AN1058 Freescale Semiconductor / Motorola, AN1058 Datasheet - Page 18

• Manufacturer Part Number: AN1058
• Description: Reducing A/D Errors in Microcontroller Applications
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
18
Errors many times this specification were encountered in the application.
Unfortunately, evaluation of the extent of the ADC errors concerned only
functional operation of the assembly and manual inspection of ADC
values read with an in-circuit emulator, making the problem more
serious. An attempt by the hardware engineers to reduce the noise by
changing the bypassing scheme yielded no apparent change in the
pattern of product shutdown. In this case, the lack of quantitative data
convinced the engineers that they had no control over the problem,
diverting attention from the actual cause.
When dealing with these types of problems, always instrument the
problem correctly — that is, ensure measuring techniques used to
observe the malfunction by following these guidelines:
Had the assembly been properly monitored, an improvement in ADC
performance with the different bypassing scheme would have been
evident (see
conversion values on the vertical axis and the hit rate (percent of total
readings landing within the boundaries of the selected ADC reading
range) on the horizontal axis. As shown, there was approximately a
3 percent improvement in the number of correct A/D conversions with
new bypassing. To detect these changes, the EPROM on the controller
PCB was probed with a fairly simple logic analyzer. The logic analyzer
was then configured to trigger on accesses to a location in memory
containing the results of A/D conversions. By utilizing the simple
statistics options given by the analyzer, each quantitative improvement
in ADC performance was observed.
1. Quantify the A/D conversion process with regard to frequency of
2. Ensure that the measurements are with sufficient resolution so
3. Ensure that the number of observed conversions is similar to
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occurrence and magnitude of error.
that minute improvements or degradations in performance may be
monitored and evaluated.
product usage or is statistically significant to allow inference from
the measured sample to actual product operation.
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Figure
8). These two histograms display ranges of A/D
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