Datasheet

LTC6405
15
6405fb
For more information www.linear.com/6405
applications inForMation
General Amplifier 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
interfacing to these ADCs easy, by providing both single-
ended to differential conversion as well as common mode
level shifting. The gain to V
OUTDIFF
from V
INM
and V
INP
is:
V
OUTDIFF
= V
+OUT
– V
–OUT
≈
R
F
R
I
• V
INP
– V
INM
( )
Note from the above equation, the differential output volt-
age (V
+OUT
– V
–OUT
) is completely independent of input
and output common mode voltages, or the voltage at the
common mode pin. This makes the LTC6405 ideally suited
for pre-amplification, 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 infinite open loop gain, the differential output
relationship is given by the equation:
V
OUTDIFF
= V
+OUT
– V
–OUT
≅
R
F
R
I
• V
INDIFF
+
∆b
b
AVG
• V
ICM
–
∆b
b
AVG
• V
OCM
where:
R
F
is the average of R
F1
, and R
F2
, and R
I
is the average
of R
I1
, and R
I2
.
b
AVG
is defined as the average feedback factor from the
outputs to their respective inputs:
b
AVG
=
1
2
•
R
I1
R
I1
+ R
F1
+
R
I2
R
I2
+ R
F2






∆b is defined as the difference in feedback factors:
∆b =
R
I2
R
I2
+R
F2
–
R
I1
R
I1
+R
F1
V
ICM
is defined as the average of the two input voltages V
INP
and V
INM
(also called the input common mode voltage):
V
ICM
=
1
2
• V
INP
+ V
INM
( )
and V
INDIFF
is defined as the difference of the input voltages:
V
INDIFF
= V
INP
– V
INM
V
OCM
is defined as the average of the two output voltages
V
+OUT
and V
–OUT
:
V
OCM
=
V
+OUT
+
V
−OUT
2
When the feedback ratios mismatch (∆b), common mode
to differential conversion occurs.
Setting the differential input to zero (V
INDIFF
= 0), the de-
gree of common mode to differential conversion is given
by the equation:
V
OUTDIFF
= V
+OUT
– V
–OUT
≈ V
ICM
– V
OCM
( )
•
∆
b
b
AVG
Figure 3. Real-World Application with
Feedback Resistor Pair Mismatch
–
+
R
F2
V
–OUT
V
+OUT
V
VOCM
V
OCM
6405 F03
R
F1
R
I2
R
I1
+
–
V
INP
–
+
V
INM
V
–IN
V
+IN