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Analog Dialogue 33-8 (1999)Bog'liq analog-to-digital-converter-architectures-and-choices1-BIT
DAC
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–
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DIGITAL
DIGITAL
FILTER
AND
DECIMATOR
BIT
STREAM
D
COMPARATOR
INTEGRATOR
A
Figure 5. Sigma-delta ADC architecture.
Design Considerations and Implications:
One of the most
advantageous features of the sigma-delta architecture is the
capability of
noise shaping
, a phenomenon by which much of the
low-frequency noise is effectively pushed up to higher frequencies
and out of the band of interest. As a result, the sigma-delta
architecture has been very popular for designing low-bandwidth
high-resolution ADCs for precision measurement. Also, since the
input is sampled at a high “oversampled” rate, unlike the other
architectures described in this paper, the requirement for external
anti-alias filtering is greatly relaxed. A limitation of this architecture
is its
latency
, which is substantially greater than that of the other
types. Because of oversampling and latency, sigma-delta converters
are not often used in multiplexed signal applications. To avoid
interference between multiplexed signals, a delay at least equal to
the decimator’s total delay must occur between conversions. These
characteristics can be improved in sophisticated sigma-delta ADC
designs by using multiple integrator stages and/or multi-bit DACs.
APPLICATION EXAMPLES
The following three examples illustrate some of the issues described
above.
Example 1: Multiple Inputs, 16-Bit Resolution
Consider an application that requires 16-bit resolution
for 4 independent signals with bandwidths of dc to 15 kHz,
15 kHz, 15 kHz, and 45 kHz. The total throughput required to
s a m p l e t h e s e s i g n a l s u n d e r t h e N y q u i s t c ri t e r i o n i s
(2
×
15 + 2
×
15 + 2
×
15 + 2
×
45) kSPS = 180 kSPS. At first
glance, the SAR-type AD974, the sigma-delta AD7722, and the
pipelined/sigma-delta AD9260 all have the required total
throughput capability. But, as has been discussed above, the
inherent latency of sigma-delta converters limits their effective
throughput when they must continually acquire new signals by
multiplexing.
Effective multiplexed throughput
can be defined as the
total throughput of a converter when two or more independent
signals are multiplexed. The following table compares the total
throughput and effective throughput for each converter and
indicates the number of converters of its type that would be
needed to serve in this application.
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