Microsecond photocurrent integrator with phase-delay interruption of integration. Encyclopedia of radio electronics and electrical engineering

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The circuit shown in Figure 1 is a two-channel microsecond photocurrent integrator with a phase delay of the integration duration, in other words, it is an optical photodetector that allows detecting stroboscopic optical pulses of different duty cycles and durations from fractions of microseconds to tens of milliseconds without tuning the integration time duration, since this parameter depends on the phase of the input signal, followed by an integration reset pulse.

(click to enlarge)

Two integration channels A1 and A2 are needed for the subsequent sum-difference processing of the signal from the output of the integrators. In this circuit, a photocurrent integrator is used, the output signal of the integrator is proportional to the area of ​​​​the section limited by the voltage amplitude and the time axis, if the input signal is direct current, then the output signal is an increasing inclined voltage plane (Fig. 2a).

Exact analog integration is carried out by OA A1 and A2 with capacitive OS - C3 and C4. The main components of the integration errors are due to the zero bias voltage Ucm and the input currents of the op-amp. To eliminate the latter, an operational amplifier was used as an integrator with field-effect transistor input stages, since their gates practically do not consume current, and the entire photocurrent generated by the photodiode PD1 and PD2 flows through the integrating capacitances C3 and C4 Fig. 1, and the rate of increase in the output voltage is determined by the value of the photocurrent. The zero offset voltage Ucm can cause significant drift in the output voltage and can cause comparator A3 to falsely trip, which would lead to a malfunction of the circuit.

Therefore, as an integrator, an operational amplifier chip from Texas Instruments OPA350 was used, which has a zero offset level of the output signal of only a few millivolts and allows you to adjust this parameter using potentiometers R7 and R8. As is known, the output voltage of the integrator achieved during the integration process does not decrease to zero at the subsequent zero input signal, but continues to remain at a given level in the absence of "parasitic" input photocurrents, otherwise it changes and reaches the maximum value Uip.

To compensate for "parasitic" input photocurrents that occur in the absence of a stroboscopic pulse, a combined optocoupler is used, consisting of a photodiode connected in reverse polarity and an LED - SD1, PD3 and SD2, PD4. Compensation adjustment is carried out by potentiometers R1 and R2 until the output signal of the integrator in the absence of an input pulse becomes a horizontal line or zero. This indicates the correct operation of the integrator, but the latter makes it practically impossible to correctly integrate subsequent signals, since the same initial conditions are required to measure and compare optical pulses before integrating them.

To eliminate this effect, the output voltage of the integrator must be periodically "reset" to Ucm. In the integrator for the "reset" reset keys are used, the DD1 microcircuit in fig. 1. K176KT1 or K561KTZ, upon closing of which the capacitances C3 and C4 are discharged, and the output voltage drops to the zero bias voltage. Here, the control "button" is the input E1 and E2. In the "reset" mode (the key is closed), the initial integration conditions are set. Such an electronic contact and its load circuit are not galvanically connected to the control signal source.

To generate a reset pulse, a circuit is used that contains a comparator chip A3, which operates as follows. From output 6 of the first integrator in Fig. 1. The signal is fed to the comparator, which is triggered when the reference signal and the signal from the integrator output are equal, the level of which is 20 mV, fig. 2a and 2c, and is adjusted by potentiometer R10. Therefore, a significant zero drift of the output signal of the previous stage of the integrator would cause false operation of the comparator and failure of the circuit.

The comparator must have an infinitely large gain with no noise in the input signal and low zero drift. Such a characteristic can be obtained using an amplifier with a very high gain, these requirements are met by the OPA350RA op-amp, which has the ability to operate from a unipolar power supply. The output is a TTL signal. Next, the output logical signal from the comparator is fed to the circuit for forming the phase delay of the integrator reset pulse, fig. 2b.

Since the delay of the reset pulse of the integrator should not depend on the frequency of the input signal, since the stroboscopic signals arriving at the input of the integrator PD1 and PD2 have different durations and duty cycles, therefore, to form the reset pulse delay, the DD2 chip of the KR1006VI1 digital timer was used to form the phase delay of the reset pulse.

The essence of the operation of the circuit is that the capacitor C13 is linearly charged through series-connected resistors R11 and R13, linearly discharged through the resistor R13. With the arrival of a signal from the comparator, the process of linear charging of the capacitor begins to the voltage Upor=1/2 Upit. When this value is reached, the capacitor begins to linearly discharge, even if there is a signal at the input. When the capacitor is discharged, a rectangular signal is generated at the output of the microcircuit; it is this signal that is the phase delay signal. This circuit generates a phase delay φ and works stably at 0<φ<180 degrees.

To increase the frequency range, the capacitance of the capacitor is better to take 1 uF. The resistance of the resistor R11 in most cases can be taken equal to 100 kOhm. The phase shift is corrected with a potentiometer R13 and it is better to choose a value of 100 kOhm. Further, on the negative edge of the pulse from the output of the timer, the waiting multivibrator DD3 is started.

Using different values ​​of the elements R12 and C11, you can set another required time for the multivibrator to work. The multivibrator generates a pulse with a duration of 20 ms, fig. 2d, supplied to the control inputs of electronic switches E1 and E2 of the DD1 microcircuit, shunting the capacitances of the integrators C3 and C4, and nulling the signals at the outputs of 6 integrators, thereby creating the initial conditions for processing subsequent stroboscopic pulses. From the outputs 6, the signals of the integrators are received for subsequent total difference processing.

Author: Altair NTPC; Publication: cxem.net

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