NI PCI-6115, problems in high frequencies

Hi,
I'm not sure if this is the right place to make this question but let's try..
I try to measure the frequency response of the output impedance from a certain switched-mode converter using NI PCI-6115 (I inject sinusoids into output voltage and measure the output current). I get a nice plot but there are some problems in high frequencies. It seems that the measurement card makes some unexpected peaks. I use the network analyzer (Venable) to verify the results and beyond 200 kHz the results do not match. I attached the plot with the results from both methods (MC = PCI-6115, Venable = network analyzer). Does anyone have any idea what could cause this difference in high frequencies? The measurement set ups where exactly identical.
Thanks!
Attachments:
Doc1.pdf ‏87 KB

looks like a resonace at 400kHz...
Are you shure that the input impedances of both setup are the same? Are you injecting with a transformer and build a LC resonator with the MC?
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