AN1058 Freescale Semiconductor / Motorola, AN1058 Datasheet - Page 6

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

Application Note
ADC Noise Susceptibility
6
For example, some of the faster and higher resolution ADCs now employ
a hybridized technique which utilizes FLASH conversion prescaling
followed by an SAC. In this case, almost instantaneous prescaling is
accomplished and easily interfaced to an existing SAC design. Hybrid
converters are a very viable alternative as an MCU peripheral and may
find eventual popularity in MCU designs when higher resolution
converters are needed.
The comparator is the cornerstone of the A/D conversion process. The
ability of the comparator to announce the presence of small
voltage/current differentials with large changes in its output voltage
make the comparator invaluable to the A/D conversion process. Yet, this
same feature also accounts for the largest potential source of ADC
malfunction. Of course, degradation of the comparator’s desired action,
and hence the ADC, is most usually caused by unwanted noise.
Two basic characteristics of the comparator affect noise susceptibility:
Wide bandwidth comparators easily respond to noise as well as to
signals. Even in the low-speed integrating converter, the accuracy of
measurement is heavily contingent upon the comparator’s speed of
operation. To illustrate, imagine that a very slowly varying input signal
has been applied to an input of such a comparator. For a single-slope
integrating converter, the other comparator input will have a linearly
increasing voltage (or other convenient shape) applied to it. As this
voltage ramp increases, an independent digital counter (started at the
same time the voltage ramp began) will count clock pulses provided by
some timebase. When the voltage ramp finally exceeds the input
voltage, the comparator will change state. If the comparator fails to
respond to the voltage ramp in a timely fashion, the digital counter will
register an incorrect count when compared to the results obtained by a
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MOTOROLA
AN1058