LT1994 Linear Technology, LT1994 Datasheet - Page 11

• Manufacturer Part Number: LT1994
• Description: Low Noise - Low Distortion Fully Differential Input/Output Amplifier/Driver
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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
OUT
Note from the above equation that the differential output
voltage (V
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:
V
V
β
OUTDIFF
OUTDIFF
–
∆
AVG
. The gain to V
β
V
CM
•
V
OUT
V
V
INM
INP
ICM
=
=
+
V
– V
V
V
–
OCM
OUT
OUT
R
R
I
0.1µF
I
OUT
β
∆
AVG
U
Figure 1. Test Circuit
V
V
OUTDIFF
+
+
β
IN
IN
–
–
+
1
2
8
–
–
) is completely independent of input
V
7
V
SHDN
+
R
–
R
•
V
V
OCM
F
F
V
OUT
OUT
U
OCM
3
6
V
V
+
–
LT1994
from V
+
–
–
–
4
5
,
0.1µF
0.1µF
≅
≈
R
R
R
R
INM
F
F
I
I
W
•
•
V
V
V
OUT
OUT
(
0.1µF
and V
INDIFF
V
+
–
1994 F01
INP
R
R
BAL
BAL
–
R
V
R
INP
+
L
OUTCM
L
V
U
INM
is:
)
+
and
where: R
average of R
β
from the outputs to their respective inputs:
Δβ is defi ned as the difference in feedback factors:
V
V
voltage):
and V
voltages:
When the feedback ratios mismatch (Δβ), common mode
to differential conversion occurs.
Setting the differential input to zero (V
gree of common mode to differential conversion is given
AVG
ICM
INP
β
∆ =
V
V
ICM
INDIFF
AVG
β
is defi ned as the average of the two input voltages,
is defi ned as the average feedback factor (or gain)
and V
INDIFF
=
=
F
R
V
V
I
=
2
INM
is the average of R
1
2
1
INP
2
INM
R
I1
•
•
+
V
is defi ned as the difference of the input
I
(
⎛
⎜
⎝
INP
Figure 2. Real-World Application
2
V
R
and R
R
INP
V
R
R
F
OCM
I
(also called the input common mode
I2
I1
2
2
0.1µF
–
R
+
–
+
V
I
2
V
V
INM
I2
R
IN
IN
R
V
–
+
.
F
1
2
8
INM
I
V
1
2
SHDNB
SHDN
7
R
V
R
R
+
–
+
F2
OCM
F1
+
I
1
R
)
3
6
R
V
F
S
LT1994
–
+
I
1
1
R
4
5
F1
+
I
1
R
www.DataSheet4U.com
and R
F
1
⎞
⎟
⎠
V
V
OUT
OUT
1994 F02
INDIFF
0.1µF
+
–
F2
, and R
R
R
L
L
LT1994
= 0), the de-
11
I
is the
1994fa