I have an application that uses two hall effect ratiometric linear sensors. One sensor occasionally "sees" a magnetic pole (either south or north). The second sensor is located in the same vicinity, but doesn't "see" the magnet. The idea being that the ambient condition can be filtered out. In other words, the natural magnetic fields, both earth's and nearby objects are no longer part of the output signal.

Currently, I feed these two analog signals into a 8051 derivative's 8-bit ADC. The two ratiometric linear devices are very integrated, temperature compensated sensors. The input voltage is a 9 volt battery.

I'm now wanting to increase the detection distance, which requires some analog work. I'm still learning and haven't done much with analog circuits, so I'm wanting to know if I'm heading down the right path.

So, I'm thinking that this is the job for an instrumentation amplifier combined with some additional ADC precision. I would like to obtain as high a gain as possible, but one problem...

The sensor outputs are not "matched". What I mean by that is that sitting in normal ambient (no magnet), one sensor may read 3.40 volts and the second sensor may read 3.57 volts. It's enough that a gain of say, 10, would result in the offset becoming a major problem.

Is there such a thing as adjustable input offset? IE: during the zero'ing process, I would like to "adjust" one signal to equal that of the other. Is this typically how such a condition is handled?

Any suggestions for a integrated IC that meets this condition? I've been looking at the AD624, but it's about $10. I *think* it generally does what I need. I was hoping to find something more around the $3-4 range.

As far as the ADC, I'm looking at the Silicon Labs F350. It's priced right ($4) and gives 24-bit precision.

Thanks for the help
Bruce