Datasheet

LT1994
12
1994fb
V
ICM
is defi ned as the average of the two input voltages,
V
INP
and V
INM
(also called the input common mode
voltage):
VVV
ICM INP INM
=+
()
1
2
•
and V
INDIFF
is defi ned as the difference of the input
voltages:
V
INDIFF
= V
INP
– V
INM
When the feedback ratios mismatch (Δβ), common mode
to differential conversion occurs.
Setting the differential input to zero (V
INDIFF
= 0), the de-
gree of common mode to differential conversion is given
by the equation:
VVV
VV
V
OUTDIFF OUT OUT
ICM OCM
AVG
INDIFF
z

$


–
–•
–
B
B
0
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 provide about
28dB 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 reference
for both the input signal source and the V
OCM
pin. A direct
short of V
OCM
to this ground plane or bypassing the V
OCM
with a high quality 0.1µF ceramic capacitor to this ground
plane will further mitigate against common mode signals
from being converted to differential.
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. 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:
RR
R
R
RR
INP INM
I
F
IF
==
+
⎡
⎣
⎢
⎤
⎦
⎥
⎛
⎝
⎜
⎞
⎠
⎟
1
1
2
–•
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 3):
R
RR
RR
INM S
INM S
1=
−
•
According to Figure 3, the input impedance looking into
the differential amp (R
INM
) refl ects the single-ended source
case, thus:
R
R
R
RR
INM
I
F
IF
=
+
⎡
⎣
⎢
⎤
⎦
⎥
⎛
⎝
⎜
⎞
⎠
⎟
1
1
2
–•
R2 is chosen to balance R1||R
S
:
R
RR
RR
S
S
2
1
1
=
+
•
Figure 3. Optimal Compensation for Signal-Source Impedance
1994 F03
R
I
R
F
R
S
V
S
+
–
–
+
LT1994
R
I
R
F
R2 = R
S
|| R1
R1 CHOSEN SO THAT R1 || R
INM
= R
S
R2 CHOSEN TO BALANCE R1 || R
S
R1
R
INM
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