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Chapter 7:
Testing ADCs on Image Sensors
Quite a few excellent publications explain ADC and DAC testing in detail. For example, Walt Kester and Dan Sheingold of Analog Devices wrote Chapter 5 of Kester’s ADC Handbook (published by Analog Devices, Inc.) exclusively on testing ADCs. This handbook is particularly helpful and free to access online. This final chapter of this book complements the convertercentric publications by providing image-sensor-centric ADC test information and covering techniques that might be of most use to image sensor professionals. ADCs must be compared against the same yardstick for any comparison or conclusions to have meaning. This yardstick is the IEEE Standard 1241-2010, Terminology and Test Methods for Analog-to-Digital Converters. The older standard 1241-2000 is also acceptable. When image sensor designers ignore this important standard, they run the risk of comparing their ADC to other ADCs using the wrong reference parameters. A great example of this problem occurs for SNR and the resultant ENOB. Numerous image sensor papers have published that their ADC has a 9-dB better SNR than it really does, according to the standard. For example, Lee et al. and Dupont and Robert both do this, though their published work is otherwise very good. The 9-dB problem can also result in electronics designers for analog sensors recommending external ADCs that are 1.5 ENOB greater than needed, which creates a higher power consumption, more-expensive, or slower-data-rate product.
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