Datasheet

LTC6404
22
6404f
APPLICATIONS INFORMATION
According to Figure 4, 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
IS
IS
2
=
+
•
Input Common Mode Voltage Range
The LTC6404’s input common mode voltage (V
ICM
) is
defi ned as the average of the two input voltages, V
IN
+
, and
V
IN
–
. It extends from V
–
to 1.4V below V
+
. The operating
input common mode range depends on the circuit con-
fi guration (gain), V
OCM
and V
CM
(Refer to Figure 5). For
fully differential input applications, where V
INP
= –V
INM
,
the common mode input voltage is approximately:
V
VV
V
R
RR
V
R
R
ICM
IN IN
OCM
I
IF
CM
F
F
=
+
≈
+
⎛
⎝
⎜
⎞
⎠
⎟
+
+ –
•
•
2
++
⎛
⎝
⎜
⎞
⎠
⎟
R
I
With singled ended inputs, there is an input signal com-
ponent to the input common mode voltage. Applying only
V
INP
(setting V
INM
to zero), the input common voltage is
approximately:
V
VV
V
R
RR
V
R
R
ICM
IN IN
OCM
I
IF
CM
F
F
=
+
≈
+
⎛
⎝
⎜
⎞
⎠
⎟
+
+ –
•
•
2
++
⎛
⎝
⎜
⎞
⎠
⎟
+
+
⎛
⎝
⎜
⎞
⎠
⎟
R
VR
RR
I
INP F
FI
2
•
Output Common Mode Voltage Range
The output common mode voltage is defi ned as the aver-
age of the two outputs:
VV
VV
OUTCM OCM
OUT OUT
==
+
+ –
2
The V
OCM
pin sets this average by an internal common
mode feedback loop which internally forces V
OUT
+
= –V
OUT
–
.
The output common mode range extends from 1.1V above
V
–
to 1V below V
+
(see the Electrical Characteristics table
for the LTC6404-4 output common mode voltage range).
The V
OCM
pin sits in the middle of a voltage divider which
sets the default mid-supply open circuit potential.
Figure 5. Circuit for Common Mode Range
V
–
V
–
V
+
0.1µF
0.1µF 0.1µF
0.1µF
0.1µF
V
CM
–
+
1
SHDN
5 6
IN
–
7
OUT
+
8
OUTF
+
16 15
IN
+
NC
NC
14
OUT
–
13
OUTF
–
V
OUTF
–
R
F
V
OUTF
+
V
OUT
–
V
OUT
+
2
V
+
3
V
–
V
+
V
+
V
–
V
+
V
–
4
V
OCM
V
SHDN
V
VOCM
V
OCM
12
V
–
11
V
+
10
V
+
9
V
–
V
–
V
–
6404 F05
LTC6404
SHDN
0.1µF
0.01µF
R
F
R
I
R
I
+
–
V
INP
–
+
V
INM