--- In "Yan Seiner" <> wrote:
>
>
> I hooked up a handful of LM34 temp probes to my MAX197 A/D. The LM34 is a
> temp probe that puts out 10mv/degree, so at 72 degrees it would put out
> .72V.
>
> Pretty simple.
>
> However, I'm seeing very weird behavior. At 65 degrees and below, the A/D
> starts reporting .29V. This is consistent over 6 LM34s and 6 channels.
>
> As soon as the LM34 gets above 65 or 66 degrees, it starts reporting
> normally.
>
> Also, I get 0.29V on any unconnected pin of the max 197.
>
> I have no idea why. Can anyone shed any light on this?
>
> --
> If you have eight hours to chop down a tree
> spend six sharpening your axe.
> --Abraham Lincoln
>
Hi Yan,
you don't say how you are connecting the LM34 so I'll assume straight in. Again
, look at input resistance. Can the LM34 supply enough current to drive the
inputs of the Max197? I have not looked (that's your job ;) ) but is suspect
that's the problem.
This may also be a case of sinking current as well as sourcing current. IIRC
the 197 in unipolar mode is biased at half full scale so voltages less than
that will probably need the sampled source to sink current. Many applications
supply current better than they sink it.
As all correspondents suggested last time : buffer with an op-amp.
(and check the amp's ability to sink).
regards , Jacques.
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