AN1058 Freescale Semiconductor / Motorola, AN1058 Datasheet - Page 5

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

AN1058
MOTOROLA
Although the sigma-delta variation of the servo converter provides high-
resolution conversions and maintains a relatively high throughput rate,
the fastest type is the FLASH converter. By stringing together several
voltage comparators (one per desired level to be detected), conversion
bandwidths in excess of 100 MHz are now quite commonplace (see
Figure
the input voltage is applied to one input of all the comparators, a set of
reference voltages is applied to the other comparator inputs. After a
period of time has elapsed, determined primarily by the propagation
delays through the comparators, the discrete-level representation of the
input voltage is available at the comparator outputs. FLASH conversion,
although incredibly fast, requires a tremendous number of devices to
implement even modest-resolution converters. In addition to the number
of transistors necessary to implement each comparator, the outputs of
each comparator are typically input to a combinational logic array to form
a desired output code. Consequently, this converter, which consumes
much silicon area when compared to other converter types, has not
been widely accepted by MCU designers and users.
The fourth ADC type is the successive-approximation converter (SAC).
Of all current converter types, the SAC is the most popular. (See
Figure
circuit requirements, medium-to-fast conversion speeds, and the
medium- to high-resolution applications (8 to 16 bits). Like other
converter types, the SAC uses a differential voltage comparator to
compare the input signal with a reference voltage. By performing a
binary search, conversion rates of one bit per clock are possible.
Because only one comparator is typically required and the output code
is inherent in the conversion process, circuitry and silicon surface area
are reduced when compared with other conversion methods. Although
the exact implementation varies from silicon-chromium-based to charge-
redistribution, this ADC is currently the most prevalent type found on
MCUs.
The fifth ADC category is the hybrid converter. In this case, the term
hybrid is not used to reference a specific implementation approach, but
rather implies combining one or more ADC types to form an ADC with
different performance characteristics.
Freescale Semiconductor, Inc.
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1(c)), albeit at lower resolutions (four to six bits are common). As
1(d)). This popularity is primarily due to its applicability to smaller
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Application Note
ADC Types
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