Comparator/DAC Combinations Solve Data-Acquisition Problems - AN647

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). Figure 1. As the brute-force approach to transient analysis, an ADC circuit is power-hungry and expensive.