Datasheet

100k
10M
FREQUENCY (Hz)
-48
-24
12
GAIN (dB)
1G
100M
1M
0
-12
-36
6
-6
-18
-30
-42
C
L
= 10pF
C
L
= 22pF
C
L
= 47pF
V
S
= 10V
C
L
= 0pF
C
L
= 4.7pF
+
-
V
+
V
-
R
IN
BUFFER INTERNAL
CONNECTIONS
C
L
-1
C
L
-2
R
SERIES
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 50Ω system as is common
in high frequency circuits a 50Ω 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 1kΩ, and a capacitor of hundreds of pF's provides the expected response at lower
frequencies.
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