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VTL5C1 Datasheet(PDF) 70 Page - PerkinElmer Optoelectronics |
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VTL5C1 Datasheet(HTML) 70 Page - PerkinElmer Optoelectronics |
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70 / 76 page  65 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 G e out e in -------- R 2 R 3 + () R 2 ---------------------- == |
Similar Part No. - VTL5C1 |
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Similar Description - VTL5C1 |
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