Datasheet

LTC6403-1
12
64031fa
APPLICATIONS INFORMATION
the non-linear capacitance, the outputs still have the ability
to sink and source small amounts of transient current if
exposed to signifi cant voltage transients. The inputs (+IN
and –IN) appear as anti-parallel diodes which can conduct
if voltage transients at the input exceed 1.4V. The inputs
also have steering diodes to either supply. The turn-on
time between the shutdown and active states is typically
4μs, and turn-off time is typically 350ns.
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 operated from a single supply voltage which can
be as low as 3V (2.7V min), and will have an optimal com-
mon mode input range near mid-supply. The LTC6403-1
makes interfacing to these ADCs trivial, by providing both
single ended to differential conversion as well as common
mode level shifting. The front page of this datasheet shows
a typical application. Referring to Figure 1, the gain to
V
OUTDIFF
from V
INM
and V
INP
is:
VVV
R
R
VV
OUTDIFF OUT OUT
F
I
INP INM
=≈
()
+
–•–
–
Note from the above equation, the differential output
voltage (V
+OUT
– V
–OUT
) is completely independent of
input and output common mode voltages. This makes
the LTC6403-1 ideally suited for pre-amplifi cation, level
shifting and conversion of single-ended input signals to
differential output signals in preparation for driving dif-
ferential input ADCs.
Effects of Resistor Pair Mismatch
Figure 3 shows a circuit diagram with takes into consid-
eration that real world resistors will not perfectly match.
Assuming infi nite open loop gain, the differential output
relationship is given by the equation:
VVV
R
R
V
V
OUTDIFF OUT OUT
F
I
INDIFF
AVG
I
=≈+
Δ
+
–•
•
–
β
β
NNCM
AVG
OCM
V–•
Δβ
β
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 (or gain)
from the outputs to their respective inputs:
β
AVG
I
IF
I
IF
R
RR
R
RR
=
+
+
+
⎛
⎝
⎜
⎞
⎠
⎟
1
2
1
11
2
22
•
Δβ is defi ned as the difference in feedback factors:
Δ=
++
β
R
RR
R
RR
I
IF
I
IF
2
22
1
11
–
V
INCM
is defi ned as the average of the two input voltages
V
INP
, and V
INM
(also called the source-referred input com-
mon mode voltage):
VVV
INCM INP INM
=+
()
1
2
•
and V
INDIFF
is defi ned as the difference of the input
voltages:
V
INDIFF
= V
INP
– V
INM
Figure 3. Real World Application With Feedback Resistor
Pair Mismatch
V
–
V
–
V
+
0.1μF
0.1μF
0.1μF
0.1μF
0.1μF
–
+
1
SHDN
5 6
–IN
7
+OUT
8
+OUTF
16 15
+IN
NC
NC
14
–OUT
13
–OUTF
V
–OUTF
R
F2
V
+OUTF
V
–OUT
V
+OUT
2
V
+
3
V
–
V
+
V
+
V
–
V
+
V
–
4
V
OCM
V
SHDN
V
VOCM
V
OCM
12
V
–
11
V
+
10
V
+
9
V
–
V
–
V
–
64031 F03
LTC6403-1
SHDN
0.1μF
0.01μF
R
F1
R
I2
R
I1
+
–
V
INP
–
+
V
INM
V
–IN
V
+IN