Hmmm... a summing amp in this configuration means I'll have to ground=20
the unused input, effectively just becoming a voltage divider at the=20
input... right?

I could possibly add the zener (or equiv clamp) on the input side.

Think I should go rummage through the op-amp bin.

FWIW, the sensor does already have an amp, but it's not adjustable, so=20
impedances should be good already.

Cheers,
-Neil.



On 5/5/2016 7:27 AM, Isaac Marino Bavaresco wrote:
> 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
>> actually be using a Chipkit board) for a low-voltage input signal.
>> Official spec =3D as good as I can get it (in limited time). The
>> sensor/amp puts out a 0-10V signal for 0-to-10 Kg force, but I only need
>> to measure about 0 to 0.5kg (up to 0.5V), so only 5% of the full range.
>> Voltage-dividing the signal reduces the resolution/range, so I'll
>> 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
>> 0.5V nets only 8-bit resolution.  I'd really like to get better. Other
>> than adding an op-amp into the circuit, is there any other trick to bump
>> the resolution up?
>>
>> If I do happen to have a workable op-amp on hand (though I doubt) to
>> amplify the signal x4, I know that the zener will affect the linearity
>> of the signal as it gets close to the zener voltage, but would a 3-ish
>> volt zener cause any noticeable error at around 2V? Or is there some
>> parameter that indicates that a zener would have a "sharper" knee?
>>
>> Cheers,
>> -Neil.
>>
>>
>
>

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