I'm currently using FOD2711A optocoupler as an isolation between the PWM generator and the rest of the circuit. I connected the optocoupler as shown bellow. My problem is that the output voltage is not the normal logic signal. At frequency less than 5KHZ it works nice since the output voltage is a logic signal with 10V peak but in the case of high frequency the Vout looks like a capacitor charging voltage( goes up with delay). Attached to you Can my circuit. you please help me.