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:
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
causes A to be at a maximum positive output.
REF
4
Gain = 1 + R / R
Maximum forward current is injected into the LED, driving the cell to a
2
PHOTOCELL
low resistance and the gain of A to the maximum where it stays until
there is a signal. A signal at the input terminal is amplified, rectified and
1
With R = 100k ohms, the minimum gain is one since the cell “off”
2
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
algebraically summed with V
at the inverting terminal of the
REF
integrator. The control loop will then act to make the absolute value of
the rectified signal equal to the reference voltage. V may be a fixed
REF
options. R can be increased, the LED drive current for the AOI can be
2
value or a function of some other parameter.
increased or a lower resistance AOI such as the VTL5C4 can be used.
Electrically Controlled Gain
Amplifier A together with diodes D and D and resistors R , R , and
2
1
2
3
4
R form a full-wave rectifier. The amplifier has a gain of one so the
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
5
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.
volts. Since the DC output of A is not referenced to ground, op-amp
2
The signal amplifier consists of op-amp A , resistors R and R which
1
3
2
A and resistors R , R , R , and R form a fully differential amplifier
3
6
7
8
9
set the gain and the input resistor R . The gain of this amplifier is given
5
which shifts the DC reference to ground.
by:
Op-amp A is used as an integrator. The signal from the full-wave
4
eout (R2 + R3)
G = -------- = ----------------------
ein R2
rectifier is summed with a reference voltage V and integrated. The
REF
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
Figure 9. AGC Circuit with Electrical Setpoint
65
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
