I am currently working on isolating SPI bus.The SCLK for my non isolated version of design is 5MHz .How do I calculate whether the optocoupler is suitable for operation at this frequency of operation.Can somebody explain me design steps with PC817 say for 20KHz frequency so that in similar way I can design my circuit.

  • \$\begingroup\$ That sort of information is usually in the datasheet for the part. \$\endgroup\$ Feb 18, 2016 at 13:08
  • \$\begingroup\$ You determine the Iforward current with respect to nominal value, frequncy characteristics and aging. Then you calculte the resitor, its all in datasheet. \$\endgroup\$ Feb 18, 2016 at 13:13

1 Answer 1


It's depend on the rise and fall time of the optocoupler, or rather I should say the propagation delay.
In the best case you'll get 143 kHZ (tr+tf)
enter image description here (see the datasheet).
If your frequency is 5MHZ, the PC817 optocoupler isn't the best choice. The optocoupler propagation delay is more than the SPI clock period (1/frequency).
In this link you'll understand how to choose a suitable optocoupler to your SPI bus.

  • 1
    \$\begingroup\$ You forgot td and ts (which are not specified, yay); any difference will influence the pulse width distortion. The cut-off frequency is not really useful because a −3 dB sine wave signal isn't digital anymore. \$\endgroup\$
    – CL.
    Feb 18, 2016 at 13:34
  • \$\begingroup\$ The td and ts are about few nano seconds. For the cut-off frequency, i agree with you it for AC signals. \$\endgroup\$
    – R Djorane
    Feb 18, 2016 at 13:43

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