On Thursday 08 May 2003 23:12, Jinx scribbled:
> > Yep -- Param A25, Sym VAin, on page 136 of ds30221B.  Table
> > 14-9: A/D  Converter Characteristics.  I don't see any reference to
> > this being specific to RA3, though.
>
> I was looking at the F877. Which one are you using ? We may be
> at complete cross-purposes here

Doc 30221b, for the 16F872.


> > I interpreted param A25 to mean "The input voltage on *any* analog
> > input pin (except those used for external Vrefs) must not go less tha=
n
> > 0.3V below Vss  and must not go higher than 0.3V above Vref+".
>
> Referring to the Analogue Input Model diagram -  ...

Okay, looking at it.


> ... AN inputs have the
> usual protection diodes to Vss and Vdd.

Correct, so the diodes will clamp any inputs greater than Vdd or less tha=
n Vss=20
to Vdd and Vss respectively.  Plus/minus the diode threshold voltages=20
accordingly, of course.


> I don't believe that putting
> Vdd on an analogue input pin causes any physical harm. A25 is not
> very well explained is it, but I can't say I'm totally au fait with eve=
ry
> aspect of the A/D, just helping you kick a ball around

It's good you did, cause I hadn't looked at the analog i/p model until yo=
u=20
mentioned it.  I'm sure they mention the max limit on the input signal wi=
th=20
reference to Vref+ for a good reason.  I'm sure the answer will pop up=20
sometime, even if I have to ask Microchip's tech support directly.

In the meanwhile, I'm designing with a zener added to the input.  However=
,=20
seeing the input model fresh in my mind, I have an alternate idea -- perh=
aps=20
I can add my own protection diode (non-zener) to the input and going to m=
y=20
+2.5V ref instead of Vdd (as the internal upper input-protection diode do=
es). =20
I've given up trying to derive a formula for the sensor for various reaso=
ns,=20
so I'm using a table to derive the measured data based on the a/d convers=
ion=20
readings.  With this system, I can calibrate around any effects presented=
 by=20
any diode(s) I add to the input pin.

Cheers,
-Neil.

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