Benjamin, your circuit is designed for diferential output, basicaly it's a
bridge. The schematic as you design it hasn't too much sense because:
-- the input signal in adc is using only half bridge so the sensitivity
will be one half than the case of a full bridge usage
-- limiting the adc input voltage can be done using a resistor and a 0.5W
glass package 5v6 zenner at the output of an operational amplifier which
inputs are connected at middle of the bridge network.
-- the bridge operating modes are described in any measuring tehniques
book.

best regards, Vasile


On Fri, 15 Feb 2002, Benjamin Bromilow wrote:

> Dear all,
>
> I wonder if anyone has any ideas on how this circuit will act?
> I'm sure it won't do what I want to do but I'm curious as to what anyone
> thinks it will do!!
>
> I've using a 2.5v reference, using an op-amp voltage doubler to bump up the
> current output and double the voltage.. I feed this through a potential
> divider with the bottom resistor being of variable resistance (actually a
> sensor). The sensor is usually between 0 and 200 ohms so I've used a 200 ohm
> resistor for the top resistor..... This gives between 0 and 2.5v from the
> middle of the pot divider....
> However, if it gets really cold the resistance will go >200 and the voltage
> goes >2.5v. The resistance could easily go much higher and the PIC ADC will
> see more than Vref+0.3v (from the 16F877 datasheet)....
> So I drew up this circuit. Theoretically if the middle of the second pot
> divider is 2.5v, any voltage on the first greater than this will cause the
> diode to conduct... But where will the "volts" go???
> My gut instinct on this circuit is that it will become an oscillator very
> quickly... Any ideas??
> How else does anyone limit the voltage... Low value zener diode? I know
> recently these have been discussed and no-one was impressed.....
>
> Thanks for any input,
>
> Ben
>

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