Datasheet

LTC6405
16
6405fb
For more information www.linear.com/6405
applications inForMation
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
OCM
pin. Bypassing the V
OCM
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 amplifier appear worse than specified.
The apparent input referred offset induced by feedback
factor mismatch is derived from the above equation:
V
OSDIFF(APPARENT)
≈ (V
ICM
– V
OCM
) • ∆b
Using the LTC6405 in a single supply application on a
single
5V supply with 1% resistors, and the input common
mode grounded, with the V
OCM
pin biased at 2.5V, the
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
INP
or V
INM
input
of Figure 1 depends on whether or not the sources V
INP
and V
INM
are fully differential or not. For balanced input
sources (V
INP
= –V
INM
), the input impedance seen at either
input is simply:
R
INP
= R
INM
= R
I
For single ended inputs, because of the signal imbalance
at the input, the input impedance actually increases over
the balanced differential case. The input impedance looking
into either input is:
R
INP
= R
INM
=
R
I
1–
1
2
•
R
F
R
I
+ R
F












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 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):
R1=
R
INM
• R
S
R
INM
– R
S
According to Figure 4, the input impedance looking into
Figure 4. Optimal Compensation for Signal Source Impedance
V
S
+
–
–
+
R
F
R
F
R
I
R
INM
R
S
R
I
R2 = R
S
|| R1
R1 CHOSEN SO THAT R1 || R
INM
= R
S
R2 CHOSEN TO BALANCE R1 || R
S
R1
6405 F04
the differential amp (R
INM
) reflects the single ended source
case, thus:
R
INM
=
R
I
1–
1
2
•
R
F
R
I
+ R
F












R2 is chosen to equal R1 || R
S
:
R2 =
R1• R
S
R1+ R
S