PHASE (Degrees)

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

-2.0

-2.5

0

45

90

135 180 225 270 315 360

OUT+

OUT-

(OUT+) - (OUT-)

1.25VP

2.5VPP

5VPPD (DIFFERENTIAL)

SVA-30206562

GAIN 0-4

5

LMH6523

5V

L1

L2

L1, L2 = 22nH

C1 = 4pF

C1

SOURCE IMPEDANCE = 100

f = 200MHz

ZAMP = (130 + j10)

ZIN = (97.8 - j1.5)

ZIN

SVA-30206569

LMH6523

www.ti.com

SNOSC88 – DECEMBER 2012

Excel can also be used for simple circuits; however, the “Analysis ToolPak” add-in must be installed to calculate

complex numbers.

http://www.circuitsage.com/matching/matcher2.html

Figure 45. Differential LC Conversion Circuit

OUTPUT CHARACTERISTICS

The LMH6523 has a low impedance output very similar to a traditional Op-amp output. This means that a wild

range of loads can be driven with good performance. Matching load impedance for proper termination of filters is

as easy as inserting the proper value of resistor between the filter and the amplifier. This flexibility makes system

design and gain calculations very easy.

By using a differential output stage the LMH6523 can achieve very large voltage swings on a single 5V supply.

This is illustrated in Figure 46. This figure shows how a voltage swing of 5VPPD is realized while only swinging 2.5

VPP on each output. The LMH6523 can swing up to 10 VPPD which is sufficient to drive most ADCs to full scale

while using a matched impedance anti alias filter between the amplifier and the ADC. The LMH6523 has been

designed for AC coupled applications and has been optimized for operation above 5 MHz.

Figure 46. Differential Output Voltage

Like most closed loop amplifiers the LMH6523 output stage can be sensitive to capacitive loading. To help with

board layout and to help minimize sensitivity to bias inductor capacitance the LMH5522 output lines have internal

10

Ω resistors. These resistors should be taken into account when choosing matching resistor values. This is

shown in as using 40.2

Ω resistors instead of 50Ω resistors to match the 100 Ω differential load. Best practise is

to place the external termination resistors as close to the DVGA output pins as possible. Due to reactive

components between the DVGA output and the filter input it may be desirable to use even smaller value resistors

than a simple calculation would indicate. For instance, at 200 MHz resistors of 30

Ω provide slightly better OIP3

performance on the LMH6523EVAL evaluation board and may also provide a better match to the filter input.

Copyright © 2012, Texas Instruments Incorporated

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Product Folder Links: LMH6523