Thanks John, you were exactly right.

DC coupling, offset to view in the millivolt range, and everything looks right again.

BTW, it's a TEK TDS3014B... last thing I would've suspected.

-Mark


>
> From: "John N. Power" <jpower@BCPL.NET>
> Date: 2004/02/11 Wed PM 04:43:39 EST
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC:] A/D module gain

>
> The common elements in all of this are the signal generator and the scope.
> Perhaps the output
> of the generator changes with frequency. You may say that you are using the
> scope to keep
> the output constant as the frequency changes. Here is a subtle point: the
> frequency range
> from 0.5 to 10 Hz may be at the low end of the AC range for your scope. If
> you are using AC coupling,
> you may be seeing the fall off of the scope response, which you are
> compensating for by
> cranking up the output of the generator. By the time you get to 20Hz, the
> input is above the
> knee of the input response. Perhaps you are using AC coupling to prevent
> the 2.5 volt DC bias
> from complicating the display. Try using DC and shifting the display
> accordingly.
>
> John Power

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