Neil,


What I use in such situations is a rail-to-rail op-amp powered by the
same voltage as the PIC, so the PIC will never see an excessive voltage.

Configure the op-amp as a summing amplifier, so the input voltage will
be first divided then amplified, this way the op-amp too will be
protected from out-of-limits voltages.

Other benefits are the high impedance seen by the sensor and the low
input impedance seen by the PIC, due to the op-amp.


Cheers,

Isaac



Em 05/05/2016 02:53, Neil escreveu:
> Hi all,
>
> I'm looking for a way to get more resolution on a PIC32 A/D input (I'll=20
> actually be using a Chipkit board) for a low-voltage input signal.=20
> Official spec =3D as good as I can get it (in limited time). The=20
> sensor/amp puts out a 0-10V signal for 0-to-10 Kg force, but I only need=
=20
> to measure about 0 to 0.5kg (up to 0.5V), so only 5% of the full range. =
=20
> Voltage-dividing the signal reduces the resolution/range, so I'll=20
> protect the A/D input with a zener to clamp the signal instead.
>
> Now, with a 2V external reference (lowest allowed by datasheet), 0 to=20
> 0.5V nets only 8-bit resolution.  I'd really like to get better. Other=20
> than adding an op-amp into the circuit, is there any other trick to bump=
=20
> the resolution up?
>
> If I do happen to have a workable op-amp on hand (though I doubt) to=20
> amplify the signal x4, I know that the zener will affect the linearity=20
> of the signal as it gets close to the zener voltage, but would a 3-ish=20
> volt zener cause any noticeable error at around 2V? Or is there some=20
> parameter that indicates that a zener would have a "sharper" knee?
>
> Cheers,
> -Neil.
>
>



--=20
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