Sean Breheny wrote:

>No the scope really is a diferential amplifer when used in A-B mode. By your
>reasoning, you would not be able to measure the voltage of a dry cell using the
>scope inputs in A-B mode.  :-)

AIUI only if it's a differential scope. Otherwise B is inverted and added to A.
          Fine for small signals but looking for noise on larger signals would
          lead to clipping or distortion.