ltc6404-2 Linear Technology Corporation, ltc6404-2 Datasheet - Page 17

• Manufacturer Part Number: ltc6404-2
• Description: 600mhz, Low Noise, Ac Precision Fully Differential Input/output Ampli?er/driver
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
R
higher output noise, but improved distortion due to less
loading on the output. The optimal feedback resistance for
the LTC6404-2 is between R
The differential fi ltered outputs OUTF
a little higher spot noise than the unfi ltered outputs (due to
the two 50Ω resistors which contribute 0.9nV/√Hz each),
but actually will provide superior Signal-to-Noise in noise
bandwidths exceeding 139MHz due to the noise-fi ltering
function the fi lter provides.
Layout Considerations
Because the LTC6404-2 is a very high speed amplifi er, it is
sensitive to both stray capacitance and stray inductance.
Three pairs of power supply pins are provided to keep the
power supply inductance as low as possible to prevent
degradation of amplifi er 2nd Harmonic performance. It is
critical that close attention be paid to supply bypassing. For
single supply applications (Pins 3, 9 and 12 grounded) it
is recommended that 3 high quality 0.1μF surface mount
ceramic bypass capacitors be placed between pins 2 and
3, between pins 11and 12, and between pins10 and 9 with
direct short connections. Pins 3, 9 and 10 should be tied
directly to a low impedance ground plane with minimal
routing. For dual (split) power supplies, it is recommended
that at least two additional high quality, 0.1μF ceramic
capacitors are used to bypass pin V
F
resistor values (but still less than 400Ω) will result in
Figure 10. LTC6404-2 Output Spot Noise vs Spot Noise
Contributed by Feedback Network Alone
100
0.1
10
1
10
TOTAL (AMPLIFIER AND
FEEDBACK NETWORK)
OUTPUT NOISE
100
R
F
FEEDBACK RESISTOR
NETWORK NOISE ALONE
= 2 • R
F
= 200Ω to 400Ω.
I
(Ω)
1k
+
+
to ground and V
and OUTF
64042 F10
10k
–
will have
–
to
ground, again with minimal routing. For driving large loads
(<200Ω), additional bypass capacitance may be needed for
optimal performance. Keep in mind that small geometry
(e.g. 0603) surface mount ceramic capacitors have a much
higher self resonant frequency than do leaded capacitors,
and perform best in high speed applications.
Any stray parasitic capacitances to ground at the sum-
ming junctions IN
minimum even if it means stripping back the ground plane
away from any trace attached to this node. This becomes
especially true when the feedback resistor network uses
resistor values >400Ω in circuits with R
sive peaking in the frequency response can be mitigated
by adding small amounts of feedback capacitance (~1pF)
around R
LTC6404-2, and that it is critical that the load impedances
seen by both outputs (stray or intended) should be as bal-
anced and symmetric as possible. This will help preserve
the natural balance of the LTC6404-2, which minimizes the
generation of even order harmonics, and preserves the
rejection of common mode signals and noise.
It is highly recommended that the V
tied to a low impedance ground plane (in split supply
applications), or bypassed to ground with a high quality
ceramic capacitor whose value exceeds 0.01μF . This will
help stabilize the common mode feedback loop as well as
prevent thermal noise from the internal voltage divider and
other external sources of noise from being converted to
differential noise due to divider mismatches in the feedback
networks. It is also recommended that the resistive feed-
back networks be comprised of 1% resistors (or better)
to enhance the output common mode rejection. This will
also prevent V
common mode amplifi er path (which cannot be fi ltered)
from being converted to differential noise, degrading the
differential noise performance.
Feedback factor mismatch has a weak effect on distortion.
Using 1% or better resistors should prevent mismatch
from impacting amplifi er linearity. However, in single
supply level shifting applications where there is a voltage
difference between the input common mode voltage and
the output common mode voltage, resistor mismatch can
F
. Always keep in mind the differential nature of the
OCM
+
, and IN
referred common mode noise of the
–
should be kept to an absolute
OCM
LTC6404-2
pin be either hard
F
= 2 • R
I
. Exces-
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