100k

10M

FREQUENCY (Hz)

-48

-24

12

1G

100M

1M

0

-12

-36

6

-6

-18

-30

-42

+

-

V

+

V

-

BUFFER INTERNAL

CONNECTIONS

L-1

L-2

R-1

R-2

LMH6559

www.ti.com

SNOSA57C – APRIL 2003 – REVISED MARCH 2013

Figure 44.

Consider a frequency sweep over the entire spectrum for which the LMH6559 high frequency buffer is active. In

the first instance peaking occurs due to the parasitic capacitance connected at the load whereas at higher

frequencies the effects of the series combination of L and C become noticeable. This causes a distinctive dip in

the output frequency sweep and this dip varies depending upon the particular capacitor as seen in Figure 45.

Figure 45.

To minimize peaking due to CL a series resistor for the purpose of isolation from the output stage should be

used. A low valued resistor will minimize the influence of such a load capacitor. In a 50

Ω system as is common

in high frequency circuits a 50

Ω series resistor is often used. Usage of the series resistor, as seen in Figure 46

eliminates the peaking but not the dip. The dip will vary with the particular capacitor. Using a resistor in series

with a capacitor creates in a single pole situation a 6dB/oct rolloff. However, at high frequencies the internal

inductance is appreciable and forms a series LC combination with the capacitor. Choice of a higher valued

resistor, for example 500 to 1k

Ω, and a capacitor of hundreds of pF's provides the expected response at lower

frequencies.

Copyright © 2003–2013, Texas Instruments Incorporated

Submit Documentation Feedback

21

Product Folder Links: LMH6559