VT33CT datasheet(70/76 Pages) PERKINELMER

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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:

Gain = 1 + R2 / RPHOTOCELL

With R2 = 100k ohms, the minimum gain is one since the cell “off”

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. R2 can be increased, the LED drive current for the AOI can be

increased or a lower resistance AOI such as the VTL5C4 can be used.

Amplifier A2 together with diodes D1 and D2 and resistors R3, R4, and

R5 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

volts. Since the DC output of A2 is not referenced to ground, op-amp

A3 and resistors R6, R7, R8, and R9 form a fully differential amplifier

which shifts the DC reference to ground.

Op-amp A4 is used as an integrator. The signal from the full-wave

rectifier is summed with a reference voltage VREF and integrated. The

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

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 VREF causes A4 to be at a maximum positive output.

Maximum forward current is injected into the LED, driving the cell to a

low resistance and the gain of A1 to the maximum where it stays until

there is a signal. A signal at the input terminal is amplified, rectified and

algebraically summed with VREF at the inverting terminal of the

integrator. The control loop will then act to make the absolute value of

the rectified signal equal to the reference voltage. VREF may be a fixed

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 A1, resistors R3 and R2 which

set the gain and the input resistor R5. The gain of this amplifier is given

by:

Figure 9. AGC Circuit with Electrical Setpoint

out

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