AN1058 Freescale Semiconductor / Motorola, AN1058 Datasheet - Page 8

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

Freescale Semiconductor, Inc.
Application Note
As an illustration, consider noise levels normally found on the power
supply of a typical high-speed HCMOS digital system. It is not unusual
to find 100 mV
broadband noise riding on top of the positive voltage
PP
rail. With a nominal 5-volt HCMOS system, the resulting voltage drop,
down to 4.9 volts, is above the V
for HCMOS. Thus, the 100-mV
OH
signal will probably not upset circuit operation. When present in such a
robust digital system (HCMOS), this 100-mV noise signal is a mere
visual nuisance on the oscilloscope. Because of the theoretically infinite
signal-to-noise ratio of digital gates, the presence of the 100-mV noise
poses no practical threat.
However, when such a noise signal is inserted into an ADC system, the
results can be much more dramatic. In an 8-bit ADC system with a
nominal 5-volt reference, this same 100-mV noise can result in a greater
than 5-bit error in the ADC reading. Thus, an MCU system utilizing an
ADC assumes a different electrical character that requires application of
design practices not traditionally used in the design of digital systems.
What design practices should be used?
To correct or avoid a noisy ADC/MCU design, separate the noisy signals
from the sensitive ones. The challenge is to design a system in which
this separation is practically realized. The closer to the ideal of
completely separating the noisy signals from the sensitive ones, the
better. For situations where the noisy and sensitive circuits cannot be
completely separated, reduce the noise coupling as much as possible.
Since it is difficult to axiomatically specify how to implement both
concepts in all cases, an illustration will aid understanding and provide
an analogy by which individual situations may be gauged.
Motorola tests 100 percent of the ADCs found on its MCUs. Before any
M68HC11 ADCs leave the factory, they have been tested and verified
for specified ADC performance. Even so, it is possible to operate the
M68HC11 in an environment that causes the M68HC11 ADC to
subsequently malfunction. These two scenarios in the life of such an
MCU indicate not, strictly speaking, a parts-related anomaly, but rather
a significant interaction of the part’s characteristics with the electrical
environment.
AN1058
8
MOTOROLA
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