Datasheet

LTC6405
17
6405fb
For more information www.linear.com/6405
–
+
R
F
V
–OUT
V
+OUT
V
VOCM
V
OCM
6405 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 LTC6405’s input common mode voltage (V
ICM
) is
defined 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 configuration (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






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 LTC6405 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 2.8V voltage
with a 5V supply. If desired, V
TIP
can be over-driven by
an external voltage (the Thevenin equivalent resistance is
approximately 17k).
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 specified performance (see Figure 6).
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 specified performance.
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 6. Manipulating V
TIP
to Improve Harmonic Distortion
HD2
V
TIP
= OPEN
HD3
V
TIP
= OPEN
FREQUENCY (MHz)
6405 F06
DISTORTION (dBc)
V
S
= 5V
V
OCM
= V
ICM
= 2.5V
R
LOAD
= 800Ω
R
I
= R
F
= 499Ω
V
OUTDIFF
= 2V
P-P
SINGLE-ENDED INPUT
QFN PACKAGE
–30
–40
–50
–60
–70
–80
–90
–110
–100
1 10010
HD3
V
TIP
= 1V
HD2
V
TIP
= 1V