Datasheet
11
1994fb
LT1994
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 that can
be as low as 2.5V and will have an optimal common mode
input range near mid-supply. The LT1994 makes interfac-
ing to these ADCs trivial, by providing both single-ended
to differential conversion as well as common mode level
shifting. Figure 1 shows a general single-supply application
with perfectly matched feedback networks from OUT
+
and
OUT
–
. 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 that the differential output
voltage (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 LT1994 ideally
suited 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 2 shows a circuit diagram that 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
VV
OUTDIFF OUT OUT
F
I
INDIFF
AVG
ICM
AVG
OCM
=≅+
ΔΔ
+
–•
•– •,
–
β
β
β
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
2
22
1
11
•
Δβ is defi ned as the difference in feedback factors:
Δ=
++
β
R
RR
R
RR
I
IF
I
IF
2
22
1
11
–
Figure 1. Test Circuit Figure 2. Real-World Application
V
OCM
0.1µF
1994 F01
R
I
R
F
R
L
V
+
0.1µF
0.1µF
V
CM
V
SHDN
V
INM
V
INP
V
OUTCM
3
7
6
8
1
2
5
4
V
–
V
OUT
+
V
IN
–
V
OUT
–
V
IN
+
R
BAL
R
BAL
0.1µF
R
I
R
F
R
L
+
–
–
+
LT1994V
OCM
1994 F02
R
I2
R
F2
R
L
V
S
V
SHDNB
V
INM
V
INP
+
–
–
+
3
7
6
8
1
2
5
4
LT1994V
OCM
V
OCM
V
OUT
+
V
IN
–
V
OUT
–
V
IN
+
0.1µF
0.1µF
R
I1
R
F1
R
L
SHDN
APPLICATIONS INFORMATION