A/D Rounding

[Guest]

Just getting back to using Pics again after 3 years absence, got a question that I should know the answer to but can't remember.

When doing A/D conversion (8bit) you only get a value between 0 - 255 right?

Now my question - when does the value change from say 50 to 51?
Does it round up ie once the true value would be 50.51 does it round up to 51 or does it stay at 50 until the voltage on the A/D pin is at least equal to 51.

Hope you can understand what I'm asking

regards

JFK

[SherpaDoug]

The answer is dominated by the signal noise and the error level of the A/D. Most modern A/Ds are guaranteed monotonic, which measn a rising voltage will go from 49 to 50 to 51 not skipping any values, but this is not guaranteed unless the spec sheet says so. Even still the range of voltage represented by "50" could be vanishingly small, or nearly twice as large as "nominal". Many A/Ds have trouble at the point the MSB switches. The step width for 0x7F may be very different from the step width for 0x80.
Better A/Ds may be guaranteed to 1/4LSB in which case each output is at least 1/4 of nominal width. But 50.51 implies 1/100LSB which just ain't going to happen!
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