[hpsdr] Mercury Input

[stoskopf at tri.net]

Glad to see the diode/zener limiting on the input to protect me from the
occasional 'accidents' I seem to be prone to.  The LTC6400-20 should
protect the following A/D.  Guessing that an input low-pass network will
help the clamp further.  Not sure I've seen what these type devices do to
linearity.  For curiosity, anyone point to a source?  Wouldn't want to
leave it out in any case.  Thanks,  N0UU