Honored Contributor
Honored ContributorHere's the data sheet for the ADS55500. http://focus.ti.com/lit/ds/symlink/ads5500.pdf If you go to page 22 you can see the input structure of that converter. Without doing the calculations exactly a mental estimate will give you about a 50 ohm input. That tells you why you have the 2:1 voltage drop from the 50 ohm function generator with/without the A/D load. Almost all of the fast converters have low impedance inputs and that's why on page 24 TI shows you examples of how to drive it.
The chopping may be related to the sample and hold input of the ADC needing a stiff driver. Your 1MOhm probe is NOT 1MOhm at anything but DC. A 15pF probe has 212 ohms of capacitive loading at 50Mhz so that will give you even more voltage drop. If you want to run an analysis there's an excellent SPICE program that Linear Technology gives away free called LTspice/SwitcherCAD III at http://www.linear.com/designtools/software/. It is actually what Linear uses to design their chips with and is better then about anything you can buy. My best advice is for you to find a good analog/RF guy for help (yes, I said RF). Remember, THE WORLD IS ACTUALLY ALL ANALOG, your logic is just fast analog :-) Al
