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Maxim > Design Support > Technical Documents > Application Notes > A/D and D/A Conversion/Sampling Circuits > APP 647
Maxim > Design Support > Technical Documents > Application Notes > Amplifier and Comparator Circuits > APP 647
Keywords: analog to digital conversion, ADC, A to D, DACs, DAC, digital to analog converter, converters,
D/A, A/D conveter, DACs and comparators, transient voltage analysis, time-domain reflectometry, fault
detection, diagnostics
APPLICATION NOTE 647
Comparator/DAC Combinations Solve Data-
Acquisition Problems
Jan 21, 2002
Abstract: The following discussion examines an overlooked option for many existing A/D converter
applications: the A/D conversion is sometimes better implemented with a discrete comparator and D/A
converter. This substitution generally entails a different measurement approach, but the advantages can
include lower cost, higher speed, more flexibility, and lower power consumption.
Current trends, though, are in the other direction—designers who must implement A/D conversion usually
specify a packaged A/D converter (ADC) for the job. Most engineers are not aware of an alternative, and
the price/performance ratios for ADCs are falling all the time. Yet, an analog comparator plus D/A
converter (DAC), along with digital processing capability, form the core of a successive-approximation
ADC.
The discrete comparator/DAC approach is already common in certain fields. Automatic test equipment,
nuclear pulse-height discriminators, and automated time-domain reflectometers often use the technique
whereby one comparator input is driven by the DAC, and the other is driven by the signal to be
monitored. Following is a selection of general measurement problems and specific applications in which
a comparator/DAC combination is actually more appropriate than an off-the-shelf ADC.
Transient Voltage Analysis
A brute-force technique for capturing fast-changing amplitude events (transients) is simply to digitize
them with a high-speed ADC supported by a processor and fast RAM (Figure 1). Single-shot events
may compel the use of this approach, as may the need to discern fine detail in the transients. Otherwise,
if the transients are repetitive, you can measure their peak amplitude and other features with the
DAC/comparator approach (Figure 2).
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