LTC6404 Linear Technology, LTC6404 Datasheet - Page 20

• Manufacturer Part Number: LTC6404
• Description: High Precision Fully Differential Input/Output Amplifier/Driver
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC6404
if voltage transients at the input exceed 1.4V. The inputs
also have steering diodes to either supply. The turn-on and
turn-off time between the shutdown and active states is
typically less than 1μs.
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 (2.7V min), and will have an optimal
common mode input range near mid-supply. The LTC6404
makes interfacing to these ADCs easy, by providing both
single-ended to differential conversion as well as com-
mon mode level shifting. The front page of this data sheet
shows a typical application. Referring to Figure 1, 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 LTC6404 ideally
suited for pre-amplifi cation, level shifting and conversion
20
V
OUTDIFF
OUTDIFF
OUT
+
from V
– V
=
V
OUT
OUT
INM
–
+
) is completely independent of input
–
and V
V
V
V
INM
INP
OUT
Figure 3. Basic Differential Amplifi er with Feedback Resistor Pair Mismatch
+
–
–
+
–
INP
V
V
≈
VOCM
0.01μF
SHDN
0.1μF
V
V
R
is:
R
+
–
F
I
•
(
V
1
2
3
4
INP
SHDN
V
V
V
+
–
OCM
R
R
–
16
V
V
I2
5
I1
SHDN
+
–
V
NC
NC
INM
15
)
6
IN
IN
–
+
V
+
–
R
R
OCM
F2
F1
14
7
of single ended signals to differential output signals to
drive differential input ADCs.
Effects of Resistor Pair Mismatch
In the circuit of Figure 3, it is possible the gain setting
resistors will not perfectly match. Assuming infi nite open
loop gain, the differential output relationship is given by
the equation:
where:
R
of R
β
from the outputs to their respective inputs:
Δβ is defi ned as the difference in feedback factors:
OUT
OUT
AVG
F
–
+
β
V
Δ =
β
is the average of R
V
V
I1
OUTF
OUTF
OUTDIFF
Δ
AVG
β
13
AVG
8
is defi ned as the average feedback factor (or gain)
, and R
β
–
+
OUTF
OUTF
V
R
=
•
+
LTC6404
I
V
–
+
2
2
1
V
V
R
I
–
–
=
6404 F03
N N CM
+
•
I2
I
V
V
V
V
2
V
⎛
⎝ ⎜
R
–
+
+
–
.
12
11
10
9
OUT
R
F
2
–
I
1
V
V
R
–
β
+
OUT
OUT
+
I
Δβ
1
AVG
R
R
–
0.1μF
0.1μF
–
+
F1
I
F
1
V
1
R
+
, and R
OUT
•
+
I
1
R
V
R
F
OCM
–
1
I
0.1μF
2
0.1μF
0.1μF
≅
R
V
V
V
+
+
–
–
F2
I
R
R
2
R
F
, and R
I
F
2
•
V
⎞
⎠ ⎟
INDIFF
I
is the average
+
6404f