VT200 PerkinElmer Optoelectronics, VT200 Datasheet - Page 71

• Manufacturer Part Number: VT200
• Description: Photoconductive Cells and Analog Optoisolators (vactrols)
• Manufacturer: PerkinElmer Optoelectronics

Application Notes—Analog Optical Isolators
The control loop consists of op-amp A
circuit sets the LED current so that:
If we set:
and:
then:
or:
where
V
---------- -
V
REF
C
=
----------------------
R
R
R
e
e
V
1
2
out
out
C
R
3
1
+
and resistors R
= R
= R
= control voltage
1
/ e
= e
R
4
2
in
4
in
= V
(V
REF
REF
Figure 10. Electrically Programmable Gain
/ V
/ V
1
C
C
and R
)
4
. This
66
Note that R
match within 10% and track over a wide range within 5% so that the
gain is closely set by V
The limits of operation are:
and the signal must never be so large that amplifier A
the gain is at maximum.
This circuit performs a dividing operation with e
numerator and denominator respectively. The accuracy is limited by
the tracking ability of the two sides of the photocell. The error due to
matching can be eliminated by trimming R
0 < V
1
and R
C
< V
e
------- -
2
e
out
REF
in
are the two halves of the cell. These two resistors
C
when V
=
V
---------- -
V
REF
C
REF
is fixed.
=
Gain
4
.
in
1
and V
saturates when
C
as the