LTC6405
20
6405fb
For more information www.linear.com/6405
applications inForMation
The LTC6405’s input referred voltage noise contributes the
equivalent noise of a 155Ω resistor. When the feedback
network is comprised of resistors whose values are less
than this, the LTC6405’s output noise is voltage noise
dominant (see Figure 11):
e
no
≈ e
ni
• 1+
R
F
R
I
Feedback networks consisting of resistors with values
greater than about 200Ω will result in output noise which
is resistor noise and amplifier current noise dominant.
e
no
≈ 2 • I
n
• R
F
( )
2
+ 1+
R
F
R
I
• 4 • k • T • R
F
Lower resistor values (<100Ω) always result in lower noise
at the penalty of increased distortion due to increased
loading of the feedback network on the output. Higher
resistor values (but still less than <500Ω) will result in
higher output noise, but typically improved distortion due
to less loading on the output. The optimal feedback resis-
tance for the LTC6405 runs in between 100Ω to 500Ω.
The differential filtered outputs +OUTF and –OUTF will
have a little higher noise than the unfiltered outputs (due
to the two 50Ω resistors which contribute 0.9nV/√Hz
each), but can provide superior signal-to-noise due to the
output noise filtering.
Layout Considerations
Because the LTC6405 is a very high speed amplifier, it is
sensitive to both stray capacitance and stray inductance.
In the QFN package, three pairs of power supply pins are
provided to keep the power supply inductance as low
as possible to prevent any degradation of amplifier 2nd
harmonic performance. It is critical that close attention be
paid to supply bypassing. For single supply applications
it is recommended that high quality 0.1µF surface mount
ceramic bypass capacitor be placed directly between each
V
+
and V
–
pin with direct short connections. The V
–
pins
should be tied directly to a low impedance ground plane
with
minimal routing. For dual (split) power supplies, it is
recommended that additional high quality, 0.1µF ceramic
capacitors are used to bypass V
+
to ground and V
–
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 summing
junctions, +IN and –IN, should be minimized. This becomes
especially true when the feedback resistor network uses
resistor values >500Ω in circuits with R
F
= R
I
. Always keep
in mind the differential nature of the LTC6405, and that it
is critical that the load impedances seen by both outputs
(stray or intended), should be as balanced and symmetric
as possible. This will help preserve the natural balance
of the LTC6405, which minimizes the generation of even
order harmonics, and improves the rejection of common
mode signals and noise.
It is highly recommended that the V
OCM
pin be 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 feedback networks be
comprised of 1% resistors (or better) to enhance the
output common mode rejection. This will also prevent
V
OCM
input referred common mode noise of the common
mode amplifier path (which cannot be filtered) 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 will limit any mismatch from
impacting amplifier linearity. However, in single supply
level shifting applications where there is a voltage differ-
ence between the input common mode voltage and the
output common mode voltage, resistor mismatch can
make the apparent voltage offset of the amplifier appear
worse than specified.
