We began to venture forth on the path of multiplying our new hall effect sensor signal when we came across some trouble.
The new hall effect sensors were in and, according to their spec sheet, twice as sensitive as the previous versions. Still, after the Arduino digitizes the analog in signal, we would still only be working with 40-60 tics of range out of the possible 1024. To combat this problem we did two things. First, we adjusted the magnets which were inducing the magnetic field that the hall effect sensor was reading in a push-push configuration so that the field would be the densest and the signal would be the strongest. Then, the more important step was to amplify the signal by means of an op-amp. Furthermore, since we were interested in a range of number interior to the max range of the signal, we not only wanted to magnify the signal, but we also wanted to offset the voltage to more or set 0v to one end of our range (low end for non-inverting, high end for inverting op-amp). To do this we would need to produce a virtual ground and send this signal to the non-inverting pin of the op-amp. The virtual ground was a non-zero voltage that would act as a starting line for the signal voltage. For example, if our voltage range was 2-3v and the virtual ground was 1v, the resulting signal would be read as 1-2v (2-3 minus 1). We therefore wanted to send in about 2.5v into the non-inverting terminal and then scale the .93v of range we were reading from the multimeter at the armatures extremes into the full 5v the Arduino would read. We thought it would be a bright idea to send a PWM equally the equivalent of 2.5v directly into the non-inverting terminal from the Arduino to save us some time creating circuits and then pick an Rf/Ri = 5.
We tried this and it did not work. Furthermore, the output size and range of the same signal did not scale according to Rf/Ri at all, and actually did not seem to follow any trend. In fact, switching Rf for Ri also did not have any effect. We tried the following combinations of Rf and Ri with the following results:
Rf: 20k, Ri:15k Vrange:.5-.72v
Rf: 20k, Ri:20k Vrange:.2-.7v
Rf: 20k, Ri:25k Vrange:.09-.64v
Rf: 20k, Ri:47k Vrange:.07.13v
We could not find a pattern or rule to go by. We tried a number of other circuits, and even tried double up the gain, by feeding the output of one op-amp into another. This didn't have any effect at all.
We actually still don't know the reason for the trouble, but, when we replaced the PWM with a voltage divided 2.5v from another op-amp circuit, everything worked like a charm. We think that perhaps there were some current flows that we did not measure that were messing up the function of the op-amp. We were checking voltage consistently, and they all looked to be alright on the input side of the op-amps, but the current was never checked.
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