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The nitty-gritty of product specification

Posted: 22 Dec 2014 ?? ?Print Version ?Bookmark and Share

Keywords:fast Fourier Transform? spec? data acquisition? ADC?

The method of specifying any product entails combining the characteristics of that product and identifying its accepted pluralities. This may sound simple enough but it gets tricky. There is no single constant condition for testing a product and this is where the issue emerges. Varying test scenarios would basically mean that specs can either turn out good or bad. That's a pretty wide range if you ask me.

Why can that be a problem? It's not practical for manufacturers to provide performance data for every conceivable set of test conditions. Thus manufacturers often provide typical performance characteristics along with maximum or worst-case performance characteristics. The typical conditions are usually chosen to be those that the user is most likely to experience when using the device.

For instance, a typical spec might be for operating the device with a signal that is 6dB below full scale. A set of worst-case conditions may include the results of operating the device with one or more operating parameters set to an extreme. For data-acquisition and digitiser products, these may include sample rate, input signal level, multiplexing rate, temperature and more.

For example, running fast Fourier Transforms (FFTs) on data acquisition products gives meaningful information on noise, harmonics, distortion, etc. Running the systems at full scale and at maximum throughput is the toughest test, which yields slightly worse specs than running them at -6dB (half scale) and at 1kHz (versus maximum throughput at over 100kHz). The best representation would be to show the entire spectrum under minimum and maximum ranges for full scale and full throughput.

OK, problem solved. Wait! Is that true?

I am reminded of a customer in Europe using one of our imaging boards a few years ago. They had a very smooth waveform from their detector using our competitor's board. Our board showed jagged edges with higher frequency components. The customer liked the competitor's board because of the smooth output. We pointed out to the customer that the competitor's board didn't have high enough bandwidth and essentially was filtering the "real" output of its detector. Because this was a medical application, they were not seeing the actual output from their own detector. To the customer, the specs were the same, but obviously the bandwidth of the other product was not meeting spec. They quickly changed to our product.

Another area in data acquisition is ADC resolution. Today, 24bit sigma-delta ADCs are widely used to provide high-resolution measurements. They also provide filtering to prevent fictitious data known as aliasing, where high-frequency input components erroneously appear as lower frequencies after sampling.

A 24bit ADC means its output word is 24 bits, but the effective resolution of the ADC may be much less. Low-frequency sigma-delta ADCs used for weigh scales may have an effective resolution of 22 bits, yet higher bandwidth 24bit ADCs used for audio applications may provide the equivalent accuracy of only 18 bits.

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