LTC6405 Linear Technology, LTC6405 Datasheet - Page 15

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

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APPLICATIONS INFORMATION
General Amplifi er Applications
As levels of integration have increased and correspond-
ingly, system supply voltages decreased, there has been
a need for ADCs to process signals differentially in order
to maintain good signal to noise ratios. These ADCs are
typically supplied from a single supply voltage which can
be as low as 3V, and will have an optimal common mode
input range of 1.25V or 1.5V. The LTC6405 makes interfac-
ing to these ADCs easy, by providing both single-ended
to differential conversion as well as common mode level
shifting. The gain to V
Note from the above equation, the differential output volt-
age (V
and output common mode voltages, or the voltage at the
common mode pin. This makes the LTC6405 ideally suited
for pre-amplifi cation, level shifting and conversion of single
ended signals to differential output signals in preparation
for driving differential input ADCs.
Effects of Resistor Pair Mismatch
Figure 3 shows a circuit diagram which takes into consid-
eration that real world resistors will not match perfectly.
Assuming infi nite open loop gain, the differential output
relationship is given by the equation:
where:
R
of R
β
outputs to their respective inputs:
AVG
F
β
V
V
β
is the average of R
I1
OUTDIFF
OUTDIFF
Δβ
AVG
AVG
is defi ned as the average feedback factor from the
, and R
+OUT
=
• V
2
1
– V
ICM
= V
= V
I2
•
⎛
⎝ ⎜
.
–OUT
+OUT
+OUT
R
–
I1
β
R
Δβ
+ R
AVG
) is completely independent of input
I1
– V
– V
OUTDIFF
F1
F1
• V
, and R
–OUT
–OUT
+
OCM
R
I2
from V
≈
≅
R
+ R
F2
R
I2
R
R
R
F
F
I
, and R
I
F2
• V
• V
INM
(
⎞
⎠ ⎟
INDIFF
INP
and V
I
is the average
– V
+
INM
INP
)
is:
Δβ is defi ned as the difference in feedback factors:
V
and V
and V
V
V
When the feedback ratios mismatch (Δβ), common mode
to differential conversion occurs.
Setting the differential input to zero (V
gree of common mode to differential conversion is given
by the equation:
ICM
OCM
+OUT
V
Δβ =
V
V
V
INDIFF
ICM
OCM
OUTDIFF
is defi ned as the average of the two input voltages V
INDIFF
INM
is defi ned as the average of the two output voltages
and V
=
R
V
=
V
INM
(also called the input common mode voltage):
INP
= V
I2
2
1
Figure 3. Real-World Application with
Feedback Resistor Pair Mismatch
is defi ned as the difference of the input voltages:
V
R
• V
–OUT
= V
+
–
–
+
+ R
+OUT
I2
INP
(
+OUT
F2
INP
:
– V
+ V
V
2
–
VOCM
+ V
R
R
INM
I2
I1
R
– V
−OUT
I1
INM
R
+ R
V
V
–OUT
I1
+IN
–IN
)
F1
≈ V
+
–
V
R
R
OCM
(
F2
F1
ICM
6405 F03
– V
INDIFF
LTC6405
OCM
V
V
–OUT
+OUT
= 0), the de-
)
•
β
15
Δβ
AVG
6405fa
INP