LTC6405 Linear Technology, LTC6405 Datasheet - Page 16

• Manufacturer Part Number: LTC6405
• Description: Rail-to-Rail Input Differential Amplifier/Driver
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC6405
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 34dB worst case of
common mode rejection. Using 0.1% resistors will provide
about 54dB of common mode rejection. A low impedance
ground plane should be used as a reference for both the
input signal source and the V
with a high quality 0.1μF ceramic capacitor to this ground
plane will further help prevent common mode signals from
being converted to differential signals.
There may be concern on how feedback factor mismatch
affects distortion. Feedback factor mismatch from using
1% resistors or better, has a negligible effect on distortion.
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 worse than specifi ed.
The apparent input referred offset induced by feedback
factor mismatch is derived from the above equation:
Using the LTC6405 in a single supply application on a
single 5V supply with 1% resistors, and the input com-
mon mode grounded, with the V
worst case DC offset can induce 25mV of apparent offset
voltage. With 0.1% resistors, the worst case apparent
offset reduces to 2.5mV.
Input Impedance and Loading Effects
The input impedance looking into the V
of Figure 1 depends on whether or not the sources V
and V
sources (V
input is simply:
For single ended inputs, because of the signal imbalance
at the input, the input impedance actually increases over
16
V
R
OSDIFF(APPARENT)
INP
INM
= R
are fully differential or not. For balanced input
INP
INM
= –V
= R
INM
I
≈ (V
), the input impedance seen at either
ICM
OCM
– V
OCM
pin. Bypassing the V
OCM
pin biased at 2.5V, the
) • Δβ
INP
or V
INM
input
OCM
INP
the balanced 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 rec-
ommended that the input source output impedance be
compensated for. If input impedance matching is required
by the source, a termination resistor 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 equal R1 || R
Figure 4. Optimal Compensation for Signal Source Impedance
R
R1=
R
R2 =
INP
INM
R1 CHOSEN SO THAT R1 || R
R2 CHOSEN TO BALANCE R1 || R
V
S
R
= R
R
R1+ R
=
R1• R
INM
INM
⎛
⎜
⎝
INM
R2 = R
1–
R
– R
• R
S
S
S
2
=
1
S
S
S
|| R1
•
⎛
⎜
⎝
R1
⎛
⎝ ⎜
1–
R
R
I
INM
I
2
R
1
+ R
INM
F
•
) refl ects the single ended source
R
⎛
⎝ ⎜
= R
S
F
R
INM
R
⎞
⎠ ⎟
I
S
I
⎞
⎟
⎠
S
R
+ R
:
F
R
R
I
I
F
⎞
⎠ ⎟
⎞
⎟
⎠
–
+
R
R
F
F
+
–
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