ADC Calibration

ErnieM said:

I find it interesting that no one has described or linked to any sort of calibration procedure. Here is how I have done it.

First we need to know the relationship from input to output, typically this is a linear relationship, meaning a straight line as we learned in high school;

Y = m * X + B eq 1
where: X is what we input
Y is what we read out of the A/D
B is the zero offset.
m is the slope of the line

To calibrate such a system we need to take two readings, one close to the bottom (X1, Y1) and one close to the top (X2, Y2). By "close" I mean where the output is changing uniformly with the input, so we are past any offset clipping and the like.

With our two measurements we can compute the slope:
m = delta Y / delta X = (Y2 - Y1) / (X2 - X1)

Now plug in either pair into eq 1 to compute B.

If you are on a "powerful enough" controller doing this using floating point numbers is fine, but in a resource limited micro it is usually best to pick the units such that m and B are integers. Once the reading is computed the decimal point (or binary point) may be shifted to get a more useful result.

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