In this measurement the J2130A provides a DC bias to the Optocoupler LED, which returns to ground through a termination resistor. The optocoupler transistor collector is powered from a separate power supply and the emitter is terminated through a termination resistor to ground, we use equal valued termination resistors in order to normalize the CTR to 100%. The resistor value is not as critical as it is for the resistors to be the same value and type.
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