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I suppose you require to match the differential I/Os of the IC.
Take the Total Impedance (1K differential on each arm => 2K total) and match as you would for single ended. Split this into differential by halving the series impedance and doubling the shunt impedance. This proced ure works well for filters and I hope it works for matching as well.
Also check out P/h/i/l/i/p/s app note A/N/1/7/7/7. It has a good discussion on differential mixer matching. I use this all the time. I measure the differential impedance, put the S-parameters in @DS, do the match, solder the parts and then tweak the values.
Have a look at www.pcbstandards.com site articles.
Also www.pcdmag.com had an article in March99 issue title, "Terminating Differential Signals on PCBs" written by Steve Kaufer & Kellee Crisafulli.
I cannot find the PDF version of the article but I have hardcopy version.
Single ended is when there is just one conductor and ground. Differential is where there are two conductors and ground. The differential signal voltage is the difference in voltage between the two conductors. The common mode signal voltage (which is desirably zero) is the average voltage of the two wires compared to ground.