Datasheet

LTC6406
16
6406fc
–
+
R
F
V
–OUT
V
+OUT
V
VOCM
V
OCM
6406 F05
R
F
R
I
R
I
+
–
V
INP
+
–
V
CM
–
+
V
INM
V
–IN
V
+IN
APPLICATIONS INFORMATION
Input Common Mode Voltage Range
The LTC6406’s input common mode voltage (V
ICM
) is
defi ned as the average of the two input voltages, V
+IN
, and
V
–IN
. At the inputs to the actual op amp, the range extends
from V
–
to V
+
. This makes it easy to interface to a wide
range of common mode signals, from ground referenced to
V
CC
referenced signals. Moreover, due to external resistive
divider action of the gain and feedback resistors, the effective
range of signals that can be processed is even wider. The
input common mode range at the op amp inputs depends
on the circuit confi 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 is approximately:
V
ICM
=
V
+IN
+ V
–IN
2
≈ V
OCM
•
R
I
R
I
+ R
F
⎛
⎝
⎜
⎞
⎠
⎟
+
V
CM
•
R
F
R
F
+ R
I
⎛
⎝
⎜
⎞
⎠
⎟
With single-ended inputs, there is an input signal compo-
nent to the input common mode voltage. Applying only
V
INP
(setting V
INM
to zero), the input common voltage is
approximately:
V
ICM
=
V
+IN
+ V
–IN
2
≈ V
OCM
•
R
I
R
I
+ R
F
⎛
⎝
⎜
⎞
⎠
⎟
+
V
CM
•
R
F
R
F
+ R
I
⎛
⎝
⎜
⎞
⎠
⎟
+
V
INP
2
•
R
F
R
F
+ R
I
⎛
⎝
⎜
⎞
⎠
⎟
Use the equations above to check that the V
ICM
at the op
amp inputs is within range (V
–
to V
+
).
Figure 5. Circuit for Common Mode Range
Manipulating the Rail-to-Rail Input Stage with V
TIP
To achieve rail-to-rail input operation, the LTC6406 features
an NPN input stage in parallel with a PNP input stage. When
the input common mode voltage is near V
+
, the NPNs are
active while the PNPs are off. When the input common
mode is near V
–
, the PNPs are active while the NPNs are
off. At some range in the middle, both input stages are
active. This ‘hand-off’ operation happens automatically.
In the QFN package, a special pin, V
TIP
, is made available
that can be used to manipulate the ‘hand-off’ operation
between the NPN and PNP input stages. By default, the
V
TIP
pin is internally biased by an internal resistive divider
between the supplies, developing a default 1.55V voltage
with a 3V supply. If desired, V
TIP
can be overdriven by an
external voltage (the Thevenin equivalent resistance is
approximately 15k).
If V
TIP
is pulled closer to V
–
, the range over which the NPN
input pair remains active is increased, while the range over
which the PNP input pair is active is reduced. In applica-
tions where the input common mode does not come close
to V
–
, this mode can be used to further improve linearity
beyond the specifi ed performance.
If V
TIP
is pulled closer to V
+
, the range over which the PNP
input pair remains active is increased, while the range over
which the NPN input pair is active is reduced. In applica-
tions where the input common mode does not come close
to V
+
, this mode can be used to further improve linearity
beyond the specifi ed performance.