The compressor is driven via triac or via relay ?
You don't have to filter the USB line (which have small current flow)
but the parasitical generator (relay, triac and or circuit to
compressor power line).

The EMI filter on USB will stop the noise from PC (which is much noisy
than your PIC).
Take a look on Steward:
http://www.steward.com/web_prod_fam1024X768.asp?Line=EMC+%2F+EMI+Board+Level+and+Cable+Core+Products&ScrSize=1240
and here:
http://www.steward.com/applications.asp

Vasile

On 3/26/08, Mujahidin@iddhien.com <mujahidin@iddhien.com> wrote:
> Dear All,
>
> Can anyone suggest for me, how / where i can find the resource to make
> the filter for USB data line?,   I'm make circuit for USB data
> acquisition in a system mixed with large current consuming instrument,
> Like when compressor (8.9 Ampere 220VAC) ON or OFF, my USB DAQ system is
> stuck. I'm using PIC18F4550 to develop USB Data acquisition board.
>
> Thanks,
>
> iddhien.
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