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Total
Throughput EffectiveBog'liq analog-to-digital-converter-architectures-and-choices Total
Throughput Effective
Converters
(16-Bit
Multiplexed Needed for
Converter Architecture Resolution) Throughput Application
AD974
SAR
200 kSPS
200 kSPS
1
AD7722
Sigma-Delta
195 kSPS
2.3 kSPS
4
AD9260
Pipelined/
Sigma-Delta
2500 kSPS
75 kSPS
3
Whether converting a single input or several multiplexed inputs,
the AD974 achieves a throughput rate of up to 200 kSPS. Since
the application requires a total throughput of 180 kSPS, the
AD974’s performance is sufficient. In fact, this is exactly the type
of application that the AD974 was designed for: in addition to the
SAR converter and reference, it also contains an integrated
4-channel multiplexer.
The AD7722 and AD9260 both face the challenges confronted
by sigma-delta converters in multiplexing several inputs. The
AD7722’s throughput is 195 kSPS when sampling a single signal,
but it drops to just 2.3 kSPS when converting multiple signals,
due to the settling time which results from oversampling and
filtering. To use the AD7723 in this application, four converters
(one per channel) would be needed.
The AD9260 combines pipeline and sigma-delta techniques. Its
throughput rate of 2.5 MSPS makes it ideal for higher throughput
single channel systems. But in this application, its settling time of
13.35 ms limits its effective throughput to 75 kSPS. To use the
AD9260 in this application would require at least 3 converters.
Note that if the AD9260 were purely a pipelined flash converter, a
single converter would have had the required throughput, assuming
that the inputs are periodic.
Example 2: Single Input, 16-Bit Resolution
Consider now an application that converts a single 90-kHz
bandwidth input at 16-bit resolution. In this case, all three
converters from the first example would work well. Here the choice
among converters would be made on other considerations,
including ac and dc performance, system-level considerations (e.g.,
Is there a great benefit to the anti-alias performance of sigma-
delta converters in this application?), latency, and cost.
Example 3: Multiple Inputs, 14-Bit Resolution
Consider an application in which 16 inputs, each with a dc to
100-kHz bandwidth, are converted with a resolution of at least 14
bits. Three converters suitable for this application include the SAR-
type AD7865, the sigma-delta AD7722, and the pipelined AD9240.
The total throughput required under the Nyquist criterion is
2
×
100 kHz
×
16 = 3.2 MSPS. The following table shows the
throughput for each converter.
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