Application Notes—Analog Optical Isolators
The control loop consists of op-amp A and resistors R and R . This
Note that R and R are the two halves of the cell. These two resistors
1 2
2
1
4
circuit sets the LED current so that:
match within 10% and track over a wide range within 5% so that the
gain is closely set by V when V is fixed.
C
REF
VREF (R1 + R4)
---------- = ----------------------
VC R1
The limits of operation are:
0 < V < V
C
REF
If we set:
and:
then:
or:
R = R
3 4
and the signal must never be so large that amplifier A saturates when
the gain is at maximum.
1
R = R
1
2
e
e
/ e = V / V
out in REF C
This circuit performs a dividing operation with e and V as the
= e (V / V )
in
C
out
in REF
C
numerator and denominator respectively. The accuracy is limited by
the tracking ability of the two sides of the photocell. The error due to
where
V = control voltage
C
matching can be eliminated by trimming R .
4
eout
------- = ---------- = Gain
ein VC
VREF
Figure 10. Electrically Programmable Gain
66
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
