Adc

[eng1]

Since you use a single supply source, the output of the amplifier won't go below 0 V and the PIC won't see a negative voltage with respect to ground. Now you have to make sure that the input voltage of the amplifier is within the allowed range.
I would try to design the bridge in such a way that a negative differential voltage would not appear at its output.


P.S.: have you tried to simulate the circuit and/or to make any measurement?

 

[vinke]

I would try to design the bridge in such a way that a negative differential voltage would not appear at its output.
P.S.: have you tried to simulate the circuit and/or to make any measurement?

Hi eng1,
i dont know if you read my previous posts in that same thread,but i said that i have found a way to make the differential output of my bridge be always positive.here is a quote from it:But the max resistance of soil seen by the electrodes is not constant ,it may increase,and if it increases above the value ive set in the bridge,the differential output will become -ve.Ive been thinking to solve that problem by placing a resistor,Rohms, in parallel with my electrodes so that when my electrodes reads a resistance greater than R,the resultant resistant seen by the bridge will never exceed Rohms.hence the differential output will never be negative,at the expense of a lower bridge output range.
Yep ive simulated the circuit.,its gud.infact that was not my problem,the problem was would a pic adc read a -ve voltage as zero or whatsoever.and as nobody seem to know the answer,ill do that experiment and let u know

 

[ericgibbs]

Single supply.normally if the differential input becomes negatives so should be the output.

Yep,a single supply

the output with reference to the ground!
that is a differential output of 0V (V/2-V/2)

As eng1 points out if you have only single positive supplies to the instrumentation amp and the bridge supply, where is the -V out signal going to come from?