Feature: T&M
Figure 1: Direct comparison of a raw impulse signal and the ringing of a sigma-delta ADC versus the higher bandwidth (but increased noise) of an SAR ADC A top-level comparison of SAR and
delta-sigma ADC types is shown in Table 1.
Speed vs. dynamic range Engineers choosing a DAQ system for their measurements have historically been forced to choose between speed and dynamic range. In a few fields of study, such as acoustics and vibration modal analysis, for example, delta-sigma has been, and remains, the clear choice because of its dynamic range and noise performance. In other fields, such as recording transient or impulse signals, SAR has been the only choice due to its raw speed. The problem arises with applications
that require a combination of both high-speed sampling and high dynamic range with anti-aliasing filters: which to choose? Engineers have been forced either to sacrifice one of these capabilities, or to buy two completely separate DAQ systems and operate them simultaneously. Sometimes, sacrificing one of the capabilities is simply not possible, so two systems must be used, drastically increasing costs and test
Figure 2: Hybrid ADC top-level architecture
complexity. This complexity only increases after the tests are done, when two completely independent sets of data have to be merged, analysed and ultimately archived. Happily, in the last few years a
new type of DAQ technology has been developed that combines the best features of both SAR and delta- sigma ADC technology. Each channel can sample up to 15MS/s. When the sample rate is below 1MS/s, resolution is better than 24 bits, providing up to 150dB of dynamic range. In addition,
anti-aliasing filtering is built-in and automatic. This new technology allows the user to
select between two modes of operation: 1. High bandwidth mode: 5MHz bandwidth and 15MS/s sampling rate, the hybrid ADC can acquire impulse, step and square signals without any ringing or overshoot. This mode is ideal for transient recording and power analysis, its behaviour similar to that of typical SAR ADCs, except with much higher sample rates and bandwidths.
www.electronicsworld.co.uk February 2022 23
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