LTC6405 Linear Technology, LTC6405 Datasheet - Page 17

• Manufacturer Part Number: LTC6405
• Description: Rail-to-Rail Input Differential Amplifier/Driver
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Input Common Mode Voltage Range
The LTC6405’s input common mode voltage (V
defi ned as the average of the two input voltages, V
V
from V
range of common mode signals, from ground referenced to
V
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
Figure 5). For fully differential input applications, where
V
With single ended inputs, there is an input signal compo-
nent to the input common mode voltage. Applying only
V
approximately:
Use the equations above to check that the V
amp inputs is within range (V
–IN
CC
INP
INP
V
V
V
V
V
referenced signals. Moreover, due to external resistive
. At the inputs to the actual op amp, the range extends
ICM
ICM
CM
CM
CM
= –V
(setting V
–
+
–
•
•
=
=
to V
⎛
⎝ ⎜
⎛
⎝ ⎜
INM
Figure 5. Circuit for Common Mode Range
V
V
R
R
V
V
+IN
+IN
INM
F
F
INP
R
R
+
, the common mode input is approximately:
. This makes it easy to interface to a wide
+ R
+ R
F
F
+
–
–
+
INM
+ V
+ V
2
2
I
I
⎞
⎠ ⎟
⎞
⎠ ⎟
–IN
to zero), the input common voltage is
–IN
+
V
VOCM
V
R
R
≈ V
≈ V
I
I
INP
2
OCM
OCM
V
V
•
+IN
–IN
⎛
⎝ ⎜
–
R
•
•
to V
⎛
⎝ ⎜
⎛
⎝ ⎜
F
R
+
–
V
R
R
+ R
R
R
OCM
F
F
F
I
I
OCM
+
R
R
+ R
+ R
).
I
I
I
⎞
⎠ ⎟
F
F
and V
⎞
⎠ ⎟
⎞
⎠ ⎟
+
+
6405 F05
ICM
CM
V
V
at the op
–OUT
+OUT
(refer to
+IN
ICM
, and
) is
Manipulating the Rail-to-Rail Input Stage with V
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
active while the PNPs are off. When the input common
mode is near V
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
that can be used to manipulate the ‘hand-off’ operation
between the NPN and PNP input stages. By default, the
V
between the supplies, developing a default 2.8V voltage
with a 5V supply. If desired, V
an external voltage (the Thevenin equivalent resistance is
approximately 17k).
If V
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
beyond the specifi ed performance (see Figure 6).
If V
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
beyond the specifi ed performance.
TIP
Figure 6. Manipulating V
TIP
TIP
–
+
pin is internally biased by an internal resistive divider
, this mode can be used to further improve linearity
, this mode can be used to further improve linearity
is pulled closer to V
is pulled closer to V
–100
–110
–30
–40
–50
–60
–70
–80
–90
1
V
V
R
–
S
OCM
LOAD
, the PNPs are active while the NPNs are
= 5V
=V
= 800
ICM
HD3
V
TIP
= 2.5V
FREQUENCY (MHz)
= OPEN
TIP
–
+
, the range over which the NPN
HD2
V
, the range over which the PNP
to Improve Harmonic Distortion
R
V
SINGLE-ENDED INPUT
QFN PACKAGE
TIP
I
OUTDIFF
10
= R
= OPEN
TIP
F
= 499
HD3
V
= 2V
TIP
can be over-driven by
TIP
=1V
P-P
HD2
V
, is made available
TIP
LTC6405
6405 F06
=1V
+
, the NPNs are
100
17
TIP
6405fa