HCNR200 HCNR201数据手册(5)

The circuit in Figure 19 is actually very similar to the pre-vious circuit. As mentioned above, only one optocoupler is used. Because a photodiode can conduct current in only one direction, two diodes (D1 and D2) are used to steer the input current to the appropriate terminal of input photodiode PD1 to allow bipolar input currents. Normally the forward voltage drops of the diodes would cause a serious linearity or accuracy problem. However, an additional amplifi er is used to provide an appropriate off set voltage to the other amplifi ers that exactly cancels the diode voltage drops to maintain circuit accuracy.Diodes D3 and D4 perform two diff erent functions; the diodes keep their respective amplifi ers active indepen-dent of the input signal polarity (as in the previous cir-cuit), and they also provide the feedback signal to PD1 that cancels the voltage drops of diodes D1 and D2.Either a comparator or an extra op-amp can be used to sense the polarity of the input signal and drive an inex-pensive digital optocoupler, like a 6N139.

It is also possible to convert this circuit into a fully bipolar circuit (with a bipolar output signal) by using the output of the 6N139 to drive some CMOS switches to switch the polarity of PD2 depending on the polarity of the input signal, obtaining a bipolar output voltage swing.

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