Datasheet

LTC6406
14
6406fc
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 LTC6406 makes interfac-
ing to these ADCs easy, by providing both single-ended
to differential conversion as well as common mode level
shifting. The front page of this data sheet shows a typical
application. 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 LTC6406 ideally
suited for preamplifi 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:
V
OUTDIFF
= V
+OUT
–V
–OUT
≅
R
F
R
I
•V
INDIFF
+
Δβ
β
AVG
•V
ICM
–
Δβ
β
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
.
β
AVG
is defi ned as the average feedback factor from the
outputs to their respective inputs:
β
AVG
=
1
2
•
R
I1
R
I1
+ R
F1
+
R
I2
R
I2
+ R
F2
⎛
⎝
⎜
⎞
⎠
⎟
Δβ is defi ned as the difference in feedback factors:
Δβ =
R
I2
R
I2
+ R
F2
–
R
I1
R
I1
+ R
F1
V
ICM
is defi ned 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 defi ned as the difference of the input voltages:
V
INDIFF
= V
INP
– V
INM
V
OCM
is defi ned 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 (Δβ), 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
()
•
Δβ
β
AVG
Figure 3. Real-World Application with Feedback Resistor
Pair Mismatch
–
+
R
F2
V
–OUT
V
+OUT
V
VOCM
V
OCM
6406 F03
R
F1
R
I2
R
I1
+
–
V
INP
–
+
V
INM
V
–IN
V
+IN