AN1058 Freescale Semiconductor / Motorola, AN1058 Datasheet - Page 20

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

Application Note
The Perfect Circuit
20
After sufficiently instrumenting the offending assembly, the next step is
to attempt to duplicate ideal operating conditions for the MCU/ADC.
Since every M68HC11 is 100 percent tested for ADC performance
before leaving the factory, in the absence of externally induced failure,
the M68HC11 should maintain factory performance given identical
operating conditions. By operating the M68HC11 in near perfect
conditions, the engineer learns if the failure is or is not parts related. The
motivating factor for this case, however, concerned 1 V
in time at approximately the E-clock rate of the M68HC11) found when
measuring V
the ADC comparator, it was best for system performance to reduce this
V
power busing to the M68HC11 only. The PCB foil was cut to V
V
directly to an external laboratory-grade power supply. With this
configuration, measurements were taken as before. The results of these
measurements are shown in
improvement was made over the non-bypassed assembly. Instead of a
5 percent error in the ADC readings, less than 0.5 percent of the
readings were outside of the expected range. Also evident in
the presence of full-scale errors as before. At this point, a bypass
capacitor was soldered between the M68HC11 V
resulting measurements, shown in
improvement over the previous non-bypassed assembly. However, due
to the granularity of the measurement reported by the logic analyzer, it
cannot be stated quantitatively how much the bypassing improved the
condition. Further manipulation of the bypassing network failed to
improve the readings in a discernible manner.
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Freescale Semiconductor, Inc.
, and V
noise as much as possible. The noise was reduced by isolating the
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DD
leading to the M68HC11. Discrete wires were then run
at the pins of the M68HC11. Given what is known about
Figure
Figure
9. As the graph shows, an
10, are an apparent
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and V
PP
noise (spaced
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Figure 9
MOTOROLA
pins. The
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AN1058
, V
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is
,