LT1251/LT1256
U
W U U
APPLICATIONS INFORMATION
Similarly for the inverting case where the noninverting
inputsaregroundedandtheinputvoltagesV1 andV2 drive
the normally grounded ends of RG1 and RG2, we get:
In low gain applications, R1 and R2 are small compared to
the feedback resistors and therefore we can simplify the
equation to:
1−K V
(
)
2
KV
1
+
1−K V
(
)
2
KV
1
R
R
R
R
F2
+
G1
G2
R +R
+1
R
+R
+1
G1
1
G2
2
R
R
R
R
(
)(
)
(
)(
)
G1 F1
G2 F2
F1
V = −
O
R +R
R +R
1−K
(
)
G1
F1
G2
F2
1+ sR C
K
OL
V =
O
+
+
1−K
R
R
R
R
(
)
1+ sR C
K
OL
F1
F2
OL
R +R
+1 R +R
+1
+
+
F1
1
F2
2
R
G2
R
R
R
F2
G1
OL
F1
General Equation for the Inverting Amplifier Case
Note that the denominator causes a gain error due to the
open-loop gain (typically 0.1% for frequencies below
20kHz) and for mismatches in RF1 and RF2. A 1% mis-
match in the feedback resistors results in a 0.25% error at
K = 0.5.
Note that the denominator is the same as the noninverting
case. In low gain applications, R1 and R2 are small
compared to the feedback resistors and therefore we can
simplify the equation to:
If we set RF1 = RF2 and assume ROL >> RF1 (a 0.1% error
at low frequencies) the above equation simplifies to:
1−K V
(
)
2
KV
1
+
V =KV A + 1−K V A
R
R
K
(
)
O
1 V1
2 V2
G1
G2
V = −
O
1−K
(
)
R
R
R
1+ sR C
F1
F2
OL
where A = 1+
and A = 1+
+
+
V1
V2
R
R
R
R
F2
G1
G2
OL
F1
This shows that the output fades linearly from input 2,
times its gain, to input 1, times its gain, as K goes from
0 to 1.
Again, if we set RF1 = RF2 and assume ROL >> RF1 (a 0.1%
error at low frequencies) the above equation simplifies to:
V = − KV A + 1−K V A
If only one input is used (for example, V1) and Pin 14 is
grounded, then the gain is proportional to K.
(
)
O
1 V1
2 V2
[
]
R
R
R
F1
F2
where A
=
and A
=
V2
V1
R
V
G1
G2
O
= KA
V1
V
1
The 4-resistor difference amplifier yields the same result
as the inverting amplifier case, and the common mode
rejection is independent of K.
13
Linear [ Linear ]
