As is often the case, this valuable forum produced many responses. Most people suggested that the input impedance to the digitizer had to be large--not 50 ohms. Indeed, sensitive, greatly broadened response is exactly what would be expected when the anode of a current output device goes into a large input impedance. The MCP anode output was connected directly to the digitizer (better performance than with buffer preamp), with input impedance selected as 50 ohms. The input impedance was the first thing checked. However, I stand guilty of being credulous--accepting that it was really 50 ohms as indicated without further skepticism and checking. It was not. Once that possibility was passed by, all the other ones made limited sense.
Thanks to all!
New Orleans, LA USA
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