From: Howard Shapiro (hms@shapirolab.com)
Date: Mon Feb 09 1998 - 19:11:01 EST
If you have two parameters, A and B, on a log scale, log (A/B) = log A - log B. Thus, you can generate the log of the ratio by simple subtraction. Generally speaking, you want to include some factor K to keep the data on scale, but, since log K(A/B) = log K + log A - log B, you can accomplish this by adding a constant. This works very nicely when you don't have enough resolution to go from log to linear and back, and, if you aren't using at least 16-bit ADC's you probably don't have enough resolution. As Mario Roederer pointed out, you still need some calibrator for both the numerator and the denominator parameter of the ratio. -Howard
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