[time-nuts] opto coupler isolation?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jan 3 17:33:38 EST 2009


Poul-Henning Kamp wrote:
> In message <494882EB-D714-49B4-A7E2-BEF04509D7F4 at deriesp.demon.nl>, Henk ten Pi
> erick writes:
>   
>> Hi,
>>
>> An opto coupler seems  good idea for an isolation amplifier but maybe  
>> not for the phase noise.
>>     
>
> Optocouplers have lousy dV/dt performance, so the jitter is
> horrible.
>
>   
Not true for the higher speed optoisolators (eg Avago's HCPL9000 series)
In any case its more a matter of noise to slew rate ratio.

The issue is really that the signal to noise ratio of the internal
optical signal is relatively low.
For better performance you need to use a higher power optical signal
such as that from a single mode laser.
However by the time you factor in the cost of the modulator etc its
usually better to revert to a conventional design.

Bruce




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