OPA891: test circuit description and analyzer/method for THD and IMD measurements

You are correct, but since these groups are always doing devices for the next rounds of test equipment they always have to do better than anything you can buy. In HD work, source cancellation methods are normally used, one of the very few published references is here. And actually, this is a ppt for circa 2004 techdays and the referenced article died with that web site when it shut down. 7446.Techniques to improve Distortion at Freq Less than 1MHz.ppt