VT300CT PerkinElmer Optoelectronics, VT300CT Datasheet - Page 70

• Manufacturer Part Number: VT300CT
• Description: Photoconductive Cells and Analog Optoisolators (Vactrols)
• Manufacturer: PerkinElmer Optoelectronics

Application Notes—Analog Optical Isolators
Figure 9 shows an AGC circuit which consists of three main elements:
a variable gain amplifier, full-wave active rectifier and a summing
amplifier. The variable gain amplifier consists of op-amp A1 with
potentiometric gain that is controlled by the resistance of the photocell
of the AOI. The gain of this amplifier is:
With R
resistance is several megohms. The maximum gain in only 100 since
the resistance of a typical VTL5C2 is 1000 ohms at an input current of
5.0 mA. If a range of 40 db (100:1) is not adequate, there are several
options. R
increased or a lower resistance AOI such as the VTL5C4 can be used.
Amplifier A
R
output is equal to the rectified input. There is no offset due to rectifier
forward drops so this circuit will rectify signals all the way down to zero
volts. Since the DC output of A
A
which shifts the DC reference to ground.
Op-amp A
rectifier is summed with a reference voltage V
time constant of the integrator is selected to limit the bandwidth of the
control loop as well as assure stability of the loop. If the bandwidth is
3
5
and resistors R
form a full-wave rectifier. The amplifier has a gain of one so the
2
= 100k ohms, the minimum gain is one since the cell “off”
Gain = 1 + R
2
4
2
can be increased, the LED drive current for the AOI can be
together with diodes D
is used as an integrator. The signal from the full-wave
6
, R
2
7
/ R
, R
PHOTOCELL
8
, and R
2
is not referenced to ground, op-amp
1
9
and D
form a fully differential amplifier
2
and resistors R
REF
and integrated. The
Figure 9. AGC Circuit with Electrical Setpoint
3
, R
4
, and
65
too wide, the control loop will follow the signal on an instantaneous
basis. The AOI alone is not very fast, but signals with frequencies of 30
– 60 Hz could be distorted if there were no time delay in the integrator.
The AGC circuit operates as follows. When there is no signal, the
negative V
Maximum forward current is injected into the LED, driving the cell to a
low resistance and the gain of A
there is a signal. A signal at the input terminal is amplified, rectified and
algebraically summed with V
integrator. The control loop will then act to make the absolute value of
the rectified signal equal to the reference voltage. V
value or a function of some other parameter.
Electrically Controlled Gain
The gain of an amplifier can be electrically programmed using the
circuit of Figure 10. An AOI with a center tapped photocell is used, one
side in the signal amplifier channel and the other in the control loop.
The signal amplifier consists of op-amp A
set the gain and the input resistor R
by:
REF
causes A
G
=
4
e
------- -
e
out
to be at a maximum positive output.
in
REF
1
=
to the maximum where it stays until
5
at the inverting terminal of the
. The gain of this amplifier is given
----------------------
R
2
1
R
, resistors R
+
2
R
3
REF
3
may be a fixed
and R
2
which