LT1251/LT1256
U
W U U
APPLICATIONS INFORMATION
Signal Path Description
R
R
F1
G1
I
1
R
1
2
–
I
1
1
2
K
1
+
V
1
I
8
O
V
O
+1
Σ
14
C
R
OL
V
2
+
–
I
2
1 – K
13
R
2
I
2
R
F2
R
G2
1251/56 BD
Figure 2. Signal Path Block Diagram
V
V
O
Figure 2 is the basic block diagram of the LT1251/LT1256
signal path with external resistors RG1, RF1, RG2 and RF2.
Both input stages are operating as noninverting amplifiers
with two input signals V1 and V2.
2
I =
−
2
R
R
R
F2
(
)(
)
G2 F2
R +R
+1
F2
2
R +
2
R
G
2
R
+R
G
2
F
2
I = KI + 1−K I
(
)
Each input stage has a unity-gain buffer from the nonin-
vertinginputtotheinvertinginput.Therefore,theinverting
input is at the same voltage as the noninverting input. R1
and R2 represent the internal output resistances of these
buffers, approximately 27Ω.
O
1
2
R
OL
V =I
O
O
1+ sR C
(
)
OL
Substituting and rearranging gives:
K is a constant determined by the control circuit and can
be any value between 0 and 1. The control circuit is
described in a later section.
1−K V
(
)
2
KV
1
+
By inspection of the diagram:
R
(
R
R
R
)(
)
(
)(
)
G1 F1
G2 F2
R +
R +
2
1
R +R
R
+R
G1
F1
G2
F2
V
V
O
V =
1
O
I =
−
1
1−K
(
)
1+ sR C
K
R
R
R
F1
OL
(
)(
)
G1 F1
+
+
R +R
+1
F1
1
R +
1
R
R
R
R
OL
F1
F2
G1
R +R
R +R
+1 R +R
+1
G1
F1
F1
1
F2
2
R
R
G2
G1
General Equation for the Noninverting Amplifier Case
12
Linear [ Linear ]
