ltc6404cud-2-trpbf Linear Technology Corporation, ltc6404cud-2-trpbf Datasheet - Page 13

• Manufacturer Part Number: ltc6404cud-2-trpbf
• Description: 600mhz, Low Noise, Ac Precision Fully Differential Input/output Ampli?er/driver
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
When the feedback ratios mismatch (Δβ), common mode
to differential conversion occurs.
Setting the differential input to zero (V
gree of common mode to differential conversion is given
by the equation:
In general, the degree of feedback pair mismatch is a
source of common mode to differential conversion of
both signals and noise. Using 1% resistors or better will
mitigate most problems, and will provide about 28dB worst
case of common mode rejection. Using 0.1% resistors
will provide about 48dB of common mode rejection. A
low impedance ground plane should be used as a refer-
ence for both the input signal source, and the V
A direct short of V
V
ground plane, will further prevent common mode signals
from being converted to differential.
There may be concern on how feedback ratio mismatch
affects distortion. Distortion caused by feedback ratio mis-
match using 1% resistors or better is negligible. 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 make the apparent voltage offset of the
amplifi er appear higher than specifi ed.
The apparent input referred offset induced by feedback
ratio mismatch is derived from the following equation:
Using the LTC6404-1 in a single supply application on a
single 5V supply with 1% resistors, and the input common
mode grounded, with the V
the worst case DC offset can induce 25mV of apparent
offset voltage. With 0.1% resistors, the worst case appar-
ent offset reduces to 2.5mV.
OCM
V
≈
V
OSDIFF(APPARENT)
OUTDIFF
(
V
with a high quality 0.1μF ceramic capacitor to this
INCM
=
–
V
V
OUT
OCM
OCM
+
)
•
≈ (V
–
β
V
Δ
to this ground or bypassing the
OUT
AVG
ICM
β
⏐
OCM
–
V
– V
I
N N DIFF
pin biased at mid-supply,
OCM
= 0
) • Δβ
INDIFF
= 0), the de-
OCM
pin.
Input Impedance and Loading Effects
The input impedance looking into the V
of Figure 1 depends on whether the sources V
V
(V
is simply:
For single ended inputs, because of the signal imbalance
at the input, the input impedance increases over the bal-
anced differential case. The input impedance looking into
either input is:
Input signal sources with non-zero output impedances can
also cause feedback imbalance between the pair of feedback
networks. For the best performance, it is recommended
that the source’s output impedance be compensated for.
If input impedance matching is required by the source,
R1 should be chosen (see Figure 4):
According to Figure 4, the input impedance looking into
the differential amp (R
case, thus:
R2 is chosen to balance R1 || R
INM
INP
R
R
R
R
R
INP
INP
1=
INM
2 =
= –V
are fully differential. For balanced input sources
= R
R
=
R
R
=
R R
INM
INM
I
R
I
INM
⎛
⎜
⎝
+
•
INM
INM
1
R
– •
–
•
), the input impedance seen at either input
S
S
R
R
= R
2
=
1
S
S
⎛
⎜
⎝
⎛
⎝ ⎜
I
1
R
R
– •
I
INM
I
2
R
1
+
F
R
) refl ects the single ended source
⎛
⎝ ⎜
F
R
R
⎞
⎠ ⎟
I
I
⎞
⎟
⎠
R
+
F
R
S
:
F
⎞
⎠ ⎟
LTC6404-1
⎞
⎟
⎠
INP
or V
INM
INP
13
input
64041f
and