The input end of the photoelectric coupler is the light-emitting diode. Therefore, its input characteristics can be represented by the volt ampere characteristic of the light-emitting diode. Its output end is the photosensitive triode, so the volt ampere characteristic of the photosensitive triode is the output characteristic. It can be seen that the photoelectric coupler has a nonlinear working area, so the precision is a little poor when it is used to transmit the analog quantity directly.
One of solutions is to use two photoelectric couplers with the same nonlinear transmission characteristics, T1 and T2, and the two emitter followers A1 and A2. If T1 and T2 are the photoelectric couplers of the same type and batch, it can be thought that their nonlinear transmission characteristics are completely consistent, that is, K1 (I1) =K2 (I1). As a result, the voltage gain of the amplifier G=Uo/U1=I3R3/I2R2= (R3 / R2) [K1 (I1)/K2 (I1)] =R3/ R2. It can be seen that the symmetry of the current transmission characteristics of T1 and T2 can be used to compensate their original non-linearity with the feedback principle .
Another solution of the analog volume transmission is to adopt the VFC (voltage-frequency conversion) approach. On the spot, the transmitter outputs analog signal( hypothetic voltage signal), and the voltage frequency converter converts the voltage signal sent by the transmitter to the pulse sequence. And then it's sent out through optical coupler
isolation. On the host side, the pulse sequence is reduced to the analog signal by a frequency voltage conversion circuit. At this time, it is the digital quantity which is isolated by optical coupler, and the effects of the nonlinear coupling can be eliminated. This is an effective and simple method of analog quantity transmission.
Of course, the linear optical coupling can also be selected for the design, such as the precision linear coupling TIL300 and high-speed linear coupling 6N135/6N136. The linear decoupling is generally more expensive than the normal one, but it is easy to use, and simple to design. With the decline of the device price, the use of linear decoupling will be the trend.