2012, Texas Instruments Incorporated

Product Folder Links: LMH6523