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The right digitiser: Finer resolution is better

11 Jul 2014  | Arthur Pini, Greg Tate, Oliver Rovini

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All these figures, other than baseline noise, are based on frequency-domain analysis of the digitiser output for a sinusoidal input. They are defined in IEEE Standards 1057 and a 1241. Most digitiser suppliers specify these values in their data sheets. When comparing figures of merit, make sure that they are specified for the same input frequency, input amplitude, sample rate, and bandwidth.

Figure 4: Comparison of a measurement at 11.6 (yellow trace) and 10 (blue trace) effective bits. The right hand grid shows the same data expanded (zoomed). Note the loss of detail in the blue trace due to lack of resolution.

Applications for high dynamic range
Applications that require digitisers with a large dynamic range and hence greater resolution are those where the signals encountered include both high and low amplitude components. Typical applications include:

 • Echo Ranging measurements such as radar, sonar, lidar, ultrasound and medical imaging. In these applications a large transmitted pulse is followed by a much weaker return echo(s) and the digitiser must be capable of accurately handling both amplitude signals.
 • Ripple Measurements, which require measurement of signals with high offset values and small variations riding on top of the offset. Both components need to be characterized.
 • Modulation analysis on amplitude-modulated signals (AM, SSB, QAM, etc.) exhibit wide variations in signal amplitude.
 • Mass spectrometry, where particles with significantly different mass/charge ratio need to be detected or the sensitivity of the mass spectrometer needs to be improved.
 • Phase measurements, which require measurements of very small differences in amplitude to distinguish between small phase differences./li>
 • Propagation studies, which involve measuring signal path attenuation over various paths and through different media often result in large range of amplitude values.
 • Component testing, where large voltage or current drops need to be characterized.

Measurement example
Figure 4 shows an example of measurement using a 14 bit Spectrum Instrumentation M4i digitiser. Its ENOB (effective number of bits) specification is >11.6 bits at 10MHz. Superimposed on that measurement is a simulation of a digitiser with an ENOB of 10 bits. This data is shown graphically using Spectrum's SBench software.

Both measurements are shown as acquired in the left hand grid, the digitiser trace is shown in yellow, the other trace is in blue. The right hand grid is the same data expanded both horizontally and vertically. Note that the blue trace fails to show the low amplitude details due to its more limited resolution.

Digitisers specify an ideal resolution based on the number of bits in the ADC. This ideal resolution is reduced due to the presence of noise and distortion products. The real resolution is specified in the form of baseline noise, SNR (signal-to-noise ratio), SINAD (Signal-to-noise and distortion ratio), and ENOB. When selecting a digitiser you must match the digitiser's true resolution to your measurement needs. You must also keep in mind the hardware tools incorporated into its design like low noise components and layout, multiple input ranges, specialised signal paths, and signal conditioning. With all these in mind you can safely choose the right digitisers for your job.

About the authors
Arthur Pini is an electronic measurement and analysis consultant with over 50 years experience in the test and measurement industry. He has extensive knowledge of oscilloscope, real-time spectrum analyser, frequency synthesisers, and arbitrary function generator measurements and applications.

Greg Tate and Oliver Rovini also contributed to this article.

To download the PDF version of this article, click here.

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