Tom,
Thanks for your input. I do note that you are very generous with your advice on this board - as are many others. I really appreciate it.
In this case, I think your first and third points are well made. As to the second, that is almost certainly true of a normal scope amplifier input, but in this case there are no such devices ¡°left¡± of the A/D chip itself, which I have absolved by test as the cause.
Roman
