The photoelectric coupler can effectively suppress the sharp pulse and all kinds of noise interference in the transmission signal, so that the signal-noise ratio in the channel can be improved greatly, and it is mainly for the following reasons:
(1) The input impedance of optical coupler
is very small, only a few hundred ohms, and interference source impedance is bigger, often 105 ~ 106 ohms. According to the principle of partial pressure, even if the interfering voltage amplitude is big, but the noise voltage at the input to feed to the photoelectric coupler will be small, can only form a weak current, as there were not enough energy and cannot make diode luminescence, which are suppressed.
(2) There is no electrical connection between the input circuit of the photoelectric coupler and the output circuit, and there is no common ground; The distributed capacitance between them is tiny, and insulation resistance is very big. Therefore, the loop side of the various interference noise is difficult to through the photoelectric coupler feed to the other side, avoids the production of common impedance coupling interference signals.
(3) The photoelectric coupler can play a very good security role, even when the external equipment fails, or if the input signal line is short, and it will not damage the instrument. Because the optical coupler's input circuit and output loop can withstand several thousand volts.
(4) The photoelectric coupler responds extremely fast, and its response delay time is only about 10 us, which is suitable for the occasion where the response speed is very high.