Datasheet

12
LT1251/LT1256
APPLICATIONS INFORMATION
WUU
U
Figure 2 is the basic block diagram of the LT1251/LT1256
signal path with external resistors R
G1
, R
F1
, R
G2
and R
F2
.
Both input stages are operating as noninverting amplifiers
with two input signals V
1
and V
2
.
Each input stage has a unity-gain buffer from the nonin-
verting input to the inverting input. Therefore, the inverting
input is at the same voltage as the noninverting input. R
1
and R
2
represent the internal output resistances of these
buffers, approximately 27Ω.
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.
By inspection of the diagram:
I
V
R
RR
RR
V
RR
R
R
GF
GF
O
F
F
G
1
1
1
11
11
11
1
1
1
=
+
()()
+
−
++






Substituting and rearranging gives:
I
V
R
RR
RR
V
RR
R
R
IKI KI
VI
R
sR C
GF
GF
O
F
F
G
O
OO
OL
OL
2
2
2
22
22
22
2
2
12
1
1
1
=
+
()()
+
−
++






=+−
()
=
+
()








General Equation for the Noninverting Amplifier Case
V
KV
R
RR
RR
KV
R
RR
RR
sR C
R
K
RR
R
R
K
RR
R
R
O
GF
GF
GF
GF
OL
OL
F
F
G
F
F
G
=
+
()()
+
+
−
()
+
()()
+
+
+
++






+
−
()
++






1
1
11
11
2
2
22
22
11
1
1
22
2
2
1
1
1
1
1
–
+
Σ
1
14
K
I
1
I
O
8
C
V
O
1251/56 BD
I
1
I
2
I
2
1 – K
R
1
V
1
V
2
R
2
R
G2
R
G1
R
OL
R
F1
R
F2
2
13
+1
–
+
1
2
Figure 2. Signal Path Block Diagram
Signal Path Description